D Alt WIN ISM

DAKWINISM

AN KXI'"Siri.i\ ()K THK

TIIKOIIY OF XATl'HAL SELECTION

WITH soMK OF ITS APPLICATIONS

\.\

ALFRED RUSSEL WALLACE

LL.D., 1.1..-., ETC.

WITH MAP AND ILLUSTRATIONS

ILontJon
MACMILLAN .VXD CO.

AND NEW YOKK
1889

All I'/';///,'

PRESSWORK BY JOHN WILSON ANU SON,
UNIVERSITY PRESS.

PREFACE

Tm present work treats tlu- problem of the Origin of Species
nil tin- same general lines afl were, adopted by Darwin ; lint
from the standpoint reached ;iftcr nearly thirty years of
di-cu ion, with an abundance of new facts and the advocac\
iif many new or old theorie-.

While nut attempting to deal, even in outline, with the
\.ist siilijert <>f evolution in general, an endeavour has Keen
made to uive >ueh an account of the t heoi \ of Natural Selec-
tion a> may enable any intelligent reader to ohtain a clear
conception of Darwin'- work, and to understand something
of the power and range of his great principle.

Darwin wrote for a -eiieration which had not accepted
evolution, and which poured contempt on those who upheld
the derivation of .species from -prcjcs l.y any natural law of
descent He did his work so well that "de-cent with
modification" i- now universally accepted as the order of
nature iii the organic world ; and the rising generation of
naturalists ,-an hardly realise the novelty of this idea, or that
their fathers considered it a scientific heresy to be condemned
rather than seriously discussed.

The objections now made to Dai-win's theory apply, solely,
to the particular means by which the change of species has
Keen brought about, not to the fact of that change. The
objectors >,,!< to minimise the agency of natural selection
and to subordinate it to laws of variation, of use and disuse,
of intelligence, and of heredity. 'These views and objections

vi PREFACE

are urged with much force and more confidence, and for the
most part by the modern school of laboratory naturalists, to
whom the peculiarities and distinctions of species, as such,
their distribution and their affinities, have little interest as
compared with the problems of histology and embryology,
of physiology and morphology. Their work in these depart-
ments is of the greatest interest and of the highest importance,
but it is not the kind of work which, by itself, enables one to
form a sound judgment on the questions involved in the
action of the law of natural selection. These rest mainly on
the external and vital relations of species to species in a state
of nature on what has been well termed by Semper the
"physiology of organisms," rather than on the anatomy or
physiology of organs.

It has always been considered a weakness in Darwin's
work that he based his theory, primarily, on the evidence of
variation in domesticated animals and cultivated plants. I
have endeavoured to secure a firm foundation for the theory
in the variations of organisms in a state of nature ; and as
the exact amount and precise character of these variations is
of paramount importance in the numerous problems that
arise when we apply the theory to explain the facts of nature,
I have endeavoured, by means of a series of diagrams, to
exhibit to the eye the actual variations as they are found to
exist in a sufficient number of species. By doing this, not
only does the reader obtain a better and more precise idea of
variation than can be given by any number of tabular state-
ments or cases of extreme individual variation, but we obtain
a basis of fact by which to test the statements and objections
usually put forth on the subject of specific variability ; and it
will be found that, throughout the work, I have frequently to
appeal to these diagrams and the facts they illustrate, just as
Darwin was accustomed to appeal to the facts of variation
among dogs and pigeons.

I'KKFACH vii

I have al-o made what appears to me an imp- >rtant change
ill the arrangement of the -ul'jei -t. Instead of treating tirst
tin- comparatively difficult and unfamiliar details i.f variation,
1 "innirnrr with the Struggle for Kxi-tence, which is really

the fundamental phenomenon on which natural .-election

depend-, while the particular fart- uhieh illustrate it are
eoinparatively familiar and very intei v-t in_. It ha- the
flirtlier advantage that, after di-eu--iiiir variation and the
effect- i.t artificial >electioli. We proceed at ..lice to explain
how natural selection acts.

Anion- the -ul>jeet- of novelty or interest di-cu ed in thi-
volume, and which have important lieariii",- on the theo|-\- of
natural -eh--tion, are : (1) A proof that all >;///- characl

ar I Once ha\e Keen) either 11-eflll ill t h'lll>el \ es i,|- COT

relat.'d with u>eful charaerer- (( 'hap. \ 1 i ; ( L' i a proof that
natural -election can, in certain cases, increase the >terilit\ "t
crosses (<'hap. \'II): (.") a fuller di-c-ii-.-i..n of the colour
relation- of animal-, with additional factfl and arguments "ii
the on-in of -exual ilitlereiiee- .,f colour (('hap-. \I I I X);
an attempted -olution of the difficulty pre-ented ly the
occurrence of both very .-imple and \ery c..mple\ mode- uf
-eeiirin^ the eii-- fertilisation of plant- (('hap. X 1 i ; (5) some
fie-h tact- and argument.- on the wind-carriage of seeds, and

its hearing on the wide di-per-al of many arctic ami alpine
plants (('hap. XII i; Hi (some ne\v illu.-t rat ion- of the noii
heretlity of acquired i-haracter-, and a proof that the etf'eet- \
u-e and disu-e. even if inherited, mu-t lie o\ erpowei-ed by
natural -ele.-tion (('ha|i. XIV): and (7) a new argument as to
the nature and ori-in ,.f the moral and intellectual faeuln>-
of man (< 'hap. XV).

Although 1 maintain, and e\en enforce, my ditlerences
from some of Darwin's views, my whole work tends forcibly
to illustrate the overwhelming importance of Natural Selec-
tion over all other agencies in the production of new species.

vin PREFACE

I thus take up Darwin's earlier position, from which he some-
what receded in the later editions of his works, on account
of criticisms and objections which I have endeavoured to show
are unsound. Even in rejecting that phase of sexual selection
depending on female choice, I insist on the greater efficacy
of natural selection. This is pre-eminently the Darwinian
doctrine, and I therefore claim for my book the position of
being the advocate of pure Darwinism.

I wish to express my obligation to Mr. Francis Darwin for
lending me some of his father's unused notes, and to many other
friends for facts or information, which have, I believe, been
acknowledged either in the text or footnotes. Mr. James Sime
has kindly read over the proofs and given me many useful
suggestions ; and I have to thank Professor Meldola, Mr.
Hemsley, and Mr. E. B. Poulton for valuable notes or
corrections in the later chapters in which their special subjects
are touched upon.

GODALMING, March 1889.

( OXTKXTS

CHAPTEB I

WH \T ARE "SPECIES" \N|. WHAT Is MEANT MY THF.Ii;

"ORIGIN "

I 1 imitioii of sp.T). - iM'.n Tip- .Mily transmutationista

- > ntitir opinion before I'uuin Tli-- prol !nn brfoiv llarwin Tin-
ohange of "I'inion rtlr, tnl by l)ar\\in Tip- I'arunnaii tln-oiA I'M

posed mode of treatment of the subjecl . . . 1'a-"^ 1-1:5

CHAI'TI'.i; II

I UK >Ti:i'(..I.K F"i: i:\lvrK\rK

iiiiiii>rt,in. - Tin' stru^l'- sunong plants^ Among animals ll!u.str;iti\'-

- Succession >( tn-r- in |.ii,--!> n|' l.nni:iik Tin- >lin^l' I'm

i-.\i-tciif.- on tin- I'.uni'ii- In. T.-:ISI- .if organisma in a geonn-tii.-il

r.itin Exaiiijili-s of raniil n ( animals llapi'l ini-riM-<- ami

\\iilr >pr.'a.l of plants Great (Vrtility i, ial t<- lapiil in' i

^MlL'U'l'' l"'t\\.'i'II rln-rly allifl >J i.-i.-i---, 11. M'--Tln' ''tl

aspect of the straggle for existence . . 14-4o

< II AFTER 111

Tin: \ AI;I Ai'.n.iTV OF smn IN \ STATE "F NAITUK

Importance of variability Popular Mra.s iv^anling it Variability of the
lower animals Tin- variability of insects Variation among lizards

x CONTENTS

Variation among birds Diagrams of bird- variation Number of
varying individuals Variation in the mammalia Variation in
internal organs Variations in the skull Variations in the habits of
animals The variability of plants Species which vary little Con-
cluding remarks ..... Pages 41-82

CHAPTER IV

VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED

PLANTS

The facts of variation and artificial selection Proofs of the generality of
variation -Variations of apples and melons Variations of flowers
Variations of domestic animals Domestic pigeons Acclimatisation
Circumstances favourable to selection by man Conditions favour-
able to variation Concluding remarks . . . 8-3-101

CHAPTER V

NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE

FITTEST

Effect of struggle for existence under unchanged conditions The effect
under change of conditions Divergence of character In insects In
birds In mammalia Divergence leads to a maximum of life in each
area Closely allied species inhabit distinct areas Adaptation to
conditions at various periods of life The continued existence of low
forms of life Extinction of low types among the higher animals
Circumstances favourable to the origin of new species Probable
origin of the dippers The importance of isolation On the advance
of organisation by natural selection Summary of the first five
chapters ....... 102-125

CHAPTER VI

DIFFICULTIES AND OBJECTIONS

Difficulty as to smallness of variations As to the right variations occur-
ring when required The beginnings of important organs The mam-
mary glands The eyes of flatfish Origin of the eye Useless or
non-adaptive characters Recent extension of the region of utility in

CONTENTS xi

plants The sanif in animals Uses of tails Of tin- horns of ile,r
i >f tlii- scale-ornamentation of reptiles Instability <>\' Don-adaptive

characters 1 lelluellf's 1 N "-] itir" character prove, I to he

useless The swamping etfeets of intercrossing Isolation as prevent-
ing intercrossim,' dilii-kou the effects of isolation ! which

isolation is illetleetive . . I'au'es 1'J'i-lfil

CHAl'TKK VII

ON THK INKKRTII.ITY or . ROSSES BETWEEN HISTINJ ] SPEl [ES
\M> rHE US1 LL STERILITY OF I III 1 1: llVl'.lilh "I PSPRING

Statement of the prol.l.-m Kxtreme susceptibility of the reproductive
functions Reciprocal ci Iii'livi'lual 'litl- : n r> -pe, t to

:.-rtilis.-itioii I liiiM.rphi-ni aiil triin.'rphism u nonu' plani^

'ilitvof livliriils ami of the infertility of nioie'i'ls

The elleet-, of close int.Tl>iv,-,linu' ML HuMi- objections -l-'eitili-

liyl'riils anmnj,' aniln lit I i tility of liyl'ri.K aiiion^ pluil- laSOS "f
terility of nii'ii^reU I' i: illeli-m liet-. ->in.u' aii'l elian.'e ..f

'oiniition> ll'in.ii!^ on tlie farts of liyl'ri'lity St.-rility line to
eliaii^eil romlitions ami usually eorrelated with other eliaraet,]-
( 'orn lation of colour \\itli rmistitiitioiial peeiiliaritie> The isolation
of varii-tie> l.y selective association The infliii-nee of natural selection
upon -terilitv ami fertility I'hysiolo^j ^:.mniarv ami

remarks ..... l.V.

CHAPTEE VIII

THK O|;K.IN ASH USES OF COLOUR IN \\IM\l.s

The I >ar\vinian tli.ory tlirew ne\v li^'lit on ..iL. r nn< colour Tlic probleii

lie solve'l Tli- f animal colour indicates utility Colour

and environment Arctic animals white- K\'cptionx j,i,,\-,- the rule

Desert, forest, nocturnal, ami oceanic animals (Jem-nil theories of
animal colour Variable prot.-.-tive colouring .Mr. I'oulton'.-, experi-
ments Special or loeal colour adaptations Imitation of ['articular
objects How they have been prodii- - bive ,-oloiirin^

of butterflies Protective lesemblanee ajnoii^ marine animals 1':"-
tection by terrifyim,' enemies Alluring coloration The coloration
of binls' eg^s ( 'oloiir a~ a ni'aiis of p-co^nition Summary of the
preceding exposition Influent f climate on colour

Concluding remarks . . . 187-21S1

xii CONTENTS

CHAPTER IX

WARNING COLORATION AND MIMICRY

The skunk as an example of warning coloration Warning colours among
insects Butterflies Caterpillars Mimicry How mimicry has been
produced Helicon idse Perfection of the imitation Other cases of
mimicry among Lepidoptera Mimicry among protected groups Its
explanation Extension of the principle Mimicry in other orders of
insects Mimicry among the vertebrata Snakes The rattlesnake and
the cobra Mimicry among birds Objections to the theory of mimicry
Concluding remarks on warning colours and mimicry

Pages 232-267

CHAPTER X

COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX

Sex colours in the mollusca and Crustacea In insects In butterflies and
moths Probable causes of these colours Sexual selection as a
supposed cause Sexual coloration of birds Cause of dull colours of
female birds Relation of sex colour to nesting habits Sexual colours
of other vertebrates Sexual selection by the struggles of males
Sexual characters due to natural selection Decorative plumage of
males and its effect on the females Display of decorative plumage
by the males A theory of animal coloration The origin of accessory
l>lumes Development of accessory plumes and their display The
effect of female preference will be neutralised by natural selection
General laws of animal coloration -Concluding remarks . 268-300

CHAPTER XI

THE SPECIAL COLOURS OF PLANTS : THEIR ORIGIN
AND PURPOSE

The general colour relations of plants Colours of fruits The meaning of
nuts Edible or attractive fruits The colours of flowers Modes of
securing cross-fertilisation The interpretation of the facts Summary

CONTENTS xiii

of a<Mition,il f.i- t- brai-ing n in>.-i't I'.-i tilisati.'ti - I-Vrtiliviti.m of
tl.iw.Ts by birds S.-lf-t'.-itilisatioii nf Bowers Difficulties and eon-

tr.idi<-ti.>iis Int'T.-ms..iii^ n.it : '.ly advantageous Supposed

i-vil results nf rinse interbreeding How tin- struggle tW i-\iMi'n.-e
unongflowen Flowers the product of in \ Concludim,'

remarks on aolonr in nature .... 1 'a ges '><.>'.

CHAPTEB XII

NIK GBOGRAPHICA1 lM->Ti;ii;l Tl- \ OP ORGANISMS

'I'll. i.i. 'is ti> } . .\|ihiiiifl Tin- . .'ii.liti<in> \vhi.-li 1. iiniii.-'l dis-

ti'iliutinii Thr ]-i in. in. -a. " .'!' oceans '.. mi. an.l n.iitiin-iital ai.'is

Ma.l.i.- IMMI- ami Nt-w Xi-.ilainl Tin- t'-.irhin^s ,.f th.' tli.ni-.aml-

f.itlK'iu lim- Tin- ilistriliiiti'>M !' m ai^ujiials Tin- iliMrilniti.ni nf

tapirs l'..\v.-rs .if >li^|-r>al as illu-t tatnl by insular organisms l'.iiu>

an.l iii- ! kltitudea The dispersal of plauta

I ii-|..-i - il ..f >, . -,u l.y tin- \viml Minrral matter carried by the \\in.l

Ol.ji'.-timis t.. tin- tin -in v nf wind-dispersal answnv.l - K.\|ilanatioii
of north ti-iui'i-rati- plants in tin- -,,iiili.-rn hfini>plirrf N'n pr.Hjf of
t,'la.-iati.n in 'I' 1 ' tr.ipi.-s L..\V.-I t.-nip.'iMtiin- imt m-i-ik-J to rxplain
the facts Concluding remarks . . 338-374

CHArTKK XIII
IIIK <;K<>I.<M.l''.vi. EVIDENCES OF EVOLUTION

What \\. in iv . \p. . t Th. iiumlicr of known species of extinct animals-
Causes of tlu- imp. -ill -. -timi of the geological record Geological
evidences of evoluti.ui Sh.-lls Ci..liles The rhinoceros tribe
The pr.li^n-e of the horse tribe Development of deer's horns Brain
devi-1'ipineiit Lnral relations of fossil and living animals Cause
of extinction of large animals Indications of general progress in
plants and animals The progressive development of plants Possible
cause of sudden late appearance of exogens Geological distribu-
tion of insects Geological succession of vertebrata Concluding
remark^ . 375-409

xiv CONTENTS

CHAPTER XIV

FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION

AND HEREDITY

Fundamental difficulties and objections Mr. Herbert Spencer's factors
of organic evolution Disuse and effects of withdrawal of natural
selection Supposed effects of disuse among wild animals Difficulty
as to co-adaptation of parts by variation and selection Direct action
of the environment The American school of evolutionists Origin
of the feet of the ungulates Supposed action of animal intelligence
Semper on the direct influence of the environment Professor Geddes's
theory of variation in plants Objections to the theory On the
origin of spines Variation and selection overpower the effects of use
and disuse Supposed action of the environment in imitating varia-
tions Weismann's theory of heredity The cause of variation The
non-heredity of acquired characters The theory of instinct Con-
cluding remarks ..... Pages 410-444

CHAPTER XV

DARWINISM APPLIED TO MAN

General identity of human and animal structure Rudiments and varia-
tions showing relation of man to other mammals The embryonic
development of man and other mammalia Diseases common to man
and the lower animals The animals most nearly allied to man
The brains of man and apes External differences of man and apes
Summary of the animal characteristics of man The geological
antiqiiity of man The probable birthplace of man The origin of
the moral and intellectual nature of man The argument from
continuity The origin of the mathematical faculty The origin of
the musical and artistic faculties Independent proof that these
faculties have not been developed by natural selection The inter-
pretation of the facts Concluding remarks . . 445-478

INDEX 479-494

LIST oK ILLUSTRATIONS

l'i:niAiT I>K Ar i ii"i: . . . /'/

M.VI' -II-. WIN.; nil I'm. I. KM II.. M MM . '/' ' \'.<

m..

L DIAGBAM or VABIATIONS OF LACBBTA MURALia . . ir

-. .. V \i;i \TluN IIK Ll/. \|;l.- . . I -

VxRiAlluN UK WIN..- \\n rA.lt "1 l!n:|.-
1 .. V\I:IVIMN iiK Dnl.Ii-ll.'N'i \ "KY/.IVol;

V \I:I.\TIIIN I.K A..M.M- PHCDTICBTO . . ."'.

'i. ,, V\i:iArni\ UK CMM.INU.I- VIKHINIV- . 58

7. ,. V \KI.\TIilN iiK lAK.-f- AMI TiiKS . . 60

VMIIAII.IN OF I'.IIM.- IN LEYDKN Mr- . 61

'.'. ,, VAKIAII..N UK I. i n:r- I'.AI.I IM..I:I

li". .. VvKi.viniN I.K A..PI.I*- rii' i NII i r- . . 64

11. ,, .vi.- OK VAKI A II..N . . 64

1-. ,. VARIATION OK CAI-.I.IN \i.i- \ H:I,IMANI - . 65

13. ,, VARIATION "t S. HKI- rAumiMN-i- . . 67

14. .. VARIATION OF SKI I.LS OK WOI.K 70
1".. .. VARIATION UK *Krn.s UK Uusr- i.Ar.i.\rr- . 7_
I'l. ,, VARIAII..N OK .-KTI.I.S UK Srs ri:i-r\; . 73

17. I'kiMn.A VKRI> '(.'owsliji). From Darmn's Forma of Flcnotrs . 157

18. GAZEI.I.A .SCEMMKRRINIM (to show recognition marks) . . 219

19. RECOGNITION MARKS OF AFRICAN PLOVERS (from Seebohm's

Charadriadce . .

xvi LIST OF ILLUSTRATIONS

FIG. PAGE

20. RECOGNITION OF (EDICNEMUS VERMICULATUS AND (E. SENEGA-

LENSIS (from Seebohm's Charadriadce) . . . 223

21. RECOGNITION OF CURSORIUS CHALCOPTERUS AND C. GALLICUS

(from Seebohm's Charadriadce) .... 224

22. RECOGNITION OF SCOLOPAX MEGALA AND S. STENURA (from

Seebohm's Charadriadix) ... . . . 225

23. METHONA PSIDII AND LEPTALIS ORISE .... 241

24. OPTHALMIS LINCEA AND ARTAXA SIMULAN.S (from the Official

Narrative of the Voyage of the Challenger) . . . 247

25. WINGS OF ITUNA ILIONE AND THYRIDIA MEGISTO (from Pro-

ceedings of the Entomological Society) .... 251

26. MYGNIMIA AVICULUS AND COLOBORHOMBUS FASCIATIPENNIS . 259

27. MIMICKING INSECTS FROM THE PHILIPPINES (from Semper's

Animal Life) ....... 260

28. MALVA SYLVESTRIS AND M. ROTUNDIFOLIA (from Lubbock's

British Wild Flowers in Relation to Insects) . . .311

29. LYTHRUM SALICARIA, THREE FORMS OF (from Lubbock's British

Wild Flowers in Relation to Insects) . . . .312

30. ORCHIS PYRAMIDALIS (from Darwin's Fertilisation of Orchids) . 314

31. HUMMING-BIRD FERTILISING MARCGRAVIA NEPENTHOIDES . 320

32. DIAGRAM OF MEAN HEIGHT OF LAND AND DEPTH OF OCEANS 345

33. GEOLOGICAL DEVELOPMENT OF THE HOKSE TRIBE (from Huxley's

American Addresses) ...... 388

34. DIAGRAM ILLUSTRATING THE GEOLOGICAL DISTRIBUTION OF

PLANTS (from Ward's Sketch of Palceobotany) . . . 402

35. TRANSFORMATION OF ARTEMIA SALINA TO A. MILHAUSENII

(from Semper's Animal Life) ..... 426

36. BRANCHIPUS STAGNALIS AND ARTEMIA SALINA (from Semper's

Animal Life) . . ... 427

37. CHIMPANZEE (TROGLODYTES NIGER) .... 454

i HAPTEB 1

WHAT AKI. "SPECIES," \\1' \\ll\l I- Ml\\l r.V
I ill n; '

] >. Mliti'ill .if .-! >]ir.-i:ll :

- ii'lltitif opinion In-l'ol'- I ' II Will- -Till' 1'

,,f opinion . i,\- IIUXMII Tin- I >.u\\ iui.m tip
Proposed lm><li ( tn .itlin-ir

'1'iiK title of Mr. I),ir\vin's -ivat work i- u ih Origin <>j
Species /// //*'//s f A"-'/'//-"/ >'/(//'/< <n\>i Hi- Preservation <>f

Favoured //" x in (In Xtrn<j<jl> J'r I. In <>r>lrr to ;iji

]>ivci:ttf fully the aim ami nlijrct nf tlii> \\ni'k, ami tin-
diaii^c which it has ctlrctcd n.>t only in natural hi-tory l>ut
in many otht-i- sciences, it is : ;\ t<> I'MIHI a clear f..n

r.-|. ti"ii 't' the incaiiin-: of the ti-fiu "species," to know what
was the -rncral l'liff regarding them at the time when Mr.
I>arwin's Look first apjieared, ami to umlerstaml what he
meant, ami what \\a- -merally meant, l>y di>eo\ ei'in- their
"origin." It is for want of thi> jireliininary kno\\lel-e that
the majority of educated pel-son- who are not naturalists are
50 ready t> accept the innumeraMe ohject ion-, criticisms, and
ditlicultii- of its opponent- as proof- that the Darwinian
theory i- un-ound, while it also renders them unaMe to ap
preeiate, or even to comprehend, the \a-; change which that
theory lias etlected in the whole ma-s of thought and opinion
on the L r reat (|iie-tion of evolution.

The term " species " \\ a- thu- defined liy the celebrated
botanist ]) Candolle: "A species is a collection of all the
individuals which resemble each other more than tl
resemble anything else, which can by mutual fecundation

e B

DARWINISM CHAP.

produce fertile individuals, and which reproduce themselves
by generation, in such a manner that we may from analogy
suppose them all to have sprung from one single individual."
And the zoologist Swainson gives a somewhat similar defini-
tion : " A species, in the usual acceptation of the term, is an
animal which, in a state of nature, is distinguished by certain
peculiarities of form, size, colour, or other circumstances, from
another animal. It propagates, ' after its kind,' individuals
perfectly resembling the parent ; its peculiarities, therefore,
are permanent." 1

To illustrate these definitions we will take two common
English birds, the rook (Corvus frugilegus) and the crow
(Corvus corone). These are distinct species, because, in the first
place, they always differ from each other in certain slight
peculiarities of structure, form, and habits, and, in the second
place, because rooks always produce rooks, and crows produce
crows, and they do not interbreed. It was therefore con-
cluded that all the rooks in the world had descended from a
single pair of rooks, and the crows in like manner from a
single pair of crows, while it was considered impossible that
crows could have descended from rooks or vice rcrsd. The
" origin " of the first pair of each kind was a mystery.
Similar remarks may be applied to our two common plants,
the sweet violet (Viola odorata) and the dog violet (Viola
canina). These also produce their like and never produce
each other or intermingle, and they were therefore each
supposed to have sprung from a single individual whose
" origin " was unknown. But besides the crow and the rook

O

there are about thirty other kinds of birds in various parts of
the world, all so much like our species that they receive the
common name of crows ; and some of them differ less from
each other than does our crow from our rook. These are all
species of the genus Corvus, and were therefore believed to
have been always as distinct as they are now, neither more
nor less, and to have each descended from one pair of ances-
tral crows of the same identical species, which themselves had
an unknown "origin." Of violets there are more than a
hundred different kinds in various parts of the world, all
differing very slightly from each other and forming distinct
1 Geography and Classification of A nimals, p. 350.

WHAT AKK SI 'Mr I MS

species oi the genus Viola. Hut, a- these al-i> each produce
their like ami do m>t intermingle, it was believed that every
one of them hail always been as distinct from all theothei-
it i- now, that all the individuals of cadi kind had descended
fn>ni nil,' ancestor, but that the " origin " of thoe hundred
Jitly diti'ering ance>tor< \\.i-, unknown. In the words of
Sir .lohn Hcrschel, <|iioted by Mi. I >arwin, the origin < f
such species ma "the my-teiy of m\ >teries."

I', niij Ti'" ' '

A few mvat naturalists, struck by the very >light difference
between many of the-e species, and tin- iiunn-rou> links that
e\i-t between the iinM dillcicnt foi'ins of animals and plants,
and al>o olis,.r\in- that .1 great many species do \ai \ con-
siderably in their forms, colours, ami haliit-. conceded the idea
that they mi-lit lie all produced one from the other. The
ino-t eminent of the-e wi-itei- was :\ great I'Veiich naturalist,
Lamarck, who pul>li.-heil an elaliorate work, the /
'/. !<>>/i</i/<', in which he endeavoured to pro\ e that all ani
iiials whatever are d.-cemled from other species of animal-.
lie attrilnited the change of >p,-cies chiefly to the etlect of
ehanue> in the conditions of life such as climate, fond, etc.
and especially to the d.->ire< and ell'orts of the animals thera-
selves to improve their condition, leading to a modification of
form or si/e in certain parts, owinu to the well known ph\-io
lo_ical law that all oiuan> are strengthened l>y constant use,
while they are weakened or even completely lost liy di>u-e.
The ai -iimeijts of Lamarck did not, however, satisfy natural;
and though a few adopted the view that closely allied species
had de-eended tVoin each other, the general belief of the
educated puMic was, that each species was a " >pecial creation"
^uite independent of all others; while the great body of
naturali-ts eijiially held, that the change from one species
to another by any known law or cause was impossible,
and that the "origin of species" was an unsolved and
probably insoluble problem. The only other important Avork
dealing with the question was the celebrated Vestiges of

//", published anonymously, but now acknowledged to
have been written by the late Robert Chambers. In this
work the action of general laws was traced throughout the

DARWINISM CHAP.

universe as a system of growth and development, and it Avas
argued that the various species of animals and plants had
been produced in orderly succession from each other by the
action of unknown laws of development aided by the action
of external conditions. Although this work had a consider-
able effect in influencing public opinion as to the extreme
improbability of the doctrine of the independent " special
creation " of each species, it had little effect upon natural-
ists, because it made no attempt to grapple with the problem
in detail, or to show in any single case how the allied species
of a genus could have arisen, and have preserved their
numerous slight and apparently purposeless differences from
each other. No clue whatever was afforded to a law which
should produce from any one species one or more slightly
differing but yet permanently distinct species, nor was any
reason given why such slight yet constant differences should
exist at all.

Scientific Opinion before Darwin.

In order to show how little effect these writers had upon
the public mind, I will quote a few passages from the
writings of Sir Charles Lyell, as representing the opinions
of the most advanced thinkers in the period immediately
preceding that of Darwin's work. When recapitulating the
facts and arguments in favour of the invariability and
permanence of species, he says : " The entire variation from
the original type which any given kind of change can pro-
duce may usually be effected in a brief period ol time, after
which no further deviation can be obtained by continuing to
alter the circumstances, though ever so gradually, indefinite
divergence either in the way of improvement or deterioration
being prevented, and the least possible excess beyond the
defined limits being fatal to the existence of the individual."
In another place he maintains that " varieties of some species
may differ more than other species do from each other
without shaking our confidence in the reality of species."
He further adduces certain facts in geology as being, in his
opinion, "fatal to the theory of progressive development,"
and he explains the fact that there are so often distinct
species in countries of similar climate and vegetation by

WHAT AKK SPECIES

-pecial creations" in each country : and the>e conclusions

were arrived at lifter ;i careful >tud\ of Lamarck's work, a full
all-tract of which is uiveii in tin 1 earlier editions of the
1'rn ' i'i. 1

I'roft --"! A. issiz, one of the greatest naturali-t- of the la-t
generation, went even further, and maintained not only that,
each species \\a- >|ieci;dly Created, but that it wa- created in
the proportions :1I1 ,1 j M the localities in which we no\v tind it

exist The follo\vin_ act from hi- \ery in-trm-the

Look on Lake Superior explain- this view: "There are in
animals peeiiliar adaptations which are characteristic of their
species, and which cannot lie -nppo-ed to have ari-en from
siiliordinate intliiciice-. Those which live in >hoal- cannot he
-nppo-ed to have been created in -in : Je paii-. Tho-e \\hidi

are made In lie the food of other- cannot have lieen created
ill the same pro|.,irti.c those which live upon them.

Tho-e which are everywhere found in innumeralile >pecim.
mii-t ha\e 1 n introduced in numbers capable of maintaining

their normal proportions to tho-e \\hich live isolated and are
comparatively and constantly fewer. For \\ e know that this
harmony in the numerical proportions between animal- i-
one of the -ivat law- of nature. The circiim-tance that

cies occur within di'tinite limits where no obstacles pi-e\ent
their wid.-r di-trilnition lead- to the further inference that
tlie-e limits were as-j-ned to them from the lie-inninu. and
30 we >hoiild come to the final coiiclii-ion that the order
which prexail- throughout nature i- intentional, that i'
le-ulated l.y the limits marked out on the tir-t day of
creation, and that it ha- lieen maintained unchanged through
ages \\itl no other moditicat ion- than tlm-e which the higher
inti-llectnal po\\er- of man enaMe him to impost; on some
few animal- more closely connected with him.

These opinion- of some of the m..-t eminent and influential
writers of the piv I )arwinian a-v seem to us, now, either
altogether oli-olrte or positively ali-urd ; but they never-
theless exhibit the mental condition of even the ino-t
advanced -ection of -vientitic men un the problem of the

i-\-]uv.--ions n.'.Mir in Chapter IX. of tin.- t-urli ma (to the

ninth of tin- /'

- J-. A.M i/., /. - , p. 377.

DARWINISM CHAP.

nature and origin of species. They render it clear that,
notwithstanding the vast knowledge and ingenious reasoning
of Lamarck, and the more general exposition of the subject by
the author of the Vestiges of Creation, the first step had not
been taken towards a satisfactory explanation of the deriva-
tion of any one species from any other. Such eminent
naturalists as Geoffroy Saint Hilaire, Dean Herbert, Professor
Grant, Von Buch, and some others, had expressed their belief
that species arose as simple varieties, and that the species of
each genus were all descended from a common ancestor ; but
none of them gave a clue as to the law or the method by
which the change had been effected. This was still " the great
mystery." As to the further question how far this common
descent could be carried ; whether distinct families, such as
crows and thrushes, could possibly have descended from each
other; or, whether all birds, including such widely distinct
types as wrens, eagles, ostriches, and ducks, could all be the
modified descendants of a common ancestor ; or, still further,
whether mammalia, birds, reptiles, and fishes, could all have
had a common origin ; these questions had hardly come up
for discussion at all, for it was felt that, while the very first
step along the road of " transmutation of species " (as it was
then called) had not been made, it Avas quite useless to
speculate as to how far it might be possible to travel in the
same direction, or where the road would ultimately lead to.

The Problem before Darwin.

It is clear, then, that what Avas understood by the " origin "
or the "transmutation" of species before DarAvin's Avork
appeared, was the comparatively simple question Avhether the
allied species of each genus had or had not been derived from
one another and, remotely, from some common ancestor, by
the ordinary method of reproduction and by means of laAvs
and conditions still in action and capable of being thoroughly
investigated. If any naturalist had been asked at that day
Avhether, supposing it to be clearly shoAvn that all the different
species of each genus had been deriA^ed from some one
ancestral species, and that a full and complete explanation
Avere to be given of IIOAV each minute difference in form,
colour, or structure might have originated, and how the

WHAT AUK SI'F.riKS

i:il peculiaritie- of lialiit and of _.-, > : ^raphical distribution
miuht ha\e Keen liroudit aliout - \\hether. if thi- were done,
tin- "origin nf species" \\<>ul<l 1"' discovered, tin- _
my-tery -olved. he would undoiil.tedlv ha\e replied in the
atlirmati\e. HI- would probalilv ha\e added tlia! lie never
expected any -udi marvelloii- discover} to lie made in
hi- lifetime. I'.nt -o much a- thi- a mvdl\ Mr. l>ar\vin lia-
dune, n.it -inly in the opinion nf hi- di-ciple- and admit-
Init liy the admi ion- of those who doubt the completeness
..f hi- explanation-. For alnm-t all their nlijert inns and
ditliculties apply t.. those lar-er ditleien.e- \\hi.-h separate
genera, familie-, and order- frnm eaeli other, not to those which
u-ate on.- species from the -].ecie- to which it i- m-t nearly
allie.l, and from the remaining -|ieeie- of t he -nine genus. They
adduce -ndi ditliciiltie- .1- the tir-t de\ elo|.ment of the eye, or
the milk |iro.luciim -land- of the mammalia ; the wonderful
in-tinct- of }:< and of ant-; the complex arrangement- for
the fertili-ation of ordiid-. and inuneroii- other point- .it
structure or halm, .1- imt l>ein. | i.-torily explained. Hut

it is e\ident that the-e | iceiil ia rit ie- had their origin at a very
remote period of the eai-th's lii-tor\. and n<> theor\. ho\\e\er
complete, can do more than atl'oi'd a probable conjecture a- to
how they were produced. Our ignorance of the state of the
earth'- -Hi-face and of the condition- of life at tho-e remote
period- is ver) great; thou-and- of animals and plant- mu-t
have e\i-ted of which we have no record : while \\ . are
usually without any information a- to the haliits and general
life hi-tor\ even of tho-e .if \\ hicli we possess -nine fragmentary
remain-: SO that the true-t and nio-t complete theory would
not enalile n- to solve nil the ditiiciilt prolilems which the

whole eOUrse of the dexelopllielit of life Upon our -lolie
presents to US.

\\"liat we may expect a true theory to do is to enalile us
to comprehend and follow out in -ome detail tho-e changes in
the form, structure, and relations of animal- and plants which
aiv etiected in -hort periods of time, geologically speaking,
and which are now -oin- on around US. \\ e may expect it
to explain .-ati-fa-'torily most of the lesser and superficial
differences which distinguish one specie- from another. A\ e
may expect it to throw !i-ht on the mutual relations of the

DARWINISM CHAP.

animals and plants which live together in any one country,
and to give some rational account of the phenomena presented
by their distribution in different parts of the world. And,
lastly, we may expect it to explain many difficulties and to
harmonise many incongruities in the excessively complex
affinities and relations of living things. All this the Darwinian
theory undoubtedly does. It shows us how, by means of
some of the most universal and ever-acting laws in nature,
new species are necessarily produced, while the old species
become extinct ; and it enables us to understand how the
continuous action of these laws during the long periods with
which geology makes us acquainted is calculated to bring
about those greater differences presented by the distinct
genera, families, and orders into which all living things are
classified by naturalists. The differences which these present
are all of the same nature as those presented by the species of
many large genera, but much greater in timmint ; and they can
all be explained by the action of the same general laws and
by the extinction of a larger or smaller number of intermediate
species. "Whether the distinctions between the higher groups
termed Classes and Sub-kingdoms may be accounted for in
the same way is a much more difficult question. The differ-
ences which separate the mammals, birds, reptiles, and fishes
from each other, though vast, yet seem of the same nature as
those which distinguish a mouse from an elephant or a
swallow from a goose. But the vertebrate animals, the
mollusca, and the insects, are so radically distinct in their
Avhole organisation and in the very plan of their structure,
that objectors may not unreasonably doubt whether they can
all have been derived from a common ancestor by means of
the very same laws as have sufficed for the differentiation
of the various species of birds or of reptiles.

The Change of Opinion, effected by Darwin.

The point I wish especially to urge is this. Before
Darwin's work appeared, the great majority of naturalists, and
almost without exception the whole literary and scientific
world, held firmly to the belief that species were realities, and
had not been derived from other species by any process
accessible to us ; the different species of crow and of violet

WHAT AUK Sl'1-VlKS

were l.elieved t<> have lieeii always &B distinct and separate as

thev a re now, and to ha\ e originated by some totally unknown
process so far ivmo\rd from ordinary reproduction thai it was

usually spoken of as "special ovation. " There was. then, no
<|ilestion of tin- origin of f.iniilir-. order-, and da.-, l.ecause
tin- very til-t step ..I all, the origin of species," was lielie\ ed
to lu 1 an insolulile ]irolilem. l!ut now this i- all changed. The
wliole -cieiitilic and literary world, even the whole educated
puMic, accepts, as a maiter of common knowledge, the oriuin
of -pecies from other allied species l>y the ordinal') process of
nitural liirth. The idea of -pecial creation or any alto-ether
e\.-eptinti;d mode of pl'oi) I let ion is a I >-olu t elv e\ t ilict ! W'

more: tin- i- held also to apply to manv higher groups as

well as to the species of a genUS, and not even Mr. harwin's
severest critics \enture to suggest that the piinie\al l.ird.

reptile, ur tish 11111-t lia\e lieell "-peciallv Healed." Alld tllJS

vast, this totally unprecedented change in puMic opinion has
lieen the result of the work of one man, and was liroii-ht

a 1 ioi it in the short space of t wentv years ! This is the answer

to those who continue to maintain t hat the "origin of >pecie- i-
not yet di-coveied : that there are still doul.t- and ilitliciiltie- ;
that there are divi-ruem-ie- of structure so great that v\e
cannot understand how they had their lie-innini:. \\ e may
admit all this, JIM as we may admit that there are enormoii-
ditlnultie.s in the way of a complete comprehension of the
oii-in and nature of all the part- of the solar system and of
the st,.|lar universe. Hut we claim for haiuin that lie i> the
Nevuon of natural history, and that, jn-t so -urely as that the
discovery and demonstration l>y Ni'Uioii of the lawof ^ravita-
ti stal.lished older in place of chaos and laid a sure founda-
tion for all future study of the starry heavens, so >ure|y has
I'arwin, l>y hi- tliseov.-i-y of the law of natural .selection
and his demonstration of the -Teat principle of the preserva-
tion of useful variation- in the struuule for life, not only thrown
a Hood of li-ht on the process u f development of the whole

inic world, lnit also established a tirm foundation for all
futuri' study of natuie.

In order to show the view Darwin took of his own work,
and what it was that he alone claimed to have done, the
concluding passage of the introduction to the Or/gin of

10 DARWINISM CHAP.

Species should be carefully considered. It is as follows :
" Although much remains obscure, and will long remain
obscure, I can entertain no doubt, after the most deliberate
and dispassionate judgment of which I am capable, that the
view which most naturalists until recently entertained and
which I formerly entertained namely, that each species has
been independently created is eiToneous. I am fully con-
vinced that species are not immutable ; but that those
belonging to what are called the same genera are lineal
descendants of some other and generally extinct species, in
the same manner as the acknowledged varieties of any one
species are the descendants of that species. Furthermore, I
am convinced that Natural Selection has been the most im-
portant, but not the exclusive, means of modification."

It should be especially noted that all which is here claimed
is now almost universally admitted, while the criticisms of
Darwin's works refer almost exclusively to those numerous
questions Avhich, as he himself says, " Avill long remain
obscure."

The Darwinian Theory.

As it will be necessary, in the following chapters, to set
forth a considerable body of facts in almost every department
of natural history, in order to establish the fundamental
propositions on which the theory of natural selection rests,
I propose to give a preliminary statement of what the theory
really is, in order that the reader may better appreciate the
necessity for discussing so many details, and may thus feel a
more enlightened interest in them. Many of the facts to be
adduced are so novel and so curious that they are sure to be
appreciated by every one who takes an interest in nature, but
unless the need of them is clearly seen it may be thought that
time is being wasted on mere curious details and strange facts
which have little bearing on the question at issue.

The theory of natural selection rests on two main classes
of facts which apply to all organised beings without exception,
and which thus take rank as fundamental principles or laws.
The first is, the power of rapid multiplication in a geometrical
progression ; the second, that the offspring always vary slightly
from the parents, though generally very closely resembling

WHAT AUK SIT.' IKS 11

them. From tin- tir>t fact <>r law there fnllnw-. nece-.-arilv, ;i
("Mutant struggle f<>r existence; because, while tin- oir>pring
alway- exceed tin- parents in number, generally t<> an enormous
extent, \ct the tntal number of living nrgani-m- in the \\orld

line-; lint, alnl callllot. ilicrea-e year by year. < 'nliM'<|llently

every year, mi the average, as many die as are born, plant- a<
well as animals; and the majnrity die premature deaths.
They kill eaeh other in a tlimi-aiid ditleivnt way- : they >tarve
each nther by >nme consuming tlie food tliat other- want ;
they are destroyed lar-.-K l.y the powers "f nature by enld
and heat, by rain and storm, by tlnnd and tire. There i> thus
a perpetual struuule amnng them which -hall li\e and which
-hall die ; and tin- .-tru ]e i- t ivmendnusly severe, because

BO few can pn--iM\ remain ali\e -une in five, one in ten, often

only mie in a liundi'ed nr even mie in a thmi-aiid.

Then ci unes the .(iie-timi, \\"hy dn -nine li\e rather than

nthers .' ]f all tllC individuals nf e-ich specie- \\ere exactly
alike in every re-pect, we cniild only -ay it i- a matter of
chance. lint they are n-'t alike. \\'e tind that they \ary in
many ditl'erent ways. Some are -tmn u e]-, -nme -\\iftei, -nine
hardier in const it ut inn, some nmre cunninu. An nli-curo
colour may render concealmenl nmre ea>\ for some, keener
-i'_ht ma\ enalile others to di-cn\ er prey or escape tVnm an
enemy better than their fellows. Amoiiu plant- the -mallest
ditlerence- may lie n-efnl or the reverse. The earlie-t and
stronue-t shoots may e-cape the -hi- ; their greater vigour
may enable them to (lower and seed earlier in a wet autumn ;
plant- lie.-t armed with spim-s or hairs may e.-cape liein^
devoured; those whose flowers are nm-t conspicuous may be
soone-t fertilised l>y insects. A\'e cannot doul>t that, on tin-
whole, any liendicial \ariations Avill m'\c the possessors of it a
greater probaliility of li\in- thmuuh the tremendmts ordeal
they ha\e to undergo. There may lie something left to
chance, l>ut on the whole //// y//A-.</ ///// survive.

Then we have another important fact to consider, the
principle of heredity or transmi inn of variatinn-. If we
grow plants from seed or breed any kind of animals year
after year, consuming or giving away all the increase we do
not wish to keep just as they come to hand, our plants or
animals will continue much the same ; but if every year we

12 DARWINISM CHAP.

carefully save the best seed to sow and the finest or brightest
coloured animals to breed from, we shall soon find that an
improvement will take place, and that the average quality of
our stock will be raised. This is the way in which all our
fine garden fruits and vegetables and flowers have been pro-
duced, as well as all our splendid breeds of domestic animals ;
and they have thus become in many cases so different from
the wild races from which they originally sprang as to be
hardly recognisable as the same. It is therefore proved that
if any particular kind of variation is preserved and bred from,
the variation itself goes on increasing in amount to an
enormous extent ; and the bearing of this on the question of
the origin of species is most important. For if in each
generation of a given animal or plant the fittest survive to
continue the breed, then whatever may be the special
peculiarity that causes " fitness " in the particular case, that
peculiarity will go on increasing and strengthening so long as
it is iisi'fal to the species. But the moment it has reached its
maximum of usefulness, and some other quality or modifica-
tion would help in the struggle, then the individuals which
vary in the new direction will survive; and thus a species may
be gradually modified, first in one direction, then in another,
till it differs from the original parent form as much as the
greyhound differs from any wild dog or the cauliflower from
any wild plant. But animals or plants which thus differ in
a state of nature are always classed as distinct species, and
thus we see how, by the continuous survival of the fittest
or the preservation of favoured races in the struggle for life,
new species may be originated.

This self-acting process which, by means of a few easily
demonstrated groups of facts, brings about change in the
organic world, and keeps each species in harmony with the
conditions of its existence, will appear to some persons so
clear and simple as to need no further demonstration. But
to the great majority of naturalists and men of science endless
difficulties and objections arise, owing to the wonderful variety
of animal and vegetable forms, and the intricate relations of
the different species and groups of species with each other ;
and it was to answer as many of these objections as possible,
and to show that the more we know of nature the more we

VHIAT AKK SPECIES 13

find it to harmoni-e with the development li ypot lie-is, that
Darwin devoted the whole <>f his life to collecting facts and
making experiment-, tin- record of a portion of \\hich he has
u- iii a series of twche masterly volume-.

It is cvidfiitly i >f the mo-t vital impoi tance to any thc,.i\
that its foundations should lie :dolutel\ BCCUTe. It is
therefore necessary to -how, l.y a wide and comprehend e
array <>f facts, that animals and plants do perpetually \ary in
the nianner and to the amount requisite ; and that this take-
place in wild animals as well as in tho-e \\hich are di.me-ti

d. It i- necessary al-o to pn>\e that all or^ani-m- <!>
tend to inciva-e .it the '_re.it rate' alleged, and thai thi-
increa>e actually occur-, under favoiiralile conditions. \\ e
have to prove, further, that \ariation- of all kind- can l>e
Increased and accumulated }>\ -election; and that the >tru-_
for exi-ience to the extent here indicated actiiallv occurs ill
nature, and lead- to the continued preservation of fa\oiiialile
\ariation-.

These matteis will lie di-cu--ed in the four succeeding
chapters, thoii-h in a somewhat ditl'erent order the stru--le
for existence and the po\\er of i-apid multiplication, which is
its cause, <>cctip\ in.u the tir.-t jilace. as comprising those facts
which are the mo-t fundamental and tho.-e which can l.e
jierfectly explained without any reference to the less generally
under-tood fact- of variation. These chapter- wi'l lie follo\\ed
liy a discussion of certain ditticulties, and of the \e\ed (|iie.-tion
of hyliridity. Then will come a rather full account of the
more important of the complex relation- of organisms to each
other and to the earth itself, which are either fully explaim d
or greatly elucidated l>y the theory. The concludin-' cha|
will treat of the origin of man and his relation- to the lower

v^

animal-.

CHAPTEE II

THE STRUGGLE FOR EXISTENCE

Its importance The struggle among plants Among animals Illustrative
cases Succession of trees in forests of Denmark The struggle for
existence on the Pampas Increase of organisms in a geometrical
ratio Examples of great powers of increase of animals Rapid
increase and wide spread of plants Great fertility not essential to
rapid increase Struggle between closely allied species most severe
The ethical aspect of the struggle for existence.

THERE is perhaps no phenomenon of nature that is at once
so important, so universal, and so little understood, as the
struggle for existence continually going on among all organ-
ised beings. To most persons nature appears calm, orderly,
and peaceful. They see the birds singing in the trees, the
insects hovering over the flowers, the squirrel climbing among
the tree-tops, and all living things in the possession of health
and vigour, and in the enjoyment of a sunny existence. But
they do not see, and hardly ever think of, the means by which
tkis beauty and harmony and enjoyment is brought about.
They do not see the constant and daily search after food, the
failure to obtain which means weakness or death ; the con-
stant effort to escape enemies ; the ever -recurring struggle
against the forces of nature. This daily and hourly struggle,
this incessant warfare, is nevertheless the very means by which
much of the beauty and harmony and enjoyment in nature is
produced, and also affords one of the most important elements
in bringing about the origin of species. We must, therefore,
devote some time to the consideration of its various aspects
and of the many curious phenomena to which it gives rise.

It is a matter of common observation that if weeds are
allowed to grow unchecked in a garden they will soon destroy

CHAP.]] THE STRUGGLE FOB EXISTENCE 15

a number of tin- flower-. It i.- not -o commonly known that
if a -arden is left to become altogether wild, the weeds that
first take possession "f it, often ro vcrin-- tin- whole surface of
the -round witli t\vn in- three ditlerent kinds, will theni-e|\ e-
lie suji|ilantfd liy other-,. MI that in a feu years iiian\ of
tin- original floun- .md ,,f the earliest weed- may alike ha\e
disappeared. Thi.- i- one of the \er\ -imple-t .MM'- of the
Iggle for existence, resulting in the succe.--i\e dis|ilaceinent

"f one >et of species by another ; hut the exact causes of this
displacement are by no nie.ni- of ^m-h a simple nature. All
the plants concerned ma\ lie perfect 1\ hardy, all iua\ uiow

freelj fn>m > 1. yet when left alone for a nmnl.er of years,

ea.h M-t i- iii turn driven out l.y a .-iirccedin- set, till at the
end ot a coii-idfi-alilt- pei iod a eentury or a f.-\\ eenturies
perhap- hardly one of the plant- \\ hieh tir-t inuiiopoli-ed
the -round wmiM ] found there.

Another phenomenon of an analogous kind i> presented }}y
'he dill. Tent behaviour of introduced \\ild plant.- or animals
into countries apparently .pute a - \\ell -uited to them as
those which they naturall\ inlial.it. ALSI-SJ/, in his work on
Lake Superior, -tales that the roadside weed- of the iioilli-

'iii 1 nited States, to the niiinlierof !."( species, are all
Knr''p'-an, the native weeds having disappeared westwards;
and in New Zealand there are no less than !'.><! species of
natnrali-ed l-'.uropean plant-, more than 1 OK species of wliidi
ha\e -|read widely over the country, often displacing the
nati\e vegetation. On the other hand, of the many hundreds
of hardy plants which produce seed freely in our gardens,
very few ever run wild, and hardly any have Income common.
K\ en attempts to naturalise suitable plants usually fail ; for
A. de Candolle states that ,-everal botanists of Paris, Geneva,
and e-pecially of Montpellier, have sown the seeds of many
hundreds of species of hardy exotic plants in what appeared
to be the most favourable situation-, but that, in hardly a
single case, has any one of them become naturalised. 1 Even
a plant like the potato so widely cultivated, so hardy, and so
well adapted to spread by means of its many-eyed tubers has
not established itself in a wild state in any part of Europe.
It would, be thought that Australian plants would easily run
1 Geographic Botanique, p. 70S.

16 DARWINISM CHAP.

wild in New Zealand. But Sir Joseph Hooker informs us
that the late Mr. Bidwell habitually scattered Australian seeds
during his extensive travels in New Zealand, yet only two or
three Australian plants appear to have established themselves
in that country, and these only in cultivated or newly moved
soil.

These few illustrations sufficiently show that all the plants
of a country are, as De Candolle says, at war with each other,
each one struggling to occupy ground at the expense of its
neighbour. But, besides this direct competition, there is one
not less powerful arising from the exposure of almost all plants
to destruction by animals. The buds are destroyed by birds,
the leaves by caterpillars, the seeds by weevils ; some insects
bore into the trunk, others burrow in the twigs and leaves ;
slugs devour the young seedlings and the tender shoots, wire-
worms gnaw the roots. Herbivorous mammals devour many
species bodily, while some uproot and devour the buried
tubers.

In animals, it is the eggs or the very young that suffer most
from their various enemies ; in plants, the tender seedlings
when they first appear above the ground. To illustrate this
latter point Mr. Darwin cleared and dug a piece of ground
three feet long and two feet wide, and then marked all the
seedlings of weeds and other plants which came up, noting
what became of them. The total number was 357, and out
of these no less than 295 were destroyed by slugs and insects.
The direct strife of plant with plant is almost equally fatal
Avhen the stronger are allowed to smother the weaker. When
turf is mown or closely browsed by animals, a number of
strong and weak plants live together, because none are allowed
to grow much beyond the rest ; but Mr. Darwin found that
Avhen the plants which compose such turf are allowed to
grow up freely, the stronger kill the weaker. In a plot of
turf three feet by four, twenty distinct species of plants were
found to be growing, and no less than nine of these perished
altogether when the other species were allowed to grow up
to their full size. 1

But besides having to protect themselves against competing
plants and against destructive animals, there is a yet deadlier
1 The Origin of Species, p. 53.

ii THK STRUGGLE I OK i:\lsTK\CK 17

enemy in the forces f inorganic nature. Kadi species can

>il-t;illi ;i certain amount of heat and cold, each requires :i
certain amount of moi-tiire ;it the ri-ht season, nidi wants

;l proper amount of li-llt or of direct sunshine, e;ieh need-
certain elements in the -oil ; the failure of a due proportion
ill the-e inorganic c. mdit ii >ns causes weakness, and thus leads
tO Speedy death. The -till Je for e\i-te|ice ill plants is,

therefoie, threefold iii charai-ter and inlinite in complexity,
and the result i- -ecu in their curioii-ly irregular distribution

OVer the face of the earth. Not only lla- each country it-
di-tinct plants, luit everj \.dle\, every hill.-ide. alnio-t e\ci\
hed-erow, ha- a ditl'ereiit -ct of plants from it- adjacent \alley,
hillside, or hed-eiow - if not always dillerent in the actual
specie^ \ et \ei\ ditlereiit in comparative abundance, some
which are rare in the one lieinu common in the other. Hence
it happen- that slight change- of conditions often produce
jie.it changes in the lloia of .1 coimti\. Thus in 1710 and
the t \vo following years the lar\ a of a moth ( 1'hala-na uramini- )
committed such de.-triictioii in many of the meadows of
Sweden that the grass >- uieatly diiniiii-heil in |iiantit\,

and many plant- which \\ere lief choked li\ the grass

spran- up, and the Around liecame varie-ated \\ith a multi-
tude of ditl'ereiit specie- of (lowers. The introduction of -oat <
into the island of St. Helena led to the entire de-t ruction of

the native forests, consisting of aboul a hundred di-tinct species

of trees and shiuli-, the VOIIIIL: plants Kein- devoured liy
the -oats as fa-t a- they -rew U]i. The camel is a still greater
eneni) to wood\- \cuetation than the goat, and^Mr. Marsh
Relieves that forests Would soon cover considel'alile tracts of
the Araliian and African deserts if the -oat and the camel
were removed from them. 1 Kven in many parts of our own
country the exi-tem-e of tree- i- dependent on the absence of
cattle. Mr. Ihirwin observed, on some extensive heaths near
Farnham, in Surrey, a few clumps of old Scotch firs, but no
\ouni: trees over hundreds of acres. Some portions of the heath
had, however, been enclosed a few years before, and these en-
closures were crowded with youn^ fir-trees growing too close
to-ether for all to live ; and these vere not sown or planted,
nothing having been done to the ground beyond enclosing it

K"itl> us Min1(ii,il l/i If limn,! A, in, n, p. 51.
C

18 DARWINISM CHAP.

so as to keep out cattle. On ascertaining this, Mr. Darwin
was so much surprised that he searched among the heather in
the unenclosed parts, and there he found multitudes of little
trees and seedlings which had been perpetually browsed down
by the cattle. In one square yard, at a point about a hundred
yards from one of the old clumps of firs, he counted thirty-
two little trees, and one of them had twenty-six rings of
growth, showing that it had for many years tried to raise its
head above the stems of the heather and had failed. Yet
this heath was very extensive and very barren, and, as Mr.
Darwin remarks, no one would ever have imagined that cattle
would have so closely and so effectually searched it for food.

In the case of animals, the competition and struggle are
more obvious. The vegetation of a given district can only
support a certain number of animals, and the different kinds
of plant-eaters will compete together for it. They will also
have insects for their competitors, and these insects will be
kept down by birds, which will thus assist the mammalia.
But there will also be carnivora destroying the herbivora ;
while small rodents, like the lemming and some of the field-
mice, often destroy so much vegetation as materially to affect
the food of all the other groups of animals. Droughts, floods,
severe winters, storms and hurricanes will injure these in
various degrees, but no one species can be diminished in
numbers without the effect being felt in various complex ways
by all the rest. A few illustrations of this reciprocal action
must be given.

Illustrative Cases of the Struggle for Life.

Sir Charles Lyell observes that if, by the attacks of seals
or other marine foes, salmon are reduced in numbers, the
consequence will be that otters, living far inland, will be
deprived of food and will then destroy many young birds or
quadrupeds, so that the increase of a marine animal may
cause the destruction of many land animals hundreds of miles
away. Mr. Darwin carefully observed the effects produced
by planting a few hundred acres of Scotch fir, in Staffordshire,
on part of a very extensive heath which had never been
cultivated. After the planted portion was about twenty-five
years old he observed that the change in the native vegetation

n TIIK STIM-CCI.K I'm; I:\ISTKXCK 19

WBS greater than is often .-ecu in pa--in^ from oiii' unite

ditlelVIlt .-oil In another. I'.e-ide- a -rcat chalice ill tilt' pl'o

]M.rtinn;il numbers of tin- native heath-plants, twelve species
which could not In- foiinil on tin- heath flourished iii the

plantations. The ctlrct on the insert life mii-t ha\e lieen still
^n-atcr, for six insectivorous l.inls which were \ery roinnion
in the plantation- weie not to be .-ecu on the heath, which

was, liowe\er. frequented by two or three diil'erent .-pecie- of
in>eeti\oroiis bird-. It \\oiild ha\e required continued study

for several yeai*S to determine all the difference- ill tlie

organic life of the t\\o areas, but the facts -tated by Mr.
I>arwin are sntticient to show how -feat a change may lie
etlected 1 >y the introduction of a sin-le kind of tree and the
keeping out of cattle.

The next case 1 \vill -.ive in Mr. I>arwin's own words:

"III >e\Clal |iait> of the \\m\i\ ill.-ects detel'lililie the existence
of cattle. l'erha|i- Taraunay oll'rrs the most ciirioii- in-talice
of this ; for llele neither cattle Imr llor.-es llor do-- ha\e e\er

run wild, tlmiiuh they swarm southward and north\\ard in a
feral state; and A/.ara and Keiii^er lia\e >ho\\ n that tlii> is
caused l.y the greater nnml>er.-, in rara-nay, of a certain tl\
which !a\s it- eggs in the nave].- of tlie-e animal- when tir-t
liorn. The im-rea-e of tiie-e Hies, numerous as they are,
mu-t lie habitually checked hy some means, ]>n>l>;d>ly ly other
jiarasitic insects. Hence, it certain insectivorous liird- were

to decrease in 1'ara-uay, the parasitic insects would piolial.ly
increase : and this would leeii the numlier of the nave]
frei|iieiitin-- Hies then cattle and horses would liecome feral,
and this would greatly alter (as indeed 1 have oliserved in
parts of South America) the \e ; _etation: this a^ain would
largely atlect the insects, and this, a- we Imvc ju.-t seen in
Staffordshire, the insectivorous birds, and so onward in ever-
iiKTea-iiii: circles of complexity. Not that under nature the
relations will ever lie as simple as this. Battle within battle
must lie continually recurring with varying success; and yet in
the long run the forces are so nicely balanced, that the face
of nature remain.- for a long time uniform, though assuredly
the merest triHe would give the victory to one organic being
over another." 1

1 The Oriijin of Sjjecies, \>. 56.

20 DARWINISM CHAP.

Such cases as the above may perhaps be thought excep-
tional, but there is good reason to believe that they are by no
means rare, but are illustrations of what is going on in every
part of the world, only it is very difficult for us to trace out
the complex reactions that are everywhere occurring. The
general impression of the ordinary observer seems to be that
wild animals and plants live peaceful lives and have
few troubles, each being exactly suited to its place and
surroundings, and therefore having no difficulty in maintain-
ing itself. Before showing that this view is, everywhere
and always, demonstrably untrue, we will consider one other
case of the complex relations of distinct organisms adduced
by Mr. Darwin, and often quoted for its striking and almost
eccentric character. It is now well known that many flowers
require to be fertilised by insects in order to produce seed,
and this fertilisation can, in some cases, only be effected by
one particular species of insect to which the flower has become
specially adapted. Two of our common plants, the wild heart's-
ease (Viola tricolor) and the red clover (Trifolium pratense), are
thus fertilised by humble-bees almost exclusively, and if these
insects are prevented from visiting the flowers, they produce
either no seed at all or exceedingly few. Now it is known that
field-mice destroy the combs and nests of humble-bees, and
Colonel Newman, Avho has paid great attention to these insects,
believes that more than two-thirds of all the humble-bees'
nests in England are thus destroyed. But the number of
mice depends a good deal on the number of Cats ; and the same
observer says that near villages and towns he has found the
nests of humble-bees more numerous than elsewhere, Avhich he
attributes to the number of cats that destroy the mice.
Hence it follows, that the abundance of red clover and wild
heart's-ease in a district will depend on a good supply of cats
to kill the mice, Avhich would otherwise destroy and keep down
the humble-bees and prevent them from fertilising the flowers.
A chain of connection has thus been found between such
totally distinct organisms as flesh-eating mammalia and sweet-
smelling flowers, the abundance or scarcity of the one closely
corresponding to that of the other !

The following account of the struggle between trees in the
forests of Denmark, from the researches of M. Hansten-

ii TIIK STKn.ci.K In); I:\ISTKXCK 21

r.langsted, >trikingly illu-t rates our subject. 1 The chief com-

l:it;lllts are ill'' beech ;iml ihr bird), tin' former being e\er\

where successful in its invasions. Forests composed wholly
of liirdi ;nv iio\v only found in sterile, sandy tract-. ; everj
where else the trees are mixed, and wherever the soil i>
favourable the beech rapidly dri\es nut the Kirch. The latter
loses its ln-anches at the touch nf the Kerch, and devotes all

ii- strength to the upper part where it towers above the beech.
It may live long in this way, l.ut it siircuinlis ultimately in
the ti-ht i'f old a-e if of nothing else, for the life of the
birch in henmark is >hurter than that of the lieech. The
writer believes that light (dr rather shade) is the cause of the
superiority of tin- latter, for it has a greater develdpinent of
its branches than the Kireh, which is ninie dpen and thus
alldus the rays ,,f the >un to pa-s thr"U-h to the snil Keldw,
\\hile the tufted, ld|>h\ tdp nf the Keech |irt-el'\es a deep
shade at its l>a-e. Hardly any \dUiiu; plant- can -I'ow under
the Keech except its dW|| .-llddts ; and while the lieech can

flourish under the >hade df the Itii-ch. the latter dies im-
mediately under the hecch. The liirch has duly 1 n saved

from total extermination l>y the facts that it had po e--idii of
the Ilanish fd|-e>ts ]on<r l.efdi'e the l.re.-li ever reached the
coiiiiti-y, and that certain districts are unfavourable to the
-n.wthdf the latter. I Jut wherever the -oil has Keen enriched
l'\ the decomposition of the lea\es df the liirch the battle
he-ins. The birch still tloiiii-he< dii the hdiders df lakes and
othei' mai-hy place-, \\here its enemy caniidt exi>t. In the
same way, in the forests df Xeelaiid, the fir fdiots are di-
appearing before the beech. Left to themselves, the firs are

sddii displaced by the 1 -h. The stru-ule between the latter

and the oak is Idii-vr and nmiv stubbdrn, fur the branches and
foliage of the oak are thicker, and dtler much resistance to the
passage of li.uht. The oak, also, has greater longevity; but,
sooner or later, it too succumbs, because it canm.t develop
in the shadow of the beech. The earliest forests of Denmark
were mainly composed of aspens, with which the birch was
apparently associated; gradually the soil was raided, and the
climate grew milder; then the fir came and termed large
f'" v Thi.- tree ruled fur centuries, and then ceded the

.Vi//(;/v, vol. xxxi. i'. 63.

DARWINISM CHAP.

first place to the holm-oak, which is now giving way to the
beech. Aspen, birch, fir, oak, and beech appear to be the
steps in the struggle for the survival of the fittest among the
forest-trees of Denmark.

It may be added that in the time of the Romans the
beech was the principal forest-tree of Denmark as it is now,
while in the much earlier bronze age, represented by the later
remains found in the peat bogs, there were no beech-trees, or
very few, the oak being the prevailing tree, while in the still
earlier stone period the fir Avas the most abundant. The
beech is a tree essentially of the temperate zone, having its
northern limit considerably southward of the oak, fir, birch,
or aspen, and its entrance into Denmark was no doubt due to
the amelioration of the climate after the glacial epoch had
entirely passed away. We thus see how changes of climate,
which are continually occurring owing either to cosmical or
geographical causes, may initiate a struggle among plants
which may continue for thousands of years, and which must
profoundly modify the relations of the animal world, since
the very existence of innumerable insects, and even of many
birds and mammals, is dependent more or less completely on
certain species of plants.

The Struggle for Existence on the Pampas.

Another illustration of the struggle for existence, in Avhich
both plants and animals are implicated, is afforded by the
pampas of the southern part of South America. The absence
of trees from these vast plains has been imputed by Mr.
Darwin to the supposed inability of the tropical and sub-
tropical forms of South America to thrive on them, and there
being no other source from which they could obtain a supply ;
and that explanation Avas adopted by such eminent botanists
as Mr. Ball and Professor Asa Gray. This explanation has
always seemed to me unsatisfactory, because there are ample
forests both in the temperate regions of the Andes and on the
\vhole Avest coast down to Terra del Fuego; and it is inconsistent
Avith what AVC know of the rapid variation and adaptation of
species to HCAV conditions. What seems a more satisfactory
explanation has been given by Mr. Edwin Clark, a civil
engineer, Avho resided nearly two years in the country and

ii THE STRl'UJLK K>K KXISTKNVK 23

paid much attention t-> its natural history. He says: "The
peculiar characteristics of the.-e \a-t le\d plains which descend
iron i t he Amies to the ;_;reat ri\ er l>a-in in unbroken monotony,
arc the absence of livers or water-storage, and the periodical

iirence uf droughts, up ' 81CCO8, ill the >iininicr inuiiths.
The-e ci.nditiiins determine the singular character huth uf its
tlora and fauna.

"ThcMiil i- naturally fertile and favuiiralile for the growth
of trees, and they grow luxuriantly wherever they are pro
tected. The eucalyptus H COVerfng lame tracts \\here\er it
is enclosed, and willows, poplar-, and tin- lig -urntiind e\ ei y

icia \\ hen fenced ill.

"The open plains are covered with droves of hor-es
and cattle, and o\errun }>y iniinliei less wild rodents, the
original tenants of the painpa-. hurinu the IOHL: periods
of drought, \\hich ale 50 i;reat a BCOlirgC to the country, the-e
animals are star\ ed Ky thousands, destroying, in their ellorts to
live, every Vestige of \.-ctation. In one of the-e 'SICCOS,' at
the time of my \isit, no le>s than 50,000 head of oxen and

sheep and horses perished from starvation and thirst, aftertearing

deep out of the soil e\ery trace of vegetation, including the
wiry roots of the pampa- jirass. I'nder >udi circumstances
the existence of an unprotected tree is inipossihle. The only
plants that hold their own, in addition to the indestructible
thi>tles, grasses, and clo\er, are a little herbaceous oxalis, pro-
ducing viviparous buds of extraordinary \ itality,a few poisonous
>pecie>, such as the hemlock, and a te\v touiili, thorny dwait
acacia- and \\iry ru>hes, \\hich even a stars in^ rat refuses.

"Although the cattle are a modern introduction, the
MlUlilierlesS indi-eiious rodents must always have etlectlially
j>re\ented the introduction of any other species of plants;
lar-e tract-- are still honeycombed ly the ubiquitous biscacho,
a uiv,aiitic raldiit ; and numerous other rodents still exist, in-
cluding rats and mice, pampas hare-, and the great nutria and
carpincho (capyliara) on the river hank-." 1

Mi. ('lark further remarks on the desperate struggle for
existence which characterises the liorderini: fertile zones,
where fixers and marshy plains permit a more luxuriant and
varied \e-etalile and animal life. After describing how the

1 A I'isit to Xoxlti Ai>i>',-i,:>i, 1878 ; also Xaturc, vol. xxxi. pp. 263-339.

24 DARWINISM CHAP.

river sometimes rose 30 feet in eight hours, doing immense
destruction, and the abundance of the larger carnivora and
large reptiles on its banks, he goes on : " But it was among
the flora that the principle of natural selection was most
prominently displayed. In such a district overrun Avith
rodents and escaped cattle, subject to floods that carried away
whole islands of botany, and especially to droughts that dried
up the lakes and almost the river itself no ordinary plant
could live, even on this rich and watered alluvial debris. The
only plants that escaped the cattle Avere such as were either
poisonous, or thorny, or resinous, or indestructibly tough.
Hence we had only a great development of solanums, talas,
acacias, euphorbias, and laurels. The buttercup is replaced by
the little poisonous yellow oxalis with its viviparous buds ; the
passion-flowers, asclepiads, bignonias, convolvuluses, and climb-
ing leguminous plants escape both floods and cattle by climb-
ing the highest trees and towering overhead in a flood of
bloom. The ground plants are the portulacas, turneras, and
cenotheras, bitter and ephemeral, on the bare rock, and almost
independent of any other moisture than the heavy dews.
The pontederias, alismas, and plantago, with grasses and
sedges, derive protection from the deep and brilliant pools ;
and though at first sight the ' monte ' doubtless impresses the
traveller as a scene of the wildest confusion and ruin, yet, on
closer examination, we found it far more remarkable as a
manifestation of harmony and law, and a striking example of
the marvellous power which plants, like animals, possess, of
adapting themselves to the local peculiarities of their habitat,
whether in the fertile shades of the luxuriant ' monte ' or on
the arid, parched-up plains of the treeless pampas."

A curious example of the struggle between plants has
been communicated to me by Mr. John Ennis, a resident in
New Zealand. The English water-cress grows so luxuriantly
in that country as to completely choke up the rivers,
sometimes leading to disastrous floods, and necessitating great
outlay to keep the stream open. But a natural remedy has
now been found in planting willows on the banks. The
roots of these trees penetrate the bed of the stream in every
direction, and the water-cress, unable to obtain the requisite
amount of nourishment, gradually disappears.

ii THK STi;n;r,LK l'(U KXISTKN. K 25

I K, * ',!.<,' ,,f n/-,/,/ /,,.-,///.- /// ,i <;,.,, H, tr/>-i/l

The fads which have now been adduced, sutliciently pro\e
that tin-re i- a continual compel itiun, and struggle, and war
-"ing "ii in nature, and that each >pccies of animal and
plant, allects many others in complex and often unexpected
way-. \Ve will now proeeed to >h<>w the fundamental cause
of this struggle, and to prove that it is ever acting over the
whole field of nature, and that no single species of animal or
plant can possibly escape from it. This results from tlie fact
of the rapid incriMM', in a geometrical ratio, of all the species
of animals and plants. In the lower orders this increase is
especially lapid, a >in;_'le flesh My (Mn-ca carnaria) piodilcin.i:
L'ii.n(ii) lar\a', and tlie-e -muin^ .-o <|iiickly that they reach
their full si/.e in the days ; hence the great S \\edish naturalist,

LinnaeUS, a>-ei-ted that a dead horse would lie devoured liy three
of these Miesas (jiiickly as liy a lion. Kachof these |ai'\ ; remains
in the pupa state al>oiit |i\c or >j\ da\ 3, so that each parent My

may be increased ten thousand-fold in a fortnight. Sii|)posin^

they \\ent on increasing at this rate during only thi-ee months
of summer, there \\onld result one hundred millions of millions
of millions for each fly at the commencement of summer, a
numlier 1:1 eater piol>al>ly than exists at any one time in the
whole world. And this is only one -pcrie-, \\ hilt; there are
thousands of other Species increasing also at an enonimus rate ;
so that, if they were unchecked, the whole atmosphere would
lie den-e \\ith Mies. ; md all animal food and much of animal
life \\oiild he destroyed 1 .y them. T,, preveni this tremendous
increase then- must lie incessant war a-ain.st lhe.se insects, 1>\
insectivorous liird- and reptiles as well as 1>\ other insects, in

the larxa as well as in the peil'ect state, liy the action of the
elements in the form of rain, hail, or drought, and liy other
unknown causes; \ et we see nothing of this e\ ei- present war.
thou-h liy its means alone, perhaps, we are sived from famine
and pestilence.

Let us now consider a less extreme and more familiar

\\ e possess ;> considerable number of liirds which,

like the redbreast, spa now, the four common titmice, the

thrush, and the blackbird, stay with us all the year round.

These lay on an average MX eggs, but, as several of them ha \e

26 DARWINISM CHAP.

two or more broods a year, ten will be below the average of
the year's increase. Such birds as these often live from fifteen
to twenty years in confinement, and we cannot suppose them to
live shorter lives in a state of nature, if unmolested ; but to
avoid possible exaggeration we will take only ten years as the
average duration of their lives. Now, if we start with a single
pair, and these are allowed to live and breed, unmolested, till
they die at the end of ten years, as they might do if turned
loose into a good-sized island with ample vegetable and insect
food, but no other competing or destructive birds or quadrupeds
their numbers would amount to more than twenty millions.
But we know very well that our bird population is no greater,
on the average, now than it was ten years ago. Year by year
it may fluctuate a little according as the winters are more
or less severe, or from other causes, but on the whole there is
no increase. What, then, becomes of the enormous surplus
population annually produced 1 It is evi Jent they must
all die or be killed, somehow ; and as the increase is, on the
average, about five to one, it follows that, if the average
number of birds of all kinds in our islands is taken at ten
millions and this is probably far under the mark then about
fifty millions of birds, including eggs as possible birds, must
annually die or be destroyed. Yet we see nothing, or almost
nothing, of this tremendous slaughter of the innocents going
on all around us. In severe winters a few birds are found
dead, and a few feathers or mangled remains show us where
a wood-pigeon or some other bird has been destroyed by a
hawk, but no one would imagine that five times as many birds
as the total number in the country in early spring die every
year. No doubt a considerable proportion of these do not die
here but during or after migration to other countries, but others
Avhich are bred in distant countries come here, and thus
balance the account. Again, as the average number of young
produced is four or five times that of the parents, we ought to
have at least five times as many birds in the country at the
end of summer as at the beginning, and there is certainly
no such enormous disproportion as this. The fact is, that the
destruction commences, and is probably most severe, with
nestling birds, which are often killed by heavy rains or blown
away by severe storms, or left to die of hunger if either of

ii THK STRUGGLE FOR EXISTENCE -j?

the parents is killed ; while they offer a defenceless prey to
jackdaws, jays, ;md magpies, and not a fe\\' are ejected from
their nests by their foster-brothers the cuckoos. As soon as
they aiv Hedged and lie-in to leave the nest great numbers
are destroyed by bu/xards, spai i -o\\ hau ks, and shrikes. Of
those which migrate in autumn a considerable proportion are
probably lo-r ;it sea or othei-wi.-e destroyed before they reach a
place of .-afety ; while tho>e which remain with us are greatly
thinned by cold and starvation during severe Winters. Exactly
the same thiiiu goes on with every species of wild animal and
plant from the ]o\\e>t to the lii-lie-t. All breed at such a rate,
that in a few year^ the progeny of any one species would, if
allowed to increase unchecked, alone monopolise the land ;
but all alike are kept within bounds by \arioiis de>tructi\e
a-encie-, go that, though the numbers of each may fluctuate,
the\ caii never permanently increase except at the expense of
some others, which must proportionately decrea-e.

///, (;,;,// j',i/r,i:< ,,/' Tncreast "

As the facts now ,-tate<l are the veiy foundation of the
theory we are considering, and the enormous increase ami
perpetual destruction continually Lioin^ on reijuire to be kept
ever present in the mind, some direct e\ idem-e of actual cases
of inerea-e mu-t be adduced. That even the larger animals.
which breed comparatively slowly, increase enormously when
placed under favourable conditions in new countries, is shown
by the rapid spread of cattle and horses in America.
Columbus, in his second voyage, left a few black cattle at St.
Domingo, and these ran wild and increased so much that.
twenty-seven years afterwards, herds of from 4000 to SOOO
head were not uncommon. Cattle were afterwards taken
from this island to Mexico and to other parts of America, and
in I ."is 7, sixty- the years after the conquest of Mexico, the
Spaniards exported rl,.")f>0 hides from that country and
'.'>'<, 1 I I trom St. Domingo, an indication of the vast numbers
of these animals which must then have existed there, since
those captured and killed could have been only a small portion
of the whole. In the pampas of Huenos Ayres there were, at
the end of the la>t century, about twelve million cows and
three million horses, besides great numbers in all other parts

DARWINISM CHAP.

of America where open pastures offered suitable conditions.
Asses, about fifty years after their introduction, ran wild and
multiplied so amazingly in Quito, that the Spanish traveller
Ulloa describes them as being a nuisance. They grazed
together in great herds, defending themselves with their
mouths, and if a horse strayed among them they all fell upon
him and did not cease biting and kicking till they left him
dead. Hogs Avere turned out in St. Domingo by Columbus
in 1493, and the Spaniards took them to other places where
they settled, the result being, that in about half a century
these animals were found in great numbers over a large part
of America, from 25 north to 40 south latitude. More
recently, in New Zealand, pigs have multiplied so greatly in
a wild state as to be a serious nuisance and injury to
agriculture. To give some idea of their numbers, it is stated
that in the province of Nelson there were killed in twenty
months 25,000 wild pigs. 1 Now, in the case of all these animals,
we know that in their native countries, and even in America
at the present time, they do not increase at all in numbers ;
therefore the whole normal increase must be kept down,
year by year, by natural or artificial means of destruction.

Rapid Increase and Wide Spread of Plants.

In the case of plants, the power of increase is even greater
and its effects more distinctly visible. Hundreds of square
miles of the plains of La Plata are now covered with two or
three species of European thistle, often to the exclusion of
almost every other plant ; but in the native countries of these
thistles they occupy, except in cultivated or waste ground, a
very subordinate part in the vegetation. Some American
plants, like the cotton-weed (Asclepias curassavica), have now
become common Aveeds over a large portion of the tropics.
White clover (Trifolium repens) spreads over all the temperate
regions of the Avorld, and in New Zealand is exterminating
many native species, including even the native flax (Phormium

1 Still more remarkable is the increase of rabbits both in New Zealand and
Australia. No less than seven millions of rabbit-skins have been exported
from the former country in a single year, their value being 67,000. In both
countries, sheep-runs have been greatly deteriorated in value by the abundance
of rabbits, which destroy the herbage ; and in some cases they have had to be
abandoned altogether.

ii THE STl;n:cLE FOR EXISTKX' K 29

tenax), a lar^e plant with iris like leaves 5 or G feet hi-h.
Mr. \\'. I,. Travers lu- paid much attention to tin- effects of
introduced plants in New Zealand, anil notes the following
species as oein- e-pecially remarkable. The common knot-
ur:i-s ( l'ol\ -oiiuni avienlare) -row- im>-t luxuriantly, sin-le
p]ant> co\erin-- a -pare 4 or ."> feet in diameter, and send
in- their mots :\ or I feet deep. A lame sul> aquatic
dork (Kiiniex ol>tusifoliiis) abound- in every ri\er lied, even
far up amon- the mountain-;. The eoninion sow thistle
(Soiielius oleraceii-l -I ou s all over the country up to an
elevation of (i(M)O feet. The water creSS ( Na-turtium ollieinalei
-ro\\> with ama/iii- \i-oiir in many of the rivers. forming
stems 1 L' feet Ion- and : , : inch in diameter, and complete!}
choking them up. It i-o-t .'.">( tn a \ear to keep the Avon
at ( 'liristehnreh tVee from it. The sorrel ( K'nmex aeetosella)
Covers hundred- of acres \\iih a sheet of red. It form- a
den>e mat, exterminating other plants, and |>re\ eiit in- eultiva-
tioii. It can, ho\\e\er, lie it>elf exterminated l.y sowin- the
ground with red clover, \\hith \\ill also \an<|iii>h the
rol\-oinim avieiilare. The most noxioii- \\eed in New
/ealand appear-. ho\\e\er. to lie the 1 1 \ | meha-ris radieata. a
coarse yellow -flowered composite not uncommon in our

meadows and \\a-te places. This has lieeii introduced with
p-ass seeils from Mn^-land. and is very de-t i ncti\ e. It is
stated that excellent pa-tnre was in three years destroyed l.y

this weed, which absolutely displaced every other plant on the
.-round. It p-ows in e\ery kind of soil, and i- ,-aid even to
drive out the white. clo\ er, which is usually so powerful in
taking jiossession of the soil.

In Australia another composite plant, called there the ( 'ape-
weedM 'ryptosteinma calendulaceiim ),did much damage, and was
noticed liy 1 la ron \"oii Ilu-vl in 1^33 as "an unexterminalile
\\eed ; luit, after forty year-' occupation, it was found to give
way to the dense herl.aue formed by lucerne and choice
gra -ses.

In Ceylon AVC are told lev Mr. Thwaites, in his Enumera
//"/>, of ('I/In/, J'lmi/.^ that a plant introduced into the
island less than fifty years a-o is helping to alter the
character of the vegetation up to an elevation of 3000 feet.
This is the Lantana mixta, a verbenaceous plant introduced

30 DARWINISM CHAP.

from the West Indies, which appears to have found in Ceylon a
soil and climate exactly suited to it. It now covers thousands
of acres Avith its dense masses of foliage, taking complete
possession of land where cultivation has been neglected or
abandoned, preventing the growth of any other plants, and
even destroying small trees, the tops of which its subscandent
stems are able to reach. The fruit of this plant is so accept-
able to frugivorous birds of all kinds that, through their instru-
mentality, it is spreading rapidly, to the complete exclusion of
the indigenous vegetation where it becomes established.

Great Fertility not essential to Rapid Increase.

The not uncommon circumstance of slow-breeding animals
being very numerous, shows that it is usually the amount
of destruction which an animal or plant is exposed to, not
its rapid multiplication, that determines its numbers in any
country. The passenger-pigeon (Ectopistes migratorius) is, or
rather was, excessively abundant in a certain area in North
America, and its enormous migrating flocks darkening the sky
for hours have often been described ; yet this bird lays only
two eggs. The fulmar petrel is supposed to be one of the
most numerous birds in the world, yet it lays only one egg.
On the other hand the great shrike, the tree-creeper, the
nut-hatch, the nut-cracker, the hoopoe, and many other birds,
lay from four to six or seven eggs, and yet are never
abundant. So in plants, the abundance of a species bears
little or no relation to its seed-producing power. Some of the
grasses and sedges, the wild hyacinth, and many buttercups
occur in immense profusion over extensive areas, although each
plant produces comparatively few seeds ; while several species
of bell-flowers, gentians, pinks, and mulleins, and even some
of the compositse, which produce an abundance of minute seeds,
many of which are easily scattered by the wind, are yet rare
species that never spread beyond a very limited area.

The above-mentioned passenger-pigeon affords such an
excellent example of an enormous bird-population kept up by
a comparatively slow rate of increase, and in spite of its
complete helplessness and the great destruction which it
suffers from its numerous enemies, that the following account
of one of its breeding-places and migrations by the celebrated

Tin: sn;rt;t;i.i: mi: K.NISTKMT. 31

American naturalist, Alexander \\iUon, will lie read with
interest :

" Not far from Shelbyville, in the State of Kentucky,

alioiit five years ago, there ua- one of the-e breeding-places,

whieh >trftclied through the woods in nearly a north and
smith direct! in, wa- seseral miles in lii-eadth. and was said to
lie upwards of 40 miles in extent. In this tract almost
every tree was furnished \\ith nests \\here\er the liraiidies
could accommodate them. 'I'he pi-eons made their tir.-t

a]ipearance there alioiit the llMh of April, and left it

altogether with their \onn- in-fore the -j.'ith of Ma\. As

soon as the \oiing were full\ grown and l.efore they left the
ne-ts, nuiiieroiis parties of the itdialiitants from all parts of
the adjacent coimtr\ came with wag-on-, axes, I".]-. rooking
utensil-, many of them accompanied liy the greater part of
then- familie-, anil encamped for ,-e\eial days at this iminen-c
nursery. Se\eial of them informed me that the noise was
SO great as to terrify their horses, and that it was ditliciilt for
one pel-son to hear another without bawling in his ear. The
Uioiind was strewed with lnoken limlis of trees, e--s, and
younu si|iial> pi-eons, which had l.cen precipitated from above,
and (Jii which herds of ho--. \\ ere fat ten in-. I la\\ ks, Im/./ai ds,
and ea-h-s weie sailing altout in -real numbers, and sei/in^
the sijlialis from the nests at plea>me; while, from _'() feet
ii]iwards to the top of the tree-, the \ iew throii-h the woods
presented a perpetual tumult of crowdin- and fluttering
multitudes of pi-eons, their win^s i-oarin- like thunder,
min-led with the frequent cia>h of falling timber; for now
the axemen were at work euttin^ down those trees that seemed
most crowded with ne-ts, and contrived to fell them in such
a manner, that in their descent they might brin- down several
others; by which means the falling of one large tree some-
times produced 200 squabs little inferior in size to the old
birds, and almost one heap of fat. On some single trees
upwards of a hundred nests were found, each containing one
sipiab only ; a circumstance in the history of the bird not
generally known to naturalists. 1 It was dangerous to walk

1 Later observers have proved that two eggs are laid and usually two
vomit: ]ini(liirril, Imt it ui.-iy be that in most cases only one of these cunies to
maturity

32 DARWINISM CHAP.

under these flying and fluttering millions, from the frequent
fall of large branches, broken down by the weight of the
multitudes above, and which in their descent often destroyed
numbers of the birds themselves ; while the clothes of those
engaged in traversing the woods were completely covered
with the excrements of the pigeons.

" These circumstances were related to me by many of the
most respectable part of the community in that quarter, and
were confirmed in part by what I myself witnessed. I passed
for several miles through this same breeding-place, where
every tree was spotted with nests, the remains of those above
described. In many instances I counted upwards of ninety
nests on a single tree ; but the pigeons had abandoned this
place for another, 60 or 80 miles off', towards Green
River, where they were said at that time to be equally
numerous. From the great numbers that were constantly
passing over our heads to or from that quarter, I had no
doubt of the truth of this statement. The mast had been
chiefly consumed in Kentucky; and the pigeons, every morn-
ing a little before sunrise, set out for the Indiana territory,
the nearest part of which was about sixty miles distant.
Many of these returned before ten o'clock, and the great body
generally appeared on their return a little after noon. I had
left the public road to visit the remains of the breeding-place
near Shelbyville, and was traversing the woods with my gun,
on my way to Frankfort, when about ten o'clock the pigeons
which I had observed flying the greater part of the morning
northerly, began to return in such immense numbers as I never
before had witnessed. Coming to an opening by the side of
a creek, where I had a more uninterrupted view, I was
astonished at their appearance : they were flying with great
steadiness and rapidity, at a height beyond gunshot, in
several strata deep, and so close together that, could shot-
have reached them, one discharge could not have failed to
bring down several individuals. From right to left, as far as
the eye could reach, the breadth of this vast procession ex-
tended, seeming everywhere equally crowded. Curious to
determine how long this appearance would continue, I took
out my watch to note the time, and sat down to observe them.
It was then half-past one ; I sat for more than an hour, but

ir Till'. STRUGGLE 1 "I; EXISTENl K 33

instead of a diminution of thi- prodigious procc--ion, it seemed
rather to increase, lioth in numbers and rapidity : and anxious
to reach I'Yankfort before ni-lit. I i o-e and \\ent on. About
four o'.-lock in the afternoon I crossed Kentucky K'iver. at the
toun of I'Yankfort, at u hieli time the Im'nu torrent abo\e m\
heail >eemed as numerous and as extensive as ever. I. on-
after this I observed them in lar-e bodies that continued to
pass for six or ei-ht minutes, ;md th. dn Were followed

by other detached liodies, ;dl mo\in- in the .-a me south-east
direction, till after >ix o'clock in the evening. The greal
breadth of front which thi- mi-lity multitude preserved would
seem to intimate a con e.-pondiii'_: l.readth of their bieediiii;
place, which, by several gentlemen \\lio had lately pa--ed

through put of it. \\.is stated to me at several miles."

1'Voin the-c various observations, \\il-on calculatotl that
the numlier of l.ird^ I'ontained in the ma-- of pi-eon- which
lie saw on this Occasion Was at lea.-t t \\ o thousand millions.
while tin- was only one of many similar a-'jre^at ion- known
to e\i-t in \arioii- part- of tlie I'nited State-. The
picture here ui\en of these defenceless 1'irds and their still

more defelicele-- young, expo-ed to the attack- of numerous
rapacioii< enemies, luin^s \i\idly liefoie n- one of the ph

of the micea-hiLi >tru;jule for existence ever ^oin^ on; Imt

wlien we consider the -low rate of increase of these
and the nioniioiis population they are neverthele-- alile to
maintain, we must lie <-on\ inct-d that in the case of the
majoiity of liirds which multiply far more rapidU.and yel
are never alile to attain such iniinliers, the stru-.Lile a-ain-t
their numerous eliemie- and a-aili-t the ad \er.-e forces of

nature mu.-t lie even more severe or mure continuous.

' fur L (it nJJiiJ AniiiKil* and

often ///' in.<t S( " re.
The stni'^le we have hitherto lieen considering has lieen

oo o

mainly that lietween an animal or plant and its direct enemies,
whether these enemies are other animals which devour it, or
the forces of nature which destroy it. But there is another
kind of struiriile often ^oinii' on at the same time between
clo-ely related species, which almost always terminates in the
destruction of one of them. As an example of what is

D

34 DARWINISM CHAP.

meant, the missel-thrush has increased in numbers in Scotland
during the last thirty years, and this has caused a decrease in
the numbers of the closely allied song- thrush in the same
country. The black rat (Mus rattus) was the common rat of
Europe till, in the beginning of the eighteenth century, the
large brown rat (Mus decumanus) appeared on the Lower
"V olga, and thence spread more or less rapidly till it overran all
Europe, and generally drove out the black rat, which in most
parts is now comparatively rare or quite extinct. This invad-
ing rat has now been carried by commerce all over the world,
and in New Zealand has completely extirpated a native rat,
which the Maoris allege they brought with them from their
home in the Pacific ; and in the same country a native fly is
being supplanted by the European house-fly. In Russia the
small Asiatic cockroach has driven away a larger native species;
and in Australia the imported hive-bee is exterminating the
small stingless native bee.

The reason why this kind of struggle goes on is apparent
if we consider that the allied species fill nearly the same place
in the economy of nature. They require nearly the same
kind of food, are exposed to the same enemies and the same
dangers. Hence, if one has ever so slight an advantage over
the other in procuring food or in avoiding danger, in its
rapidity of multiplication or its tenacity of life, it will increase
more rapidly, and by that very fact will cause the other to
decrease and often become altogether extinct. In some cases,
no doubt, there is actual war between the two, the stronger
killing the weaker ; but this is by no means necessary, and
there may be cases in which the weaker species, physically,
may prevail, by its power of more rapid multiplication, its
better withstanding vicissitudes of climates, or its greater
cunning in escaping the attacks of the common enemies.
The same principle is seen at work in the fact that certain
mountain varieties of sheep will starve out other mountain
varieties, so that the two cannot be kept together. In plants
the same thing occurs. If several distinct varieties of wheat
are sown together, and the mixed seed resown, some of the
varieties which best suit the soil and climate, or are naturally
the most fertile, will beat the others and so yield more seed, and
will consequently in a few years supplant the other varieties.

it THE STRUf:r:LE FOR EXISTKXCE 35

A- mi effect i>f this pi inciple, we seldom timl closely allied
species nt' animals or plants living together, but often in
distinct though adjacent districts \\hnv the cuiiditii>n> .,f life
are somewhat dillerent. Thus we may find cowslips (Primula
veris) growing in a meadow, ;md primroses < 1'. vulgari>) in ;m
adjoining \V(H1, each in abundance, but not often intermingled.

And f<>r the same reason the old turf of a pasture or heath
consists of a meat variety of plants matted together, so much
so that in a patch little more than a yard square Mr. I>arwin
found twenty distinct specie-., belonging to eighteen distinct
genera and to ei-ht natural orders, thus showing their extreme
diversity of organisation. For the same reason a number of
distinct grasses and clovers are -own in older to make a good
lawn instead of anyone species ; and the <|iiantit\ of hay
produced has lieen found to lie greater from a variety of very
distinct grasses than from anv one specie.-, of -la

It may lie thought that Forests are an exception to this
rule, >ince in the north-temperate and arctic re-ions we find
extensive forests of pines or of oaks. P,ut these are, after all,
exceptional, and eharaeteri.se those re-ions only where the
climate is little favourable to forest ve-etatioii. In the
tropical and all the warm temperate parts of the earth, wlieie
there i> a siitlicient supply of moisture, the forests present the
same variety of species as does the turf of our old pastures;
and in the equatorial virgin forests there is M> -real a variety
of forms, and they are so thoroii-hlv intei niin-lcd, that tin-
traveller often finds it difficult to discover a second specimen
of any particular species which In: has noticed. Even the
forests of the temperate zones, in all favourable situations,
exhibit a considerable variety of trees of distinct genera and
families, and it is only when Ave approach the outskirts of
forest vegetation, where either drought or winds or the severity
of the winter is adverse to the existence of most trees, that
we find extensive tracts monopolised by one or two species.
Even Canada has more than sixty different forest trees, and
the Eastern United States a hundred and fifty ; Europe is
rather poor, containing about eighty trees only ; while the
forests of Eastern Asia, Japan, and Manchuria are exceedingly
rich, about a hundred and seventy species being already
known. And in all these countries the trees grow inter-

36 DARWINISM CHAP.

mingled, so that in every extensive forest we have a consider-
able variety, as may be seen in the few remnants of our
primitive woods in some parts of Epping Forest and the
New Forest.

Among animals the same law prevails, though, owing to
their constant movements and power of concealment, it is not
so readily observed. As illustrations we may refer to the
wolf, ranging over Europe and Northern Asia, Avhile the jackal
inhabits Southern Asia and Northern Africa ; the tree-
porcupines, of which there are two closely allied species, one
inhabiting the eastern, the other the western half of North
America ; the common hare (Lepus timidus) in Central and
Southern Europe, while all Northern Europe is inhabited by
the variable hare (Lepus variabilis) ; the common jay (Garrulus
glandarius) inhabiting all Europe, while another species
(Garrulus Brandti) is found all across Asia from the Urals to
Japan ; and many species of birds in the Eastern United
States are replaced by closely allied species in the west. Of
course there are also numbers of closely related species in the
same country, but it will almost always be found that they
frequent different stations and have somewhat different habits,
and so do not come into direct competition with each other ;
just as closely allied plants may inhabit the same districts,
when one prefers meadows the other woods, one a chalky
soil the other sand, one a damp situation the other a dry one.
With plants, fixed as they are to the earth, we easily note
these peculiarities of station ; but with wild animals, which we
see only on rare occasions, it requires close and long-continued
observation to detect the peculiarities in their mode of life
which may prevent all direct competition between closely
allied species dwelling in the same area.

The Ethical Aspect of the Struggle for Existence.

Our exposition of the phenomena presented by the struggle
for existence may be fitly concluded by a few remarks on its
ethical aspect. Now that the war of nature is better known,
it has been dwelt upon by many writers as presenting so vast
an amount of cruelty and pain as to be revolting to our
instincts of humanity, while it has proved a stumbling-block
in the way of those who would fain believe in an all-wise and

ii THE STRUGGLE FOR K.\ISTKN< 11 37

ruler of thf universe. Tims, a brilliant writer
says : " Pain, .-riff, disease, and ilcatli, arc the-e th<- in vent inns
of a ln\iii- < lod .' That IHI animal >hall ri-f to excellence
f\cfpt by lii-iii.u fatal to thf life of other-, is this the law uf
a kind Creator 1 It is useless to .-ay that pain has its

l.fiif\olfncf, that ma acre ha- it- mercy. Why is it so

ordained that had should be the raw material of ->.id' I'.u'n
is not the less pain because it i- u-fful : murder i- not less
murder lieran-f it i.- conducive to development. Here is
blood upon the hand still, and all the pel fumes of Arabia will
not sweeten i;

K\en -o thoughtful a writer as I'lofcs-oi Huxlf.v adopts
-imilar vie\\.- In a leccnf article on "Thf Stru-ulf t"i
K\i-tf ncf " he -peak- of the ln\ I iad- of -f I if I a t joi i - of lie i bl V
OTOUS animals which "ha\f been tormented and devoured by
carnix ores : of the carnh ores and he rl>i\ ores alike " -ill iject to
all the misfrif- incidental to old age, disease, and o\cr multi-
plication": and of tin- "more or If-- cndurum snHeiin-, '
which i- thf niffd of lioth \ani|ui-hed and \ictor. And he
conclude- that, -inee thoii-and-of time- a minute, weieoiir
ear- .-harp enoii-.li, we should heal- >i-h- and -KMH- of pain
like tho-e heard 1>\ hante at the -;itf of hell, the \\orld
cannot he -ovcriied l>y what we call Kenev olence.-

Now there is, I think, -ood rea-oii to l,elie\e that all this
i- -really exaggerated; that tin- Hippo-fd " torments " and
"miseries" <>f animal- have little real e.xistfiicf. hut are the
reflection of the ima-ined >e n-at ions of ciiltixated men and
\\oinen in similar circumstances ; and that the amount of actual
sull'erin-- caused liy the stru-ule for e.xi>tence amoii- animals

i- altogether ill>i-llifie;illt. Let US, therefore, endeavour to

ascertain what are the real facts on which the.-e tremendous
accusations are founded.

In the tir-t place, we must remember that animals are
entirely spared \\\c pain \\ e sutler in the anticipation of death
a pain far greater, in most cases, than the reality. This leads.
prolialdy, to an almost perpetual enjoyment of their lives;
since their constant watchfulness against danger, and even
actual tli-ht from an enemy, "\vill be the enjoyable

1 \Yin\VODcl R.-;ulr's Mil,-tl//-t/n,ll <>f .I/""//. \>. ')20.

- ffineteenth Century, February 1888, pp. 162, 163.

38 DARWINISM CHAP.

exercise of the powers and faculties they possess, unmixed
with any serious dread. There is, in the next place, much
evidence to show that violent deaths, if not too prolonged, are
painless and easy ; even in the case of man, whose nervous
system is in all probability much more susceptible to pain than
that of most animals. In all cases in which persons have
escaped after being seized by a lion or tiger, they declare
that they suffered little or no pain, physical or mental. A
well-known instance is that of Livingstone, who thus describes
his sensations when seized by a lion : " Starting and looking
half round, I saw the lion just in the act of springing on me.
I was upon a little height ; he caught my shoulder as he sprang,
and we both came to the ground below together. Growling
horribly close to my ear, he shook me as a terrier-dog does a
rat. The shock produced a stupor similar to that which
seems to be felt by a mouse after the first shake of the cat.
It causes a sort of dreaminess, in ivhich there was no sense of
pain or feeling of terror, though I was quite conscious of all
that Avas happening. It was like what patients partially
under the influence of chloroform describe, Avho see all the
operation, but feel not the knife. This singular condition
was not the result of any mental process. The shake
annihilated fear, and allowed no sense of horror in looking
round at the beast."

This absence of pain is not peculiar to those seized by wild
beasts, but is equally produced by any accident Avhich causes
a general shock to the system. Mr. Whymper describes an
accident to himself during one of his preliminary explorations
of the Matterhorn, Avhen he fell several hundred feet, bounding
from rock to rock, till fortunately embedded in a snoAv-drift
near the edge of a tremendous precipice. He declares that
Avhile falling and feeling bloAv after bloAv, he neither lost
consciousness nor suffered pain, merely thinking, calmly, that
a feAV more bloAvs Avould finish him. AA r e have therefore a
right to conclude, that AA r hen death follows soon after any
great shock it is as easy and painless a death as possible ; and
this is certainly what happens Avhen an animal is seized by a
beast of prey. For the enemy is one Avhich hunts for food,
not for pleasure or excitement ; and it is doubtful Avhether any
carnivorous animal in a state of nature begins to seek after

M TI1K STRUGGLE I"R KXISTKXCK 39

prey till driven to (In BO l>y hunger. When an animal is
cau-ht, then-fore, it i> very -non de\ mired, mid thus tlie !ir-t
>hoek is followed liy ;HI ;dniost painless death. Neither do
those which die of cold .,r hunger sutler much. Cold is
-'nerally severest at ni-ht and has a tendency to produce
sleep and painle-> extinction. Hunger, mi the other hand, h
hardly felt during period- of excitement, and \\hen fond is

BCarce the excitement of .-eekin- for it i- at its greatest. It
i- prol.alile, also, that when hiin-er presses, nm-t animals will
de\our anything to stay their hun-er, and will die of -radual
exhaustion and weakne-s not nece arily painful, if they do
not fall an earlier prey to some enemy or to cold. 1

Now let us consider \\hat are the enjoyments of the li\es
of most animals. As a rule they come into existence at a
time of year when food is m<>>t plentiful and the climate nn,-t
suitable, that is in the >prin- of the temperate /one and at
the commencement of the di\ season in the tropics. They

grow \ Lormisly, liein- >upplied \\ith aliundaiice of f 1 ; and

when the\ reach maturity their li\e> are a continual round of
healthy excitement and exercise, alternating \\ith complete
repo-e. The daily search for the daily food employs all their
faculties and exercises e\ery OIL;. in of their liodies, while this
exercise leads to the satisfaction of all their ph \.-ical needs.
In our own case, \\ can give no more perfect detinitionof
liappiness, than this exercise and this >at i-fact ion ; and we
must therefore conclude that animals, as a rule, enjoy all the
happine of which they are capalile. And this normal state
of happiness is not alloyed, as with us, }>y lotiLC jieriod-
Avhole lives often of poverty or ill health, and of the 1111
sati-tied loii-in- for plea-iue- \\hich others enjoy luit to which
u e cannot attain. Illness, and what answers to poverty in
animals continued hunger are (jniekly followei] }>\ unantici
]ated and almost painle-s extinction. \\ here we err is, in
-i\in-- t<> animals feelings and emotions which they do not
PMC-S. Tons the \ery >i-ht of lilood and of torn or man-led
limlis is painful, while the idea of the surl'erini; implied }>y it

1 Thr Kotn-1, wliirli usually fi-i-ils on mice, birds, and fr<>:_ r N soinftiincs
stays its lumber with t/artli\vm IMS. as do some of the American buzzards.
The Hom-y-1'uzzard sometimes eats n-t only earthworms and slugs, but even
corn ; and the Buteo borealis of North America, whose usual food is small
mammals and birds, sometimes eats crayfish.

40 DARWINISM CHAP, n

is heartrending. We have a horror of all violent and sudden
death, because we think of the life full of promise cut short,
of hopes and expectations unfulfilled, and of the grief of
mourning relatives. But all this is quite out of place in the
case of animals, for whom a violent and a sudden death is in
every way the best. Thus the poet's picture of

' ' Nature red in tooth and claw
With ravine "

is a picture the evil of which is read into it by our
imaginations, the reality being made up of full and happy
lives, usually terminated by the quickest and least painful of
deaths.

On the whole, then, we conclude that the popular idea of
the struggle for existence entailing misery and pain on the
animal world is the very reverse of the truth. What it
really brings about, is, the maximum of life and of the enjoy-
ment of life with the minimum of suffering and pain. Given
the necessity of death and reproduction and without these
there could have been~ no progressive development of the
organic world, and it is difficult even to imagine a system
by which a greater balance of happiness could have been
secured. And this view was evidently that of Darwin himself,
who thus concludes his chapter on the struggle for existence :
" When we reflect on this struggle, we may console ourselves
with the full belief that the war of nature is not incessant,
that no fear is felt, that death is generally prompt, and that
the vigorous, the healthy, and the happy survive and
multiply."

CHAPTER 111

TIII-: \ \i;i Ai'.n.n v ><\- SPECIB8 IN \ STATE OF N\H'i;i-:

Import -HUT nf v.irialiility- Popular i irding it Variability <>t' thr

]n\vrr animal- 'I'll.- \ ;tri;iliilit y nf in-iTt.-. Variation among lizards

Y;iri;iti"ii aimm^ l>ii.i- 1'iaiM am* iif I'inl-variatiiin Xuml>. i \'
varying iii'liviiliials Variation in tin- mammalia Variation in

inttTii.il ui'LCan-. \' n ia: inns in tin- skull Variation.-, in tin- li.il.ii-,
<if Aniin.il.>. Tin- Varialtility <>l' plant- - i.i>-h vary lir

r< marks.

Till-: foundation of (lie I>:ir\viiii;in theory i- tin- varialiility of
species, Mini it i> i|iiitr useless to attempt r\ni to understand

that theory, milch li-ss to appreciate tin- completelie of tin-
proof uf it, Ulili'.-s \\c tir-t olit;iill ;i rlc;i|- colici ) it i. H i of tin-
ii;ituri- ;iiid extent of this \ ari;iliilit \ . The nio-t fiv.|Ueiit ;ind
the Ilio-t )ili>le;i(lin- of the ol ijeet i Hi-> to the etlie.ir\ of niitlinil
-election ;in'-M' flulu i - 1 n i|-;i I K c of tlli> >1 tl iject . Mil i^llol'Mliee

.-h;ireil l.y niMiiy iiMtiir;rli-ts. for it i- only since Mr. I);ir\\in
has tiiiiLiht us their ini|ioi1aiice that \arietie> liave lieen
.-y-tematieally collected and recorded: and even ii"\v \ny
fe\V collectors or Mlldellts liestnw Ujioii them the attention

they deserve. l!y the older naturalist>, indeed, varieties-
e-|iecially if nuineroii-, -mail, and of frequent occurrence -
were looked iiimii as an unmitigated nuisance, liecaii-e they
rendered it almost im]io--ilile to -ive jireeise definitions of
species, then cDii.-idered the chief end of systematic natural
history. Hence it was the cu>tom to de.-criKe what -was
siiji]to>ed to lie the "typical foi'in " of species, and most
collectors \\erc satisfied if they possessed this typical form
in their culiinets. Now, however, a collection is valued in
proportion as it contains illustrative specimens of all the
varieties that occur in each species, and in some cases these

42 DARWINISM CHAP.

have been carefully described, so that we possess a consider-
able mass of information on the subject. Utilising this in-
formation we will now endeavour to give some idea of the
nature and extent of variation in the species of animals and
plants.

It is very commonly objected that the widespread and
constant variability which is admitted to be a characteristic of
domesticated animals and cultivated plants is largely due to
the unnatural conditions of their existence, and that we have
no proof of any corresponding amount of variation occurring
in a state of nature. Wild animals and plants, it is said, are
usually stable, and when variations occur these are alleged to
be small in amount and to affect superficial characters only ;
or if larger and more important, to occur so rarely as not to
afford any aid in the supposed formation of new species.

This objection, as will be shown, is utterly unfounded ;
but as it is one which goes to the very root of the problem, it
is necessary to enter at some length into the various proofs of
variation in a state of nature. This is the more necessary
because the materials collected by Mr. Darwin bearing on
this question have never been published, and comparatively
few of them have been cited in The Origin of Species ; while a
considerable body of facts has been made known since the
publication of the last edition of that work.

Variability of the Lower Animals.

Among the lowest and most ancient marine organisms are
the Foraminifera, little masses of living jelly, apparently
structureless, but which secrete beautiful shelly coverings,
often perfectly symmetrical, as varied in form as those of the
mollusca and far more complicated. These have been studird
with great care by many eminent naturalists, and the late Dr.
AV. B. Carpenter in his great work the Lit mil action to the
Study of the Foraminifera thus refers to their variability :
" There is not a single species of plant or animal of which the
range of variation has been studied by the collocation and
comparison of so large a number of specimens as have passed
under the review of Messrs. Williamson, Parker, Rupert
Jones, and myself in our studies of the types of this group ;"
and he states as the result of this extensive comparison of

in YAKIAIJIUTY OF SPECIES IN A STATE OF NATURE 43

specimens : "The range of variation is so great aiming the
Foraminifera as to include not merely those dillerential char-
acters which have 1 ..... 11 usually accounted .>/, ///<, but also
thus,, upon which the greater part of the genera of this group
have Keen founded, and even in some instance.-, tho-e of its
"

( 'oming now to a higher group the Se;i- Anemones Mr. P.
H. Gosse and other writers on these creatures often refer to
variations in si/e, in the thickness and length of the tentacles,
the form of the disc and of the mouth, and the character of
sin-face of the column, while the colour varies enormously in
a great number of the specie-. Similar variations occur in all
the various groups of marine invertebrata, and in the great
sub-kingdom of the molhi-ca the\ are especially numerous.

Thus, Dr. S. 1'. Wo,,d\vard states that many pre-ent a most
perplexing amount of variation, resulting (as he supposes)
from supply of food, variety of depth and of saltings of the
water; but w know that many variations are .jiiite inde-
pendent of Midi causes, and we will now consider a few COSCS
among the land-mollusca in which they have been more care-
fully studied.

In the small forest region of ( >alm, one of the Sandwich
Mands, there have been found about 1 7 > speries of land shells
represented l.y 7<><) or sno varieties; and we are told bv the
Kev. .1. T. (iulick, who studied them carefully, that "we
frequently find a genus represented in several successive
valleys by allied species, sometimes feeding on the same, some
times on ditlei-ent plants. In every such case the vallev-
that are nearest to each other furnish the nio-t nearly allied
forms ; iiml ,i Jiill >// (//' //// rurir/it* ,,f ciii'li sjur/i's /'/V.SV///N

" nun ni gradation j _/"////.< li-(<r,,n ///,- , ni ,r<' <///< n/, //! {>//>,

t'//nl 111 tin more U-'nllij .<-yov///(/ lin-al'itl,-*." 1

In most land-shells there is a considerable amount of varia-
t|1 "i in colour, markings, sixe, form, and texture or striation
of the surface, even in specimens collected in the same
locality. Thus, a French author has enumerated no less than
I'.is varieties of the eoinmon wood-snail (Helix nemoralis),
while of the equally common garden -snail (Helix hortensi>;
ninety varieties have been described. Fresh-water shells are also
1 Foraminifera, preface, p. x.

44 DARWINISM CHAP.

subject to great variation, so that there is much uncertainty as
to the number of species ; and variations are especially frequent
in the Planorbidse, which exhibit many eccentric deviations from
the usual form of the species deviations which must often
affect the form of the living animal. In Mr. Ingersoll's Report
on the Recent Mollusca of Colorado many of these extra-
ordinary variations are referred to, and it is stated that a shell
(Helisonia trivolvis) abundant in some small ponds and lakes,
had scarcely two specimens alike, and many of them closely
resembled other and altogether distinct species. 1

The Variability of Insects.

Among Insects there is a large amount of variation, though
very few entomologists devote themselves to its investigation.
Our first examples will be taken from the late Mr. T. Vernon
Wollaston's book, On the Variation of Species, and they
must be considered as indications of very widespread though
little noticed phenomena. He speaks of the curious little
carabideous beetles of the genus Notiophilus as being
" extremely unstable both in their sculpture and hue ; " of
the common Gala thus mollis as having " the hind wings at
one time ample, at another rudimentary, and at a third nearly
obsolete ;" and of the same irregularity as to the wings being
characteristic of many Orthoptera and of the Homopterous
Fulgoridre. Mr. Westwood in his Modern Classification of
Iiivr.fs states that "the species of Gerris, Hydrometra, and
Velia are mostly found perfectly apterous, though occasionally
with full-sized wings."

It is, however, among the Lepidoptera (butterflies and
moths) that the most numerous cases of variation have been
observed, and every good collection of these insects affords
striking examples. I will first adduce the testimony of Mr.
Bates, who speaks of the butterflies of the Amazon valley
exhibiting innumerable local varieties or races, while some
species showed great individual variability. Of the beautiful
Mechanitis Polymnia he says, that at Ega on the Upper
Amazons, " it varies not only in general colour and pattern,
but also very considerably in the shape of the wings,
especially in the male sex." Again, at St. Paulo, Ithomia
* United States Geological Survey of the Territories, 1874,

HI VAKIAIHUTY 01- SlT.dKS IX A STATK or NATUIK 45

Orolina exhibits four distinct varieties, ;ill occurring together,
;ui(l these ditl'er imt niily in colour but in fnnn. one variety
being described as having the fore wings much elongated in the
male, while another i- nnirh larger and lias "the hind \\inu- in
the male different in shape." of lleliconius Nmnata Mr. I'.ate-
says: " This >pecie- i- 30 variaMe that it is ditlieiilt to find tuo
examples exactly alike," while "it varies in structure as well
as in colours. The win-> are sometimes broader, some
times narrower ; and their edue- are simple in some examples
and fe-tooned in other-.' Of another species of the -ame
genUS, II. llielpomene, ten distinct \arietie- are descrilied
all more or le-s cuniiected by intermediate forms, and four
nt the-e varieties were obtained at one locality. Serpa on
the north bank of the Ama/.on. ('eratina Ninoiiia is another
of these very unstable species exhibiting many local varieties
which are. hou. \e|, incomplete and connected ly intermediate

forms; while the several species of the genus I .\corea all

\ar\ to such an extent as alino>t to link them together, so
that Mr. Hates think- the\ miuht all fairly lie considered as
varieties of one >pecic- only.

Turning to the Ka-tern 1 lemi-phere we have in I'apilo
Se\cnis ; ( species \\hich e.xliiliits a lariie amount of simple
\ariation, in the proelice or absence of a pale patch on the

upper wings, in the In-own submarginal marks on the lower
win ITS, in the form and extent of the \ello\v Land, and in
the >i/,e of the -pecinien-. The nio-i extreme form-, as well
as the intermediate one-, are often found in one locality and
in company with each other. A -mall butterfly (Teiias hecalie)
ranges over the whole of the Indian and Malayan regions to
Australia, and everywhere e.xliiliits great variations, many of
which have been described as distinct species ; but a gentle-
man in Australia bred two of tliese distinct forms (T. hecabe
and T. .Ksiope), with .-eveial intermediates, from one batch of
caterpillars found feeding together on the same plant. 1 It is
therefore very probable that a con.-iderable number of supposed
distinct species are only individual varieties.

('ases of variation similar to those now adduced among
butterflies might be increased indefinitely, but it is as well to
note that such important characters as the neuration of the
1 Proceedings of tfa Entomological -<f L<>i>ii<>n, 1875, p. vii.

46 DARWINISM CHAP.

wings, on which generic and family distinctions are often
established, are also subject to variation. The Rev. R. P.
Murray, in 1872, laid before the Entomological Society
examples of such variation in six species of butterflies, and
other cases have been since described. The larv?e of butter-
flies and moths are also very variable, and one observer
recorded in the Proceedings of the Entomological Society for
1870 no less than sixteen varieties of the caterpillar of the
bedstraw hawk-moth (Deilephela galii).

Variation among Lizards.

Passing on from the lower animals to the vertebrata, we
find more abundant and more definite evidence as to the
extent and amount of individual variation. I will first give a
case among the Reptilia from some of Mr. Darwin's un-
published MSS., which have been kindly lent me by Mr.
Francis Darwin.

" M. Milne Edwards (Annales des Sci. Nat., 1 ser., torn,
xvi. p. 50) has given a curious table of measurements of four-
teen specimens of Lacerta muralis ; and, taking the length of
the head as a standard, he finds the neck, trunk, tail, front
and hind legs, colour, and femoral pores, all varying wonder-
fully ; and so it is more or less with other species. So ap-
parently trifling a character as the scales on the head affording
almost the only constant characters."

As the table of measurements above referred to would give
no clear conception of the nature and amount of the variation
without a laborious study and comparison of the figures, I
have endeavoured to find a method of presenting the facts to
the eye, so that they may be easily grasped and appreciated.
In the diagram opposite, the comparative variations of the
different organs of this species are given by means of variously
bent lines. The head is represented by a straight line because
it presented (apparently) no variation. The body is next
given, the specimens being arranged in the order of their size
from No. 1, the smallest, to No. 14, the largest, the actual
lengths being laid down from a base line at a suitable
distance below, in this case two inches below the centre, the
mean length of the body of the fourteen specimens being two
inches. The respective lengths of the neck, legs, and toe of

Ill

DIAGRAM OF VARIATION

47

J 3 5 ~T~ 9 1
The lengths in the table are given in millimetres, which are here reduced
to inches for the means.

Fio. 1. Variations of Lacerta muralis.

48 DARWINISM CHAP.

Length of Head famBBi^BaBa) fa/fen as : t/i;
standard in each of the above-named species

Fio. 2. Variation of Lizards.

Ill VAKIAI'.II.ITV OF SPECIES IN A STATK n|- NATURE 40

cadi specimen :iri i then laid down in tin- same manner at
convenient distances apart fur comparison; and we see that
their variations hear no definite relation to those of the liu.lv,
and nut much to thu-e ..f each other. \\'itli tin- exception of
No. ", in which all the [tart- agree in being large. there is a
marked independence uf ra.-li [Mil, shown l.y the lines often
curving in oppu-jte directions; which prove.- that in tho-e
-pecimens one part i- lame \\hile the other is -mall. Tin-
actual amount of the variation i- veiv great) ranging from

une sixth of the mean length iii the neck to considerably more

than a fourth in the hind leu, and this among only fourteen
examples which happen to lie in a particular museum.

To prove that this i- not an isolated case, Professor Milue
Kdvvards also give-- a tal>le showing the amount of variation in
tin' museum specimens uf six common specie.- of li/.ard-, also
takin- the head a- the ,-taiidard, so that the comparative
variation uf each part to the head i- uiveii. In the accompan v
ing diagiam (Fig. L' ) the variations are exhibited by means of
lines of varying length. It will lie understood that, however
much the s|,eciniens \arieil in size, if they had kept the same
y//-.y</7/><//>, the variation line would have lieen in every case
reduced to a point, as in the neck of L. velo\ which exhibits
no variation. Tin- different proportions of the variation line-
foreach s[>ecies may show a distinct mode uf variation, or may
be merely due to the small and differing numlu-r of specimens ;
for it is certain that whatever amount of variation occurs
amoni; a few specimens will be -reatlv increa-ed when a much
larger number of specimens are examined. That the amount of
variation is lar-e. may be seen l>y com paring it with the actual
length of the head (uiven lielovv the din-ram) which was used
as a standard in determining the variation, but which itself
seems not to have varied. 1

/j//v/.s-.

Coming now to the class of Birds, we find much more
copious evidence of variation. This is due partly to the fact
that Ornithology has perhaps a larger body of devotees than
any other branch of natural history (except entomology) ; to
the moderate size of the majority of birds ; and to the circum-

1 Ann. des Sci. Nut., torn. xvi. p. 50.
E

50 DARWINISM CHAP.

stance that the form and dimensions of the wings, tail, beak,
and feet offer the best generic and specific characters and can
all be easily measured and compared. The most systematic
observations on the individual variation of birds have been
made by Mr. J. A. Allen, in his remarkable memoir: " On the
Mammals and Winter Birds of East Florida, with an examina-
tion of certain assumed specific characters in Birds, and a
sketch of the Bird Faunee of Eastern North America,"
published in the Bulletin of the Museum of Comparative
Zoology at Harvard College, Cambridge, Massachusetts, in
1871. In this work exact measurements are given of all the
chief external parts of a large number of species of common
American birds, from twenty to sixty or more specimens of
each species being measured, so that we are able to determine
with some precision the nature and extent of the variation
that usually occurs. Mr. Allen says : " The facts of the
case show that a variation of from 15 to 20 per cent
in general size, and an equal degree of variation in the
relative size of different parts, may be ordinarily expected
among specimens of the same species and sex, taken at the
same locality, while in some cases the variation is even greater
than this." He then goes on to show that each part varies
to a considerable extent independently of the other parts ; so
that when the size varies, the proportions of all the parts
vary, often to a much greater amount. The wing and tail,
for example, besides varying in length, vary in the pro-
portionate length of each feather, and this causes their outline
to vary considerably in shape. The bill also varies in length,
width, depth, and curvature. The tarsus varies in length, as
does each toe separately and independently ; and all this not
to a minute degree requiring very careful measurement to
detect it at all, but to an amount easily seen without any
measurement, as it averages one-sixth of the whole length and
often reaches one -fourth. In twelve species of common
perching birds the wing varied (in from twenty-five to thirty
specimens) from 1 4 to 2 1 per cent of the mean length, and the
tail from 13 '8 to 2 3 '4 per .cent. The variation of the form of
the wing can be very easily tested by noting which feather is
longest, which next in length, and so on, the respective
feathers being indicated by the numbers 1, 2, 3, etc., com-

in VARIABILITY OK SlT.iIF.S IX A STATE OF XA'ITKK 51

iiieiicin- with the outer one. A- an example of the il
variation constantly met \\ith. the following occurred
twenty the >pecimeiis of Deiidra-ca coronata. Numbers
bracketed imply that the corresponding feathers were of

eijiial length. 1

KKLATIVK LKNI.TIIS OF PRIMARY Wi\<; Fi \mi i,"s OF
I>KM>l;.l.< A CORONATA.

1

1 in
Length.

Tliinl in
Let

h in

!i in
1 njtli.

I

3

1

I

5

6

3

2

I

1

5

6

3

1 ~
! 4

1

G

6

7

'.2 1
3 I

4

1

5

6

7

2

1 1

5

6

7

-

9

1

4 \

Here we have the \ery distinet proportionate lengths of
the wiiiL; feathers, any one of which is often tlnnmlit sutticient
to characterise a distinct species of lird ; and thotiuh thi- i-
rather an extreme case, Mr. Allen assures us that "the com-
parison, extended in the table to only :i few <pecie>, has been
carried to scores of others \\itli similar results."

A Ion- with this variation in size and proportions there occurs
a large amount of variation in colour and markings. "The
ditl'erence in intensity of colour between the extremes of a
series of fifty or one hundred specimens of any species, collected
;it a >inule locality, ;md nearly at the same season of the year,
is often as great as occurs between truly distinct species." But
there is also a great amount of individual variability in the
markings of the same species. Birds having the plumage
varied with streaks and spots differ exceedingly in different
individuals of the same species in respect to the size, shape,
and number of these marks, and in the general aspect of the
plumage resulting from such variations. "In the common

1 See Winter Birds of Florida, p. 206, Table F.

52 DARWINISM CHAP.

song sparrow (Melospiza melodia), the fox-coloured sparrow
(Passerella iliaca), the swamp sparrow (Melospiza palustris), the
black and white creeper (Mniotilta varia), the water-wagtail
(Seiurus novseboracencis), in Turdus fuscescens and its allies, the
difference in the size of the streaks is often very considerable.
In the song sparrow they vary to such an extent that in some
cases they are reduced to narrow lines ; in others so enlarged
as to cover the greater part of the breast and sides of the body,
sometimes uniting on the middle of the breast into a nearly
continuous patch."

Mr. Allen then goes on to particularise several species in
which such variations occur, giving cases in which two speci-
mens taken at the same place on the same day exhibited the
two extremes of coloration. Another set of variations is
thus described : " The white markings so common on the wings
and tails of birds, as the bars formed by the white tips of the
greater wing-coverts, the white patch occasionally present at
the base of the primary quills, or the white band crossing
them, and the white patch near the end of the outer tail-
feathers are also extremely liable to variation in respect to
their extent and the number of feathers to which, in the same
species, these markings extend." It is to be especially noted
that all these varieties are distinct from those which depend
on season, on age, or on sex, and that they are such as have
in many other species been considered to be of specific
value.

These variations of colour could not be presented to the eye
without a series of carefully engraved plates, but in order to
bring Mr. Allen's measurements, illustrating variations of size and
proportion, more clearly before the reader, I have prepared a
series of diagrams illustrating the more important facts and
their bearings on the Darwinian theory.

The first of these is intended, mainly, to show the actual
amount of the variation, as it gives the true length of the
wing and tail in the extreme cases among thirty specimens of
each of three species. The shaded portion shows the minimum
length, the unshaded portion the additional length in the
maximum. The point to be specially noted here is, that in
each of these common species there is about the same amount
of variation, and that it is so great as to be obvious at a glance.

Ill

MA'.KAM OF VARIATION

53

co

3.

eo

=3

CO

^o

Ho

-s:

S-J

O

ca

^ r

5

-O
O

co

CD

5

s-

c
o

c

3

s

a

-

5

o

-c

co

-Q
W
t3

a

-c

cu

-c:

1

_o

; l

(f>
2

v.

a

-0

a

et

a

*

<
o

I

CO

o

54 DARWINISM CHAP.

There is here no question of " minute " or " infinitesimal "
variation, which many people suppose to be the only kind of
variation that exists. It cannot even be called small ; yet
from all the evidence we now possess it seems to be the
amount which characterises most of the common species of
birds.

It maybe said, however, that these are the extreme variations,
and only occur in one or two individuals, while the great
majority exhibit little or no difference. Other diagrams will
show that this is not the case ; but even if it were so, it would
be no objection at all, because these are the extremes among
thirty specimens only. We may safely assume that these thirty
specimens, taken by chance, are not, in the case of all these
species, exceptional lots, and therefore we might expect at least
two similarly varying specimens in each additional thirty. But
the number of individuals, even in a very rare species, is
probably thirty thousand or more, and in a common species
thirty, or even three hundred, millions. Even one individual
in each thirty, varying to the amount shown in the diagram,
would give at least a million in the total population of any
common bird, and among this million many would vary much
more than the extreme among thirty only. We should thus
have a vast body of individuals varying to a large extent in
the length of the wings and tail, and offering ample material
for the modification of these organs by natural selection. We
will now proceed to show that other parts of the body vary,
simultaneously, but independently, to an equal amount.

The first bird taken is the common Bob-o-link or Rice-bird
(Dolichonyx oryzivorus), and the Diagram, Fig. 4, exhibits the
variations of seven important characters in twenty male adult
specimens. 1 These characters are the lengths of the body,
wing, tail, tarsus, middle toe, outer toe, and hind toe, being as
many as can be conveniently exhibited in one diagram. The
length of the body is not given by Mr. Allen, but as it forms
a convenient standard of comparison, it has been obtained by
deducting the length of the tail from the total length of the
birds as given by him. The diagram has been constructed
as follows : The twenty specimens are first arranged in a
series according to the body-lengths (which may be con-
1 See Table I, p. 211, of Allen's Winter Birds of Florida.

Ill

DIAGRAM OF VARIATION

55

5 10 15

FIG. 4. Dolichonyx oryzivorus. 20 Males.

20

56

DARWINISM

CHAP.

/ 5

15 20 25 30 35 40

1 5

10 15 20 25 30

FIG. 5. Agelseus phoeniceus. 40 Males.

55 40

in VARIABILITY <>l- SPECIES IN A STATE OF NATURE 57

M'.|I-I-IM| t> ui\c the si/e of tin- bird), from tin- slim-test
to the h>iiue-t, and the same number of vertical lines are
drawn, numbered from iiin> to twenty. In this case (and
wherever practicable) the bodv length is measured from the
lower line nf the diagram, so that the actual length of the lird
i- exhibited as well as the actual variations of length. These
can lie well e-tiniated by means of the hori/oiital line dra\\n

at the mean between tile t V\ o e \ 1 fell ie-, and it will lie -cell

that one tit'tli of the total immlier of >|>eciniens taken on either
-ide exhibits a \ery lar-e amount of variation, which would of
course lie very much greater if a hundred or moi e -peeimen.-
were coni]ia!-ed. The lengths of the v\ I'IIL:, tail, and other parts
are then laid down, and the diagram thus exhibit- at a glance
the comparative variation of these parts in even specimen as
well as the actual amount of variation in the t went v .-pec miens ;
and v\ e are thus enabled to arrive at some important con-
clusions.

We note, first, that the variation- of none of the part- follow
the variation- of the bodv, but are sometimes almost in an
Opposite direction. Thus the longest v\in- corresponds to a
rather small liody, the longest tail to a medium body, while
the lon-e-t leg and toes belong to only a moderately lar^e liodv .
A.gain, even relatetl part- do not constantly varv together but

pre-eiit many instances of independent variation, as .-hown by
the want of parallelism in their respective variation-lines. In

No. 5 (see Flu. I) the wilii; is Very loii^, tile tail modefateh

so; while in No. <i the win^ is much -horter while the tail is
considerably lonuei-. The tarsus pre-ent.- comparatively little
variation ; and although the three toes may lie said to vary in
general together, then' are many divergencies; thus, in passing
from No. '.i to No. ID, the outer toe becomes longer, while the
hind toe becomes considerably shorter ; while in Nos. 3 and 4
the middle toe varie.s in an opposite way to the outer and the
hind toes.

In the next diagram (Fig. 5) we have the variations in
forty males of the Red-winged Blackbird (Agelreus pha>niceus),
and here we see the same general features. One-fifth of the
whole number of specimens offer a large amount of variation
either below or above the mean ; while the wings, tail, and head
vary quite independently of the body. The wing and tail too,

58

DARWINISM

CHAP.

75 10 15 20 25 30

FIG. 6. Cardinalis virginiamis. 31 Males,

in VARIABILITY OF SPECIKS IX A STATE OF NATURE 59

though showing some iiiiiuunt of (.-on-elated variation, yet in
no le.-s than nine cases vary in opposite directions as compared

with thr preceding >pecics.

The next diagram (Fig. *'>). showing the variations of thirty-
one males of tin- ( 'ardinal bird (Cardinalis virginianus), exhibits
these fcatnrt-s much inoi-f strongly. The amount of variation
in proportion to the si/.r of tin- bird i- \ery much greater;
while tin- variations of tin- wing and tail not only have no
correspondence with that of the liody but very little with each

other. In no le-^ than twelv thirteen instances thi-y \ary

in opposite directions, while even where they correspond in
direction the amount of the variation is often very dispropor-
tionate.

As the proportions of the tar.-i and toes of birds have threat
influence on their mode of life and habits and are often used
as >pecitic i.r even generic characters, I have prepared a
diagram i Fig. 7 ) to show the variation in the-e parts only, among
t \\ enty specimens of each of four species of birds, four or five of
the most variable alone being given. The extreme divergence
of each of the lines in a \ertical direction shows the actual
amount of variation ; and if we consider the small length of
the toes of these small birds, averaging about three-quarters of
an inch, we shall see that the variation i- really very large;
while the diverging curves and angles show that each part
varies, to a greal extent, independently. It is evident that
if we compared some thousands of individuals instead of
only twenty, we should have an amount of independent
variation occurring each year which would enable almost any
modification of these important organs to be rapidly effected.

In order to meet the objection that the large amount of
variability here shown depends chiefly on the observations
of one person and on the birds of a single country, I have
examined Professor Schlegels Catalogue of the Birds in the

O O

Leyden Museum, in Avhich he usually gives the range of
variation of the specimens in the museum (which are
commonly less than a dozen and rarely over twenty) as
regards some of their more important dimensions. These
fully support the statement of Mr. Allen, since they show an
equal amount of variability when the numbers compared are

60

DARWINISM

CHAP.

Middle Toe
Outer Toe
Hind Toe

Middle Toe
Outer Toe
Hind 7be_

Middle Toe
Outer 7be_

Hind 7be_

1 2 3
Fro:n Table G. in Allen's Birds of Florida.

FIG. 7. Variation of Tarsus and Toes,

Ill

OF VARIATION

61

Phonygama atra

Orio/us galbula

Pica can data

Cn

CO

3

CO

3 I

I

Semeioptera iuallacei

III..

1 1 1

I.I

Pyrrhocorax alpinus

i I .

Fio. 8. Variation of Birds in Leyden Museum.

62 DARWINISM CHAP.

sufficient, which, however, is not often the case. The
accompanying diagram exhibits the actual differences of size
in five organs which occur in five species taken almost at
random from this catalogue. Here, again, we perceive that
the variation is decidedly large, even among a very small
number of specimens ; while the facts all show that there is
no ground whatever for the common assumption that natural
species consist of individuals which are nearly all alike, or
that the variations Avhich occur are " infinitesimal " or even
" small."

The proportionate Number of Individuals which present a
considerable amount of Variation.

The notion that variation is a comparatively exceptional
phenomenon, and that in any case considerable variations
occur very rarely in proportion to the number of individuals
which do not vary, is so deeply rooted that it is necessary to
show by every possible method of illustration how completely
opposed it is to the facts of nature. I have therefore
prepared some diagrams in which each of the individual birds
measured is represented by a spot, placed at a proportionate
distance, right and left, from the median line accordingly as
it varies in excess or defect of the mean length as regards the
particular part compared. As the object in this set of dia-
grams is to show the number of individuals which vary con-
siderably in proportion to those which vary little or not at
all, the scale has been enlarged in order to allow room for
placing the spots without overlapping each other.

In the diagram opposite twenty males of Icterus Baltimore
are registered, so as to exhibit to the eye the proportionate
number of specimens which vary, to a greater or less amount,
in the length of the tail, wing, tarsus, middle toe, hind toe, and
bill. It will be noticed that there is usually no very great
accumulation of dots about the median line which shows the
average dimensions, but that a considerable number are spread
at varying distances on each side of it.

In the next diagram (Fig. 10), showing the variation
among forty males of Agel?eus phceniceus, this approach to an
equable spreading of the variations is still more apparent;
while in Fig. 12, where fifty -eight specimens of Cardinalis

rit VAKIAIULITV OF SPECIES IX A STATE OF NATURE 63

rirginianus an- registered, we see M remarkable Dreading oul
of thrsp.it>, showing in -ome of the characters a tendency t.i

regation into two or more groups of individuals, each van
ill IT <-oii-iderabl\ from the mean.

In unltT fully to appreciate the teaching of these diagram-.

VARIATION OF

ICTERUS BALTIMORE. 20. <?

Tqil.

Wing.
>

Tarsus.

Hind

Middle Toe.

Toe.

Bill,

. ::

Bill,

Length.

I

Width.

o *o e 9

FIG. 9.

we must remember, that, whatever kind and amount of varia-
tions are exhibited by the few specimens here compared,
would be greatly extended and brought into symmetrical
form if large numbers thousands or millions were sub-
jected to the same process of measurement and registration.
"W e know, from the general law which governs variations
from a mean value, that with increasing numbers the range

64

DARWINISM

CHAP.

VARIATION

OF 40 MALES OF

AGEL/EUS PHCENICEUS.

Length

of Bill.

_ _
ooooooooooooo

Total Len

A

800

jthofBird.

Length

9 *00

of Tail.

oooo

Lengtn of Wing.
o ooo

0000 00

oo*o eo o*ooo

Amount of

BILL 7?

Variation.
LENGTH -k

TAIL, j- WING.

Fro. 10.

of variation of each part would increase also, at first rather
rapidly and then more slowly ; while gaps and irregularities

Curves of Variation
FIG. 11.

would be gradually filled up, and at length the distribution
of the dots would indicate a tolerably regular curve of double
curvature like those shown in Fig. 11. The great divergence

in VAKIAP.ILITY OF SI'KCIKS IX A STATK OK NATURE 65

of tin- 1<>K when even a fe\v -peciinens are compared, sho\vs
that the curve, with high numbers, would lie a flat one like the
lower curve in the illustratimi here given. '1'his liein- the case it
would follow that a \ery large proportion of the total number i f
individuals constituting a species would diverge considerably
from its average condition as regard.- i ach part or organ : and
a- \\-e kiio\v from the previous diagram- of variation ( Fi-s. 1
to 7) that each part varies to a considerable extent, nnl,
j" mil iitlii, the materials constantly reaily for natural selection

CARDINALIS VIRGINIANUS. 58 specimens. Florida.

Tail.

Length

of Bird.

0000

o*ooo*

V.

Wing.

O O

O 00*

a

(From Allen's Birds of Florida, p. 281)
Fio. 12

to act upon are abundant in quantity and very varied in kind.
Almost any combination of variations of distinct parts will be
available, where required ; and this, as we shall see further
on, obviates one of the most weighty objections which have
been urged auain>t the etticiency of natural selection in pro-
ducing new species, genera, and higher groups.

h/ f/ic MI in null in.

Owing to the generally large size of this class of animals,
and the comparatively small number of naturalists who study
them, large series of specimens are only occasionally examined

F

66 DARWINISM CHAP.

and compared, and thus the materials for determining the
question of their variability in a state of nature are compara-
tively scanty. The fact that our domestic animals belonging
to this group, especially dogs, present extreme varieties not
surpassed even by pigeons and poultry among birds, renders it
almost certain that an equal amount of variability exists in the
wild state ; and this is confirmed by the example of a species of
squirrel (Sciurus carolinensis), of which sixteen specimens, all
males and all taken in Florida, were measured and tabulated
by Mr. Allen. The diagram here given shows, that, both the
general amount of the variation and the independent variability
of the several members of the body, accord completely with
the variations so common in the class of birds ; while their
amount and their independence of each other are even greater
than usual.

Variation in the Internal Organs of Animals.

In case it should be objected that the cases of variation
hitherto adduced are in the external parts only, and that
there is no proof that the internal organs vary in the same
manner, it will be advisable to show that such varieties also
occur. It is, however, impossible to adduce the same amount
of evidence in this class of variation, because the great labour
of dissecting large numbers of specimens of the same species
is rarely undertaken, and we have to trust to the chance
observations of anatomists recorded in their regular course of
study.

It must, however, be noted that a very large proportion of
the variations already recorded in the external parts of
animals necessarily imply corresponding internal variations.
When feet and legs vary in size, it is because the bones vary ;
when the head, body, limbs, and tail change their proportions,
the bony skeleton must also change ; and even when the wing
or tail feathers of birds become longer or more numerous,
there is sure to be a corresponding change in the bones which
support and the muscles which move them. I will, however,
give a few cases of variations which have been directly
observed.

Mr. Frank E. Beddard has kindly communicated to me
some remarkable variations he has observed in the internal

Ill

KIACRA.M OF VARIATION

67

30 32

5 10 15 20 25 30 32

Fio. 13. Sciurus carolinensis. 32 specimens. Florida.

68 DARWINISM CHAP.

organs of a species of earthworm (Perionyx excavatus). The
normal characters of this species are-
Setae forming a complete row round each segment.
Two pairs of spermathecse spherical pouches without

diverticulce in segments 8 and 9.
Two pairs of testes in segments 1 1 and 1 2.
Ovaries, a single pair in segment 13.
Oviducts open by a common pore in the middle of

segment 14.
Vasa deferentia open separately in segment 18, each

furnished at its termination with a large prostate

gland.

Between two and three hundred specimens were examined,
and among them thirteen specimens exhibited the following
marked variations :

(1) The number of the spermathecse varied from two to

three or four pairs, their position also varying.

(2) There were occasionally two pairs of ovaries, each

with its own oviduct ; the external apertures of
these varied in position, being upon segments 13
and 14, 14 and 15, or 15 and 16. Occasionally
when there was only the normal single oviduct
pore present it varied in position, once occurring on
the 10th, and once on the llth segment.

(3) The male generative pores varied in position from

segments 14 to 20. In one instance there were two
pairs instead of the normal single pair, and in this
case each of the four apertures had its own
prostate gland.

Mr. Beddard remarks that all, or nearly all, the above
variations are found normally in other genera and species.

When we consider the enormous number of earthworms
and the comparatively very small number of individuals ex-
amined, we may be sure, not only that such variations as these
occur with considerable frequency, but also that still more
extraordinary deviations from the normal structure may often
exist.

The next example is taken from Mr. Darwin's unpublished
MSS.

in VARIABILITY OF Sl'KdKS IX A STATE OF NATURE 69

"In some >pecie- of Shrew- iSore\) and in some field-mice
i Ar\ icola), the Kev. L .lenyn- ( Ann. .\"f. Ifixf., vol. vii. pp. i'G7,
'2'-) found the proportional length uf the intestinal canal to
\i\r\ considerably. He found the same variability in the

number of the caudal Vertebra'. In three s| lecimeilS of all

Ar\ icola he found the gall-bladder having a very different
decree of development, and there is reason to believe it is
M'lin-tinies ;dseiit. 1 Vote or Owen has shown that this is
the case with the -all Madder of the uiratVe."

Dr. Crisp (Proc. Zool. Soc., 1862, p. 137) found the -all
bladder present in some specimens of ( Jen us .-uperciliaris while
absent in others; and he found it to be ab-ent in three
-ir.illes which he di.-.-ected. A double -all bladder was
found in a >heej, and in a small mammal pre-erved in the
llunterian Mii-eiim there are three di-tinct -all bladder-.

The length of the alimentary canal varie- -reatly. In three
adult ijnilles de-cribt-d 1,\ rrofeor <>\\en it was from 124 to
!."; feet lonir ; one dissected in l-'ram-e had thi> canal I'll
feet loim : while Dr. I'ri.-p mea>uretl une uf the extraordinary
length of L'^4 feet, and similar variations are recorded in
other animals. 1

The number of ribs varies in many animals. Mr. St. George
.Mix-art says: "In the highest forms of the Primates, the
number of true ribs is seven, but in Hylobates there are some-
times eight pairs. In Semnopithecus and L'olobus there are
generally seven, but sometimes eight pair- of true ribs. In
the ( Vhid;e there are generally seven or eight pairs, but in
A.teles sometimes mae" (Proc. Zool. >''., ls<i.">, p. .~><;s). In
the same pa}>er it is stated that the number of dorsal vertebra?
in man is normally twelve, very rarely thirteen. In the
Chimpan/ee there are normally thirteen d<>r-al vertebra-, but
occasionally there are fourteen or only twelve.

In fin >'/'////.

Among the nine adult male Orang-utans, collected by
myself in Borneo, the skulls dillered remarkably in size and
proportions. The orbits varied in width and height, the
cranial ridge was either single or double, either much or little
developed, and the zygomatic aperture varied considerably in

Zool. Soc., 1864, i>. 6-1.

70

DARWINISM

CHAP.

Length

Width

Orbits

Length of
Lower Jaw

8

10

3 4 5 G 7 8 9

Fio. 14. Variation of Sknll of Wolf. 10 specimens

10

in VAKIAlllLITY OF Sl'KCIF.s IX A STATE <>|' X.VITKK 71

I noted particularly that the-e \ariations hore n<>
necessary relation ti each ntlicr, so that a lar-e temporal
mil-de and /\ -]ii;itic aperture mi-lit e\i-t either with a
la r i, r e r a small rraniuni ; and thu> W9 explained the curious
diHerence hetweeii the simje croted and tin- doiihle crested
>klllls, \\ liidi had IK TII suppo-ed t < i cha ract eri-e di-t inrt Species.
AJB an instance of tin- amount of \ariation in tin- .-kulls of
fully adult male uiaiiu-, I found the width het \\een tin- urliits
I'xti-i -nally to In- only 4 inches in one sjierimen and fully
~> inrhrs in another.

lv\ai-t iiira-uii'iiient- of laiue 861168 of i-omparalth' >klllls of

the mammalia are not easily found, l>ut from those a\ailaMe

I liave prepared three diagram- ( l''i--. I I. !">. and 1 li ), in order
to f.\hil)ir the facts of \aiiatioii in thi> \er\ important or-an.
The tir-t sho\\s the variation in ten >peeimen- of the c iinmon
wolf (I'anis lu])ii.-) from one district in North Amei'ica, and
we see that it is not only lai'^e in amount, lnit that each
part e\hiltit> a con>ideralile independent \arialiility. 1

In I>ia-ram 1 > \\ e ha\c the \ariationsof ci-ht skulls of
the Indian lloiie\ hear ( I 'r>u~ lahiatu-i, as tahulatcd hy the
late Dr. .1. I''., (iiay of the Ilritish .Mu>eum. l-'or such a
small numher of specimens the amount of \ariation is \ery
lar-e from otieei-hth to one-fifth of the mean si/e, while
there are an extraordinary numher of instances of inde
pendent variability. In Hia^raiu hi we have the len-th and
width of twelve skulls of adult males of the Indian wild hoar
(Sus cristatus), also ui\en hy |)r. (Iray, cxhihitiiiL; in hot h sets
of measurements ;) variation of more than one -i\th. comhincd
with a very con>idcrahle amount of independent variahility.-

Thc few facts now ^iven, as to variations, of the internal
parts of animals, miidit he multiplied indefinitely hy a search
throu-h the voluminous writings of comparative anatomists.

Hut the evidence ali'eady adduced, taken in conjunction with
the much fuller evidence of variation in all external or-an-,
le.nl- n> to the conclusion that wherever variations are looked
for amonu a coiisiderahle numher of individuals of the inure

-

1 .1. A. Alli'ii, mi ( M'Mu r ia]ihi<-;il Variation ;UII<>H;_; North American Mammals,
H,,ll. I'. S. '"!. and <;.<!. Survey, \<>\. ii. p. 314 (1876).
- Proc. Z"nl. Soc. Land., IStil, p. TOO. and 1568, p. 28.

72

DARAVINISM

CHAP.

4

8

Length...

Mean 11^ in.

Width

Mean

n.

Palate^

(length)
Mean 6y 2 in.

Orbits.

(Width)
Mean 2

n.

Nose

(Width)
Mean 2% in.

7 2 3 4^ 5 6

(From Table by Dr. J.E. Gray. P.Z.S. 1864. p. 700.)

FIG. 15. Variation of 8 skulls (Ursus labial us).

8

Ill

PIAfiRAM OF VARIATION

73

74 DARWINISM CHAP.

common species they are sure to be found ; that they are
everywhere of considerable amount, often reaching 20 per
cent of the size of the part implicated ; and that they are to
a great extent independent of each other, and thus afford
almost any combination of variations that may be needed.

It must be particularly noticed that the whole series of
variation-diagrams here given (except the three which illustrate
the number of varying individuals) in every case represent the
actual amount of the variation, not on any reduced or enlarged
scale, but as it were life-size. Whatever number of inches or
decimals of an inch the species varies in any of its parts is
marked on the diagrams, so that with the help of an ordinary
divided rule or a pair of compasses the variation of the
different parts can be ascertained and compared just as if the
specimens themselves were before the reader, but with much
greater ease.

In my lectures on the Darwinian theory in America and
in this country I used diagrams constructed on a different
plan, equally illustrating the large amount of independent
variability, but less simple and less intelligible. The present
method is a modification of that used by Mr. Francis Galton
in his researches on the theory of variability, the upper line
(showing the variability of the body) in Diagrams 4, 5, 6, and
13, being laid down on the method he has used in his experi-
ments with sweet-peas and in pedigree moth-breeding. 1 I be-
lieve, after much consideration, and many tedious experiments
in diagram-making, that no better method can be adopted for
bringing before the eye, both the amount and the peculiar
features of individual variability.

Variations of the Habits of Animals.

Closely connected with those variations of internal and
external structure which have been already described, are the
changes of habits which often occur in certain individuals or
in whole species, since these must necessarily depend upon some
corresponding change in the brain or in other parts of the
organism ; and as these changes are of great importance in
relation to the theory of instinct, a few examples of them will
be now adduced.

1 See Trans. Entomological Society f London, 1887, p. 24.

in VAKIA11ILITV OF Sl'K.rlKS IN A STATK OF NA'ITIIK 75

The Ke;i (Xestor notabilis) is a curious parrot inhabiting the
mountain ran-cs of the Middle Island of New Zealand. It

belongs tn the family of I'.rush hm-ned parrots, and naturally
feeds en tin- honey i>f flowers and the inserts \\lndi frequent
thfin, together with such fruits or Lorries ;is are found in tho

; on. Till quite recently this comprised its whole diet, Imt

si nee the country it in ha Kits has becoi ecu pied liy Kuropeans

it has developed a taste for a carnivorous diet, with alarming
results. It began by picking the -lieepskins huni: out to dry
or the meat in process of being cured. Alioiit ISOS it Mas
first observed to attack living sheep, which had freijiiently
lieen found with raw and bleeding wounds on their hacks.
Since then it is stated that the hird actually Inn-rows into the
living sheep, eating its way down to the kidne\ -, which form
its special delicacy. As a natural consequence, the liird is
being destroyed as rapidly as po-sjMe. and one of the rare
and curious meniliers of the New Zealand fauna will no
doubt shortly cease to e\; The case all'ords a remark

able instance of how the climbing feet and powerful hooked
beak dexelc.ped for one set of purposes call be applied to
another altogether ditl'erent purpose, and it also shows how
little real stability there may be in what appear to us the
most fixed habits of life. A somewhat similar change of diet
has been recorded by the Duke of Argyll, in which a goose,
reared by a -olden ea-le, was tan-lit by its foster-parent to
eat flesh, which it continued to do regularly and apparently
with -rent relish. 1

Change of habits appears to be often a result of imitation,
of which Mi-. TegetmeiiT gives some good examples. He states
that if pigeons are reared exclusively with small grain, as
wheat or barley, they will stane before eating beans. But
when they are tints starving, if a bean eating pigeon is put
among them, they follow its example, and thereafter adopt
the habit, So fowls sometimes refuse to eat mai/.e, but on
seeing others eat it, they do the same and become excessively
fond of it. Many persons have found that their yellow
crocuses were eaten by sparrows, while the blue, purple, and
white coloured varieties were left untouched ; but Mr. Teget-
nieier, who grows only these latter colours, found that after

1 .\'n/i'i-i', vol. xix. [>. "p."i4.

76 DARWINISM CHAP.

two years the sparrows began to attack them, and thereafter
destroyed them quite as readily as the yellow ones ; and he
believes it was merely because some bolder sparrow than the
rest set the example. On this subject Mr. Charles C. Abbott
well remarks : " In studying the habits of our American birds
;md I suppose it is true of birds everywhere it must at all
times be remembered that there is less stability in the habits
of birds than is usually supposed ; and no account of the habits
of any one species will exactly detail the various features of
its habits as they really are, in every portion of the terri-
tory it inhabits." l

Mr. Charles Dixon has recorded a remarkable change in the
mode of nest-building of some common chaffinches which were
taken to New Zealand and turned out there. He says : " The
cup of the nest is small, loosely put together, apparently lined
with feathers, and the walls of the structure are prolonged for
about 18 inches, and hang loosely down the side of the
supporting branch. The whole structure bears some re-
semblance to the nests of the hangnests (Icteridse), with the
exception that the cavity is at the top. Clearly these New
Zealand chaffinches were at a loss for a design Avhen fabricat-
ing their nest. They had no standard to work by, no nests of
their own kind to copy, no older birds to give them any instruc-
tion, and the result is the abnormal structure I have just
described." 2

These few examples are sufficient to show that both the
habits and instincts of animals are subject to variation ; and
had we a sufficient number of detailed observations we should
probably find that these variations were as numerous, as
diverse in character, as large in amount, and as independent
of each other as those which we have seen to characterise
their bodily structure.

The Variability of Plants.

The variability of plants is notorious, being proved not only
by the endless variations which occur whenever a species is
largely grown by horticulturists, but also by the great difficulty
that is felt by botanists in determining the limits of species in

1 Nature, vol. xvi. p. 163 ; and vol. xi. p. 227.
2 Ibid., vol. xxxi. (1885), p. 533.

in VARIABILITY OF SPECIES IN A STATK <>F NATURE 77

many ! _ enera. A- examples we may take tin- roses, the
brambles, ami the willow- as well illn.-trat in- this fact. In Mr.
Kaker's /,'> ris/nii uf t!i, /!i-lfi.<!/ Roses (published by the Linnean
Society in lSli:>), he includes under tin- single specie-. Kosa

callilia - the common do u n.-e no le-s tliall t WelltV -eight

named varieties distinguished by more or less c. m-tant character-
and often routined to special localities, and to these are
referred about seventy of the spedes of I'.ritish and continental
liiitaiiists. Of the -enus Kuliiis or bramble, fir, Urit i-li -| iecie~
are -i\en in I'.eiitliam'- //./////-",./. f tin Hritidi I'lum, while,
in the fifth edition of Babington's M<nniiil "f /.V/7/>7/ 1'xiliniii,
published about the same time, no !-> than fnfiii j!r. species
are de-cril'ed. ( >f \villo\\-, (Salix) the same two works
enumerate ///'/"// and fliirtf/-<>n> species respectively. The
hawkweeds ( I lieracium ) are equally pux/.lin-, for while Mr.
lleiitham admits nnly >e\en Uritish .-pecie-, I'rofesvnr Haliin-
ton describes no less than thirty t\\o. l.esidi-s several named
varieties.

A French botanist, Mm is. A- Jordan, has collected numerous
forms of a common little plant, the spring Avhitlow -rass
(DraKa verna) ; he has cultivated these for >e\cral successive
years, and declares that they preserve their peculiarities un-
changed; he al-o save that they each come true from seed,
and thus possess all the characteristics of true species. He
has described no less than titty two such species or permanent
varieties, all found in the south of France ; and lie urv.es
botanists to follow his example in collect in--, describing, and
cultivating all such varieties as may occur in their respective
di-tricN. Now, as the plant is very common almost all over
Furope and ranges from North America to the Himalayas,
the number of similar forms over this wide area would prob-
ably have to be reckoned by hundreds if not by thousands.

The class of facts now adduced mu>t certainly be held
to prove that in many lar-e genera and in some single species
there is a very large amount of variation, which renders it
impossible for experts to agree upon the limits of species.

We will now adduce a few striking cases of individual
variation.

The distinguished botanist, Alp. de Candolle, made a special
study of the oaks of the whole world, and lias Mated some

DARWINISM CHAP.

remarkable facts as to their variability. He declares that on
the same branch of oak he has noted the following variations :
(1) In the length of the petiole, as one to three ; (2) in the form
of the leaf, being either elliptical or obovoid ; (3) in the margin
being entire, or notched, or even pinnatifid ; (4) in the ex-
tremity being acute or bhmt ; (5) in the base being sharp,
blnnt, or cordate ; (6) in the surface being pubescent or
smooth ; (7) the perianth varies in depth and lobing ; (8)
the stamens vary in number, independently ; (9) the anthers
are mucronate or blunt; (10) the fruit stalks vary greatly
in length, often as one to three; (11) the number of fruits
varies; (12) the form of the base of the cup varies ; (13) the
scales of the cup vary in form; (14) the proportions of the
acorns vary ; (15) the times of the acorns ripening and falling
vary.

Besides this, many species exhibit well-marked varieties
which have been described and named, and these are most
numerous in the best-known species. Our British oak (Quercus
robur) has twenty- eight varieties; Quercus Lusitanica has
eleven ; Quercus calliprinos has ten ; and Quercus coccifera
eight.

A most remarkable case of variation in the parts of a
common flower has been given by Dr. Hermann Miiller. He
examined two hundred flowers of Myosurus minimus, among
which he found thirty-five different proportions of the sepals,
petals, and anthers, the first varying from four to seven, the
second from two to five, and the third from two to ten. Five
sepals occurred in one hundred and eighty-nine out of the two
hundred, but of these one hundred and five had three petals,
forty-six had four petals, and twenty-six had five petals ; but
in each of these sets the anthers varied in number from three
to eight, or from two to nine. AVe have here an example of
the same amount of " independent variability " that, as we
have seen, occurs in the various dimensions of birds and
mammals ; and it may be taken as an illustration of the kind
and degree of variability that may be expected to occur
among small and little specialised flowers. 1

In the common wind-flower (Anemone nemorosa) an almost
equal amount of variation occurs ; and I have myself gathered

1 Nature, vol. xxvi. p. 81.

in VAKIAI'.ILITY OF Sl'lJ IKS IN A STATE OF NATTHK 79

in Din.- locality tlower- varying from ; inch to 1 : , : incli in
diameter ; the In-art-; varying from 1.1 inch to I inches across;
and tin- petaloid sepals cither broad or narrow, and \ar\in-'
in nunilicr from five t<> ten. Though generally pure white
on their upper surface, gome -pecimen- are a full pink, while
others have a decided liluish tini:' 1 .

Mr. 1 >ar\vin states that he carefully examined a laiir number
of plants nf ( Jeranium pha-uin and ( 1. pyrenaiciim (not perhaps
truly I'.ritish but frequently found wild), which had escaped
from cultivation, and had spread by seed in an open planta
tion ; and he declares that "the seedling \aried in alino-r
e\eiy single chai-acter, both in their Howers and foliage, to a
decree which I have never >een exceeded ; yet they eniild not
have been expired tu any great change df their ciinditiuns." 1

The following example- df \ariation in important part- of
plants woe collected by Mr. Darwin and have been copied
from his unpublished MSS. : -

"I>e Candolle (Mem. Soc. /'////>-. <1< Genbve, tom. ii. jiai-t ii.

}). 1'17) state- that l'ap.i\ei bracteatUID and 1'. orientate present
indifferently tWO -pal- and four petals, or three sepals and
six petals, which is sufficiently rare witli other species of the
genus."

"In the Primulacea- and in the pvat das* to which this
family belongs the unilociilar oxarinm is free, but M. I Mibury
(Mini. Soc. /'////.<. //< Geneve, torn. ii. p. -\0(\) has often found
individuals in Cyclamen hedersefolium, in which the base of
the ovary was connected for a third part of its length with
the inferior part of the calyx."

" M. Auu. St. Ililaire (Sur la (lynobase, Man. di'S Mn.<.
</'/[i-;f. \"f., torn. x. p. 134), speaking of some bushes of the
(iomphia olea-folia, which he at first thought formed a quite
distinct species, says: 'Yoila done dans un meme individu
des loges et un style qui se rattadient tantot a un axe vertical,
et tantot a un gynoba^e ; done celui-ci n'est qu'un axe veri-
table ; mais cet axe cst deprime au lieu d'etre vertical." He
adds (p. 151), 'Does not all this indicate that nature has
tried, in a manner, in the family of Rutaceae to produce from
a single multilocular ovary, one-styled and symmetrical,
several unilociilar ovaries, each with its own style.' And he
1 A ninw.h anJ 1'lmtts under Domestication, vol. ii. p. 258.

80 DARWINISM CHAP.

subsequently shows that, in Xanthoxylum monogynum, ' it
often happens that on the same plant, on the same panicle,
we find flowers with one or with two ovaries ;' and that this is
an important character is shown by the Rutaceae (to which
Xanthoxylum belongs), being placed in a group of natural
orders characterised by having a solitary ovary."

" De Candolle has divided the Cruciferse into five sub-orders
in accordance with the position of the radicle and cotyledons,
yet Mons. T. Gray (Ann. d?s Scien. Nut., ser. i. torn. vii. p. 389)
found in sixteen seeds of Petrocallis Pyrenaica the form of the
embryo so uncertain that he could not tell whether it ought
to be placed in the sub-orders ' Pleurorhizee' or 'Notorhizee' ;
so again (p. 400) in Cochlearia saxatilis M. Gay examined
twenty-nine embryos, and of these sixteen were vigorously
' pleurorhizees,' nine had characters intermediate between
pleuro- and notor- hize'es, and four were pure notorhizees."

"M. Raspail asserts (Ann. des Scien. Nat., ser. i. torn. v. p.
440) that a grass (Nostus Borbonicus) is so eminently variable
in its floral organisation, that the varieties might serve to
make a family with sufficiently numerous genera and tribes
a remark which shows that important organs must be here

variable."

Species which vary little.

The preceding statements, as to the great amount of
variation occurring in animals and plants, do not prove
that all species vary to the same extent, or even vary at
all, but, merely, that a considerable number of species in
every class, order, and family do so vary. It will have
been observed that the examples of great variability have
all been taken from common species, or species which have
a wide range and are abundant in individuals. Now Mr.
Darwin concludes, from an elaborate examination of the floras
and faunas of several distinct regions, that common, wide
ranging species, as a rule, vary most, while those that are
confined to special districts and are therefore comparatively
limited in number of individuals vary least. By a similar
comparison it is shown that species of large genera vary more
than species of small genera. These facts explain, to some
extent, why the opinion has been so prevalent that variation
is very limited in amount and exceptional in character. For

in VARIABILITY OF SPECIES IX A STATE OF XATVKK 81

naturalists of the old school, and all mere collectors, were
interested in species in proportion to their rarity, and would
often have in their collections a larger number of specimen.-
of a raiv species than of a species that was very common.
Now as these rare species d<> really vary much less than the
common specie-, and in many cases hardly vary at all, it was
very natural that a belief in the fixity of species should
prevail. It is not, however, as we shall see presently, the
rare, but the common and widespread specie- which become
the parents of new forms, and thus the mm variability of any
number of rare or local species offers no ditiiculty whatever in
the way of the theory of evolution.

Concluding

"We have in>\v shown in some detail, at the risk of being
tedious, that individual variability is a general character of all
common and wide.-pread species of animals or plants; and,
further, that this variability extends, so far as we know, to
every part and organ, whether external or internal, as well as
to every mental faculty. Vet more important is the fact that
each part or organ varies to a considerable extent inde-
pendently of other j tarts. Again, we have shown, by abundant
e\ idence, that the variation that occurs is very large in
amount usually reaching 10 or 20, and sometimes even iTi
per cent of the average size of the varying part; while
not one or two only, but from 5 to 10 per cent of the speci-
mens examined exhibit nearly as large an amount of variation.
These facts have been brought clearly before the reader by
means of numerous diagrams, drawn to scale and exhibiting
the actual variations in inches, so that there can be no pos-
sibility of denying either their generality or their amount.
The importance of this full exposition of the subject will be
seen in future chapters, when we shall frequently have to
refer to the facts here set forth, especially when we deal with
the various theories of recent writers and the criticisms that
have been made of the Darwinian theory.

A full exposition of the facts of variation among wild
animals and plants is the more necessary, because compara-
tively few of them were published in Mr. Darwin's works.
while the more important have only been made known since

a

82 DARWINISM CHAP, in

the last edition of The Oritjin of Species was prepared ; and it
is clear that Mr. Darwin himself did not fully recognise the
enormous amount of variability that actually exists. This
i.s indicated by his frequent reference to the extreme slowness
of the changes for which variation furnishes the materials,
and also by his use of such expressions as the following : "A
variety when once formed must again, perhaps after a long
interval of time, vary or present individual differences of the
same favourable nature as before" (Oritjin, p. G6). And
again, after speaking of changed conditions " affording a better
chance of the occurrence of favourable variations," he adds :
" Unless such occur natural selection can do nothing" (Origin,
p. G4). These expressions are hardly consistent with the
fact of the constant and large amount of variation, of every
part, in all directions, which evidently occurs in each genera-
tion of all the more abundant species, and which must afford
an ample supply of favourable variations whenever required ;
and they have been seized upon and exaggerated by some
writers as proofs of the extreme difficulties in the way of the
theory. It is to show that such difficulties do not exist, and
in the full conviction that an adequate knowledge of the
facts of variation affords the only sure foundation for the
Darwinian theory of the origin of species, that this chapter
has been written.

CHAPTER IV

VARIATION (>K iHiMKSTH'ATKH LNIMALS \\l>
CULTIVATED PLANTS

Tin- t.n-t.s of variation ami artificial selection Proofs of tlir ^i-ncrality of
variation Variations of aj>pl<-s and im-lons - Vai iaiimis of llo\\n>
Variations of domestic animals IioimMj,- pilous Ai-climatK-ition
('iiviinistaiii'i'x favourable to srl.-rtion l>y man Conditions favoiir-
al'lc to variation Concluding remarks.

HAVFMJ so fully discussed variation under nature it will be
unneces>ary to devote so much space to domesticated animals
and cultivated plants, especially as Mr. I>ar\\in has published
t\vo remarkable volumes on the subject where those who
desire it may obtain ample information. A general sketch of
the more important facts will, however, be given, for the
purpose of showing ho\v closely they correspond with those
described in the preceding chapter, and also to point out the
general principles which they illustrate. It will also be
necessary to explain how these variations have been im-rea-ed
and accumulated by artificial selection, since we are thereby
better enabled to understand the action of natural selection, to
be discussed in the succeeding chapter.

The fdds of Variation and Artificial Selection.

Every one knows that in each litter of kittens or of
puppies no two are alike. Even in the case in which several
are exactly alike in colours, other differences are always
perceptible to those who observe them closely. They will
differ in size, in the proportions of their bodies and limbs, in
the length or texture of their hairy covering, and notably
in their disposition. They each possess, too, an individual

84 DARWINISM CHAP.

countenance, almost as varied when closely studied as that of
a human being ; not only can a shepherd distinguish every
sheep in his flock, but we all know that each kitten in the
successive families of our old favourite cat has a face of its
own, with an expression and individuality distinct from all
its brothers and sisters. Now this individual variability
exists among all creatures whatever, which we can closely
observe, even when the two parents are very much alike and
have been matched in order to preserve some special breed.
The same thing occurs in the vegetable kingdom. All plants
raised from seed differ more or less from each other. In
every bed of flowers or of vegetables we shall find, if we look
closely, that there are countless small differences, in the size,
in the mode of growth, in the shape or colour of the leaves,
in the form, colour, or markings of the flowers, or in the size,
form, colour, or flavour of the fruit. These differences are
usually small, but are yet easily seen, and in their extremes
are very considerable ; and they have this important quality,
that they have a tendency to be reproduced, and thus by
careful breeding any particular variation or group of varia-
tions can be increased to an enormous extent apparently to
any extent not incompatible with the life, growth, and re-
production of the plant or animal.

The way this is done is by artificial selection, and it is
very important to understand this process and its results.
Suppose we have a plant with a small edible seed, and we
want to increase the size of that seed. We grow as large a

O O

quantity of it as possible, and when the crop is ripe we
carefully choose a few of the very largest seeds, or we may
by means of a sieve sort out a quantity of the lai-gest seeds.
Next year we sow only these large seeds, taking care to give
them suitable soil and manure, and the result is found to be
that the average size of the seeds is larger than in the first
crop, and that the largest seeds are now somewhat larger and
more numerous. Again sowing these, we obtain a further
slight increase of size, and in a very few years we obtain a
greatly improved race, which will always produce larger seeds
than the unimproved race, even if cultivated without any
special care. In this way all our fine sorts of vegetables,
fruits, and flowers have been obtained, all our choice breeds

iv VARIATION UNDER DOMESTICATION 85

of cattle or of poultry, our wonderful race-horses, and our
endless varieties of dogs. It is a very common but mistaken
idea that this improvement is due to crossing and feeding in
the case of animals, and to improved cultivation in the case
of plants. Crossing is occasionally used in order to obtain a
combination of qualities found in two distinct breeds, and
also because it is found to increase the constitutional vigour ;
but every breed possessing any exceptional quality is the
result of the selection of variations occurring year after year
and accumulated in the manner just described. Purity of
breed, with repeated selection of the best varieties of that
breed, is the foundation of all improvement in our domestic
animals and cultivated plants.

<>f tin 1 (iiinrnlify of J'nri<il//i.

Another very common error is, that variation is the
exception, and rather a rare exception, and that it occurs
only in one direction at a time that is, that only one or t\v>
of the numerous possible modes of variation oeeiir at the same
time. The experience of breeders and cultivators, however,
proves that variation is the rule instead of the exception, and
that it occurs, more or less, in almost every direction. This is
shown by the fact that different species of plants and animals
have required different kinds of modification to adapt them to
our use, and we have never failed to meet with variation /'//
Hint jiii/lii'/il/n' ilii'i'i-fion, so as to enable us to accumulate it and
so to produce ultimately a large amount of change in the
required direction. Our gardens furnish us with numberless
examples of this property of plants. In the cabbage and
lettuce we have found variation in the size and mode of
growth of the leaf, enabling us to produce by selection the
almost innumerable varieties, some with solid heads of foliage
quite unlike any plant in a state of nature, others with
curiously wrinkled leaves like the savoy, others of a deep
purple colour used for pickling. From the very same species
as the cabbage (Brassica oleracea) have arisen the broccoli
and cauliflower, in which the leaves have undergone little
alteration, while the branching heads of flowers grow into a
compact mass forming one of our most delicate vegetables.
The brussels sprouts are another form of the same plant, in

86 DARAVINISM CHAP.

which the whole mode of growth has been altered, numerous
little heads of leaves being produced on the stem. In other
varieties the ribs of the leaves are thickened so as to become
themselves a culinary vegetable ; while, in the Kohlrabi, the
stem grows into a turnip-like mass just above ground. Now
all these extraordinarily distinct plants come from one original
species which still grows wild on our coasts ; and it must have
varied in all these directions, otherwise variations could not
have been accumulated to the extent we now see them. The
flowers and seeds of all these plants have remained nearly
stationary, because no attempt has been made to accumulate
the slight variations that no doubt occur in them.

If now we turn to another set of plants, the turnips,
radishes, carrots, and potatoes, we find that the roots or under-
ground tubers have been wonderfully enlarged and improved,
and also altered in shape and colour, while the stems, leaves,
flowers, and fruits have remained almost unchanged. In the
various kinds of peas and beans it is the pod or fruit and the
seed that has been subjected to selection, and therefore greatly
modified ; and it is here very important to notice that while
all these plants have undergone cultivation in a great variety
of soils and climates, with different manures and under
different systems, yet the flowers have remained but little
altered, those of the broad bean, the scarlet-runner, and the
garden-pea, being nearly the same in all the varieties. This
shows us how little change is produced by mere cultivation,
or even by variety of soil and climate, if there is no select inn.
to preserve and accumulate the small variations that are con-
tinually occurring. When, however, a great amount of modifi-
cation has been effected in one country, change to another
country produces a decided effect. Thus it has been found
that some of the numerous varieties of maize produced and
cultivated in the United States change considerably, not only
in their size and colour, but even in the shape of the seed when
grown for a few successive years in Germany. 1 In all our
cultivated fruit trees the fruits vary immensely in shape, size,
colour, flavour, time of ripening, and other qualities, Avhile the
leaves and flowers usually differ so little that they are hardly
distinguishable except to a very close observer.

1 Darwin, Animals and Plants under Domestication, vol. i. p. 322.

,v IATION UNDER DOMESTICATION 87

1't riii finitx af Api'l * mill <>/ Mi'l'

The most remarkable varieties arc afforded by the apple
and the melon, and some account of these will be given as
illustrating the eH'ects of slight variations accumulated by
selection. All our apples are known to have descended from
the common crab of our hedges (1'yrus mains), and from this
at least a thousand distinct varieties have been produced.
These dill'er greatly in the sixe and form of the fruit, in its
colour, and in the texture of the skin. They further ditl'er in
the time of ripening, in their flavour, and in their keeping
properties; but apple trees also difi'er in many other ways.
The foliage of the different varieties can often be distinguished
by peculiarities of form and colour, and it varies considerably
in the time of its appearance; in some hardly a leaf appears
till the tree is in full bloom, while others produce their leaves
so early as almost to hide the flowers. The flowers ditl'er in
size and colour, and in one, case in structure also, that of the
St. Yalery apple having a double calyx with ten divisions, and
fourteen styles with oblique stigmas, but without stamens or
corolla. The flowers, therefore. ha\e to l.r fertilised with the
pollen from other varieties in order to produce fruit. The
pips or seeds differ also in shape, sixe, and colour; some
varieties are liable to canker more than others, while the
Winter Majetin and one or two others have the strange con-
stitutional peculiarity of never being attacked by the mealy
bug even when all the other trees in the same orchard are in-
fested with it.

All the cucumbers and gourds vary immensely, but the
melon (Cucumis melo) exceeds them all. A French botanist,
M. Naudin, devoted >i\ \ears to their study. He found that
previous botanists had described thirty distinct species, as they
thought, which were really only varieties of melons. They
differ chiefly in their fruits, but also very much in foliage and
mode of growth. Some melons are only as large as small
plums, others weigh as much as sixty-six pounds. One variety
has a scarlet fruit. Another is not more than an inch in
diameter, but sometimes more than a yard in length, twisting
about in all directions like a serpent. Some melons are
exactly like cucumbers ; and an Algerian variety, when ripe,

88 DARWINISM CHAP.

cracks and falls to pieces, just as occurs in a wild gourd
(C. momordica). 1

Variations of Flowers.

Turning to flowers, we find that in the same genus as our
currant and gooseberry, which Ave have cultivated for their
fruits, there are some ornamental species, as the Ribes sanguinea,
and in these the flowers have been selected so as to produce deep
red, pink, or white varieties. When any particular flower be-
comes fashionable and is grown in large quantities, variations
are always met with sufficient to produce great varieties of tint
or marking, as shown by our roses, auriculas, and geraniums.
When varied leaves are required, it is found that a number of
plants vary sufficiently in this direction also, and we have
zonal geraniums, variegated ivies, gold and silver marked
hollies, and many others.

Variations of Domestic Animal*.

Coming now to our domesticated animals, we find still more
extraordinary cases ; and it appears as if any special quality or
modification in an animal can be obtained if we only breed it
in sufficient quantity, watch carefully for the required varia-
tions, and carry on selection with patience and skill for a
sufficiently long period. Thus, in sheep we have enormously
increased the wool, and have obtained the power of rapidly
forming flesh and fat ; in cows we have increased the produc-
tion of milk ; in horses we have obtained strength, endurance,
or speed, and have greatly modified size, form, and colour ; in
poultry we have secured various colours of plumage, increase
of size, and almost perpetual egg-laying. But it is in dogs and
pigeons that the most marvellous changes have been effected,
and these require our special attention.

Our various domestic dogs are believed to have originated
from several distinct wild species, because in every part of
the world the native dogs resemble some wild dogs or wolves
of the same country. Thus perhaps several species of wolves
and jackals were domesticated in very early times, and from
breeds derived from these, crossed and improved by selection,

1 These facts are taken from Darwin's Domesticated Animals and Cultivated
Plants, vol. i. pp. 359, 360, 392-401 ; vol. ii. pp. 231, 275, 330.

iv V AIM ATI ON rXDHK DOMESTICATION 89

our existing dogs have descended. But this intermixture of
distinct species will go a very little way in accounting for the
peculiarities of the different breeds of dogs, many of which are
totally unlike any wild animal. Such is the case with grey-
hounds, bloodhounds, bulldogs, Blenheim spaniels, terriers,
pugs, turnspits, pointers, and many others; and these differ
so greatly in size, shape, colour, and habits, as well as in the
form and proportions of all the dilferent parts of the body,
that it seems impossible that they could have descended from
any of the known wild dogs, wolves, or allied animals, none
of which dift'er nearly so much in size, form, and proportions.
We li:i\c liei-e :i remarkable proof that variation is not con-
fined to superficial characters to the colour, hair, or external
appendage*, when \ve see how the entire skeletons of such
forms as the greyhound and the bulldog have been gradually
changed in opposite directions till they are both completely
unlike that of any known wild animal, recent or extinct.
These changes have been the result of some thousands of years
of domestication and selection, different breeds being used and
preserved for different purposes ; but some of the best breeds
are known to have been improved and perfected in modern
times. About the middle of the last century a new and im-
proved kind of foxhound was produced ; the greyhound was
also greatly improved at the end of the last century, while the
true bulldog was brought to perfection about the same period.
The Xewfotindland dog has been so much changed since it was
first imported that it is now quite unlike any existing native
dog in that island. 1

Domestic Pigeon*.

The most remarkable and instructive example of variation
produced by human selection is afforded by the various races
and breeds of domestic pigeons, not only because the varia-
tions produced are often most extraordinary in amount and
diverse in character, but because in this case there is no
doubt whatever that all have been derived from one wild
species, the common rock-pigeon (Columba livia). As this is a
very important point it is well to state the evidence on which
the belief is founded. The wild rock-pigeon is of a slaty-blue

1 See Darwin's Animals itmJ Plants under Domestication, vol. i. pp. 40-42.

90 DARWINISM CHAP.

colour, the tail has a dark band across the end, the wings
have two black bands, and the outer tail-feathers are edged
with white at the base. No other wild pigeon in the world
has this combination of characters. Now in every one of the
domestic varieties, even the most extreme, all the above
marks, even to the white edging of the outer tail-feathers,
are sometimes found perfectly developed. When birds
belonging to two distinct breeds are crossed one or more
times, neither of the parents being blue, or having any of the
above-named marks, the mongrel offspring are very apt to
acquire some of these characters. Mr. Darwin gives instances
which he observed himself. He crossed some white fantails
with some black barbs, and the mongrels were black, brown,
or mottled. He also crossed a barb with a spot, which is a
white bird with a red tail and red spot on the forehead, and
the mongrel offspring were dusky and mottled. On now
crossing these two sets of mongrels with each other, he
obtained a bird of a beautiful blue colour, with the barred
and white edged tail, and double-banded wings, so as almost
exactly to resemble a wild rock-pigeon. This bird was
descended in the second generation from a pure white and
pure black bird, both of which when unmixed breed their
kind remarkably true. These facts, well known to ex-
perienced pigeon -fanciers, together with the habits of the
birds, which all like to nest in holes, or dovecots, not in trees
like the great majority of wild pigeons, have led to the general
belief in the single origin of all the different kinds.

In order to afford some idea of the great differences which
exist among domesticated pigeons, it will be well to give a
brief abstract of Mr. Darwin's account of them. He divides
them into eleven distinct races, most of which have several
sub-races.

RACE I. Pouters. -These are especially distinguished by
the enormously enlarged crop, which can be so inflated in
some birds as almost to conceal the beak. They are very long
in the body and legs and stand almost upright, so as to
present a very distinct appearance. Their skeleton has
become modified, the ribs being broader and the vertebrae
more numerous than in other pigeons.

iv VARIATION UNDER DOMESTICATION 91

IT. Curriers. --These are large, long-necked birds,
with a long pointed beak, and the eyes surrounded with a
naked carunculated skin or wattle, which is also largely
developed at the base of the beak. The opening of the
mouth is unusually \\ide. There are several sub-races, one
being called Dragons.

BACK III. Runts. These are very large-bodied, long-beaked
pigeons, with naked skin round the eyes. The wings are
usually very long, the legs long, and the feet large, and tin-
skin of the neck is often red. There are several sub races,
and these ditt'er very nnieh, forming a series of links between
the wild rock-pigeon and the carrier.

RAf'K IV. Hnrfi*. --These are remarkable for their very
short and thick beak, so unlike that of most pigeons that
fanciers compare it with that of a bullfinch. They ha\e aUo
a naked carunculated skin round the eyes, and the skin over
the nostrils swollen.

RACK V. F<n>fnil.<. Short-bodied and rather small-beaked
pigeons, with an enormously developed tail, consisting usually
of from fourteen to forty feathers instead of twelve, the
regular number in all other pigeons, wild and tame. The
tail spreads out like a fan and is usually carried erect, and
the bird bends Lack its slender neck, so that in highly-bred
varieties the head touches the tail. The feet are small, and
they walk stiffly.

RACE VI. Tin-lit* mtil OH-!*. --These are characterised by
the feathers of the middle of neck and breast in front
spreading out irregularly so as to form a frill. The Tiirbits
a l-o have a crest on the head, and both have the beak
exceedingly short.

RACE VII. Tiniillrr*. These have a small body and short
beak, but they are specially distinguished by the singular
habit of tumbling over backwards during flight. One of the
sub -races, the Indian Lotan or Ground tumbler, if slightly
shaken and placed on the ground, will immediately begin
tumbling head over heels until taken up and soothed. If not
taken up, some of them Anil go on tumbling till they die.

92 DARWINISM CHAP.

Some English tumblers are almost equally persistent. A
writer, quoted by Mr. Darwin, says that these birds generally
begin to tumble almost as soon as they can fly ; " at three
months old they tumble well, but still fly strong ; at five or
six months they tumble excessively ; and in the second year
they mostly give up flying, on account of their tumbling so
much and so close to the ground. Some fly round with the
flock, throwing a clean summersault every few yards till they
are obliged to settle from giddiness and exhaustion. These
are called Air- tumblers, and they commonly throw from
twenty to thirty summersaults in a minute, each clear and
clean. I have one red cock that I have on two or three
occasions timed by my watch, and counted forty summer-
saults in the minute. At first they throw a single summer-
sault, then it is double, till it becomes a continuous roll,
which puts an end to flying, for if they fly a few yards over
they go, and roll till they reach the ground. Thus I had one
kill herself, and another broke his leg. Many of them turn
over only a few inches from the ground, and will tumble two
or three times in flying across their loft. These are called
House -tumblers from tumbling in the house. The act of
tumbling seems to be one over which they have no control,
an involuntary movement Avhich they seem to try to prevent.
I have seen a bird sometimes in his struggles fly a yard or
two straight upwards, the impulse forcing him backwards
Avhile he struggles to go forwards." l

The Short-faced tumblers are an improved sub-race which
have almost lost the power of tumbling, but are valued for
possessing some other characteristics in an extreme degree.
They are very small, have almost globular heads, and a very
minute beak, so that fanciers say the head of a perfect bird
should resemble a cherry with a barleycorn stuck in it. Some
of these weigh less than seven ounces, whereas the wild rock-
pigeon weighs about fourteen ounces. The feet, too, are
very short and small, and the middle toe has twelve or
thirteen instead of fourteen or fifteen scutellse. They have
often only nine primary wing-feathers instead of ten as in all
other pigeons.

1 Mr. Brent in Journal of Horticulture, 1861, p. 76 ; quoted by Darwin,
Animals and Plants under Domestication, vol. i. p. 151.

iv VARIATION UNDER DOMESTICATION 93

RACE VIII. //></ 'in n Frill-lack. In these birds the beak is
very short, and the feathers of the whole body are reversed
or turn backwards.

RACK IX. JitcoliJn. These curious birds have a hood of
feathers almost enclosing the head and meeting in front <>f

O O

the neck. The wings and tail are unusually Ion

Or

O'

RACE X. Tnti/i//n: Distinguished by a tuft of feathers
curling forwards over the beak, and the feet very much
feathered. They obtain their name from the peculiar voice
unlike that of any other pigeon. The coo is rapidly repeated,
and is continued for several minutes. The feet are covered
with feathers so large as often to appear like little wings.

RACK XI. comprises I<<in<ili> r.<, /'//'// -//'/.>, X>/n.^ Spots, ml
Sici.t/lti:-<. They are all very like the common rock -pigeon,
but have each some slight peculiarity. The Laughers have a
peculiar voice, supposed to resemble a laugh. The Nuns are
white, with the head, tail, and primary \\ inn feathers black or
red. The Spots are white, with the tail and a spot on the
forehead red. The Swallows arc slender, white in colour,
with the head and wings of some darker colour.

Besides these races and sub-races a number of other kinds
have been described, and about one hundred and fifty varieties
can be distinguished. It is interesting to note that almost
every part of the bird, whose variations can be noted and
selected, has led to variations of a considerable extent, and
many of these have necessitated changes in the plumage and
in the skeleton quite as great as any that occur in the
numerous distinct species of large genera. The form of the
skull and beak varies enormously, so that the skulls of the
Short -faced tumbler and some of the Carriers differ more
than any wild pigeons, even those classed in distinct genera.
The breadth and number of the ribs vary, as well as the
processes on them ; the number of the vertebra? and the
length of the sternum also vary ; and the perforations in the
sternum vary in size and shape. The oil gland varies in
development, and is sometimes absent. The number of the
wing-feathers varies, and those of the tail to an enormous
extent. The proportions of the leg and feet and the number

94 DARWINISM CHAP.

of the scutellae also vary. The eggs also vary somewhat in
size and shape ; and the amount of downy clothing on the
young bird, when first hatched, differs very considerably.
Finally, the attitude of the body, the manner of walking, the
mode of night, and the voice, all exhibit modifications of the
most remarkable kind. 1

Acclimatisation.

A very important kind of variation is that constitutional
change termed acclimatisation, which enables any organism to
become gradually adapted to a different climate from the
parent stock. As closely allied species often inhabit different
countries possessing very different climates, we should expect
to find cases illustrating this change among our domesticated
animals and cultivated plants. A few examples will therefore
be adduced showing that such constitutional variation does
occur.

Among animals the cases are not numerous, because no
systematic attempt has been made to select varieties for this
special quality. It has, however, been observed that, though
no European dogs thrive well in India, the Newfoundland dog,
originating from a severe climate, can hardly be kept alive.
A better case, perhaps, is furnished by merino sheep, which,
when imported directly from England, do not thrive, while those
which have been bred in the intermediate climate of the Cape
of Good Hope do much better. When geese were first intro-
duced into Bogota, they laid few eggs at long intervals, and
few of the young survived. By degrees, however, the fecundity
improved, and in about twenty years became equal to what
it is in Europe. According to Garcilaso, when fowls were
first introduced into Peru they were not fertile, whereas now
they are as much so as in Europe.

Plants furnish much more important evidence. Our
nurserymen distinguish in their catalogues varieties of fruit-
trees which are more or less hardy, and this is especially the
case in America, where certain varieties only will stand the
severe climate of Canada. There is one variety of pear, the
Forelle, which both in England and France withstood frosts

1 This account of domestic pigeons is greatly condensed from Mr.
Darwin's work already referred to.

iv VARIATION UNDER DOMESTICATION 05

that killed the flower* and buds of all other kinds of pears.
Wheat, which H -n>\\ n over so largo a portion of the world, has
become adapted to special climates. AVheat imported from
India and sown in good wheat soil in England produced the
nio>t meagre, ears; while wheat taken from France to the
West Indian Islands produced cither wholly barren spikes or
spikes furnished with two or three miserable seeds, while
\\ '--I Indian seed by its >ide yielded an enormous harvest. The
orange was very tender when first introduced into Italy, and
continued so as long as it Wa- propagated by Drafts, but
when trees were raised from seed many of these were found
to be hardier, and the orange i> now perfectly acclimatised in
Italy. S\\ eet pea- ( Fathyrus od oral us) imported from Fn^land
to the Calcutta llotanic (iardrns produced few blossoms and
no seed ; those from France (lowered a 1 it t le better, but still
produced no seed, but plants raised from seed brought from
Darjeeling in the Himalayas, but originally derived from
Filmland, flower and seed profusely in ( 'alciitta. '

An observation by Mr. I >arwin himself is perhaps even
more instructive. He says: "On I'lth May ls'| there
was a se\cre frost in Kent, and two rows of scarlet runners
(Phaseolus multiflorus) in my garden, containing 390 plants of
the same age and equally exposed, were all blackened and
killed except about a dozen plants. In an adjoining row of
Fulmer's dwarf bean (Phaseolus vulgaris) one single plant
escaped. A still more severe frost occurred four days after
wards, and of the dozen plants which had previously escaped
only three survived ; these were not taller or more vigorous
than the other young plants, but they escaped completely,
with not even the tips of their leaves browned. It was im-
possible to behold these three plants, with their blackened,
withered, and dead brethren all around them, and not see at
a glance that they differed widely in their constitutional power
of resisting frost.'

The preceding sketch of the variation that occurs among
domestic animals and cultivated plants shows how Avide it is
in range and how great in amount ; and we have good reason
to believe that similar variation extends to all organised beings.
In the class of fishes, for example, we have one kind which has
1 Animals and Plants under Domestication, vol. ii. pp. 307-311.

DARWINISM CHAP.

been long domesticated in the East, the gold and silver carps ;
and these present great variation, not only of colour but in the
form and structure of the fins and other external organs. In
like manner, the only domesticated insects, hive bees and silk-
worm moths, present numbers of remarkable varieties which
have been produced by the selection of chance variations just
as in the case of plants and the higher animals.

Circumstances favourable to Selection ly Man.

It may be supposed, that the systematic selection which
has been employed for the purpose of improving the races
of animals or plants useful to man is of comparatively recent
origin, though some of the different races are known to have
been in existence in very early times. But Mr. Darwin has
pointed out, that unconscious selection must have begun to
produce an efect as soon as plants were cultivated or animals
domesticated by man. It would have been very soon observed
that animals and plants produced their like, that seed of early
wheat produced early wheat, that the offspring of very swift
dogs were also swift, and as every one would try to have a
good rather than a bad sort this would necessarily lead to the
slow but steady improvement of all useful plants and animals
subject to man's care. Soon there would arise distinct breeds,
owing to the varying uses to which the animals and plants
were put. Dogs would be wanted chiefly to hunt one kind
of game in one part of the country and another kind else-
where ; for one purpose scent would be more important, for
another swiftness, for another strength and courage, for yet
another watchfulness and intelligence, and this would soon
lead to the formation of very distinct races. In the case of
vegetables and fruits, different varieties would be found to
succeed best in certain soils and climates ; some might be
preferred on account of the quantity of food they produced,
others for their sweetness and tenderness, while others might
be more useful on account of their ripening at a particular
season, and thus again distinct varieties would be established.
An instance of unconscious selection leading to distinct results
in modern times is afforded by two flocks of Leicester sheep
which both originated from the same stock, and were then bred
pure for upwards of fifty years by two gentlemen, Mr. Buckley

iv VARIATION UNDER DOMESTICATION 97

i IK I Mr. I'mrgess. Mr. Youatt, one of the greatest authorities
on In-ceding domestic animals, says: "There is not a suspicion
existing in the mind of any one at all acquainted with the
subject that the owner of either of them has deviated in any
one instance from the pure blood of Mr. Bakewell's original
flock, and yet the difference between the sheep possessed by
these two gentlemen is so great that they have the appearance
of being quite different varieties." In this case there was no
.lesire tu deviate frmn the original breed, and the difference
must have arisen from some slight difference of taste or judg-
ment in selecting, each year, the patents for the next year's
stuck, combined perhaps with some direct effect of the slight
differences of climate and soil on the two farms.

Most of our domesticated animals and cultivated plants
have come to us from the earliest seats of civilisation in
\Ve-tern A.-ia or lyjvpt, and have therefore been the subjects
of human care and selection for some thousands of years, tin-
result being that, in many cases, we do not know the wild
stock from which they originally sprang. The horse, the
camel, and the common bull and cow are nowhere found in a
wild state, and they lia\e all been domesticated from remote
antiquity. The original of the domestic fowl is still wild in
India and the Malay Islands, and it was domesticated in India
and China before 1 400 B.C. It was introduced into Knrope
about 600 B.C. Several distinct breeds were known to the
Romans about the commencement of the Christian era, and
they have since spread all over the civilised world and been
subjected to a vast amount of conscious and unconscious
selection, to many varieties of climate and to differences of
food ; the result being seen in the wonderful diversity of breeds
which differ quite as remarkably as do the different races of
pigeons already described.

In the vegetable kingdom, most of the cereals wheat,
barley, etc. are unknown as truly wild plants ; and the same
is the case with many vegetables, for De Candolle states that
out of 157 useful cultivated plants thirty-two are quite un-
known in a wild state, and that forty more are of doubtful
origin. It is not improbable that most of these do exist
wild, but they have been so profoundly changed by thousands
of years of cultivation as to be quite unrecognisable. The

H

98 DARAVINISM CHAP.

peach is unknown in a wild state, unless it is derived from
the common almond, on which point there is much difference
of opinion among botanists and horticulturists.

The immense antiquity of most of our cultivated plants
sufficiently explains the apparent absence of such useful
productions in Australia and the Cape of Good Hope, not-
withstanding that they both possess an exceedingly rich and
varied flora. These countries having been, until a com-
paratively recent period, inhabited only by uncivilised men,
neither cultivation nor selection has been carried on for a
sufficiently long time. In North America, however, where
there was evidently a very ancient if low form of civilisation,
as indicated by the remarkable mounds, earthworks, and
other prehistoric remains, maize was cultivated, though it
was probably derived from Peru ; and the ancient civilisation
of that country and of Mexico has given rise to no fewer than
thirty-three useful cultivated plants.

Conditions favourable to the production of Variations.

In order that plants and animals may be improved and
modified to any considerable extent, it is of course essential
that suitable variations should occur with tolerable frequency.
There seem to be three conditions which are especially favour-
able to the production of variations: (1) That the particular
species or variety should be kept in very large numbers ; (2)
that it should be spread over a wide area and thus subjected
to a considerable diversity of physical conditions ; and (3)
that it should be occasionally crossed with some distinct but
closely allied race. The first of these conditions is perhaps
the most important, the chance of variations of aiiy partic-
ular kind being increased in proportion to the quantity of
the original stock and of its annual offspring. It has been re-
marked that only those breeders who keep large flocks can
effect much improvement ; and it is for the same reason that
pigeons and fowls, which can be so easily and rapidly increased,
and which have been kept in such large numbers by so great
a number of persons, have produced such strange and numer-
ous varieties. In like manner, nurserymen who grow fruit and
flowers in large quantities have a great advantage over private
amateurs in the production of new varieties.

iv VARIATION UNDER DOMESTICATION 99

Although I believe, for reasons Avhich will be given further
on, that some amount of variability is a constant and
necessary property of all organisms, yet there appears to be
good evidence to show that changed conditions of life tend to
increase it, both by a direct action on the organisation and
by indirectly affecting the reproductive system. Hence the
extension of civilisation, by favouring domestication under
altered conditions, facilitates the process of modification. Yet
this change does not seem to be an essential condition, for
nowhere has the production of extreme varieties of plants and
flowers been carried farther than in Japan, where careful
selection continued for many generations must have been the
chief factor. The effect of occasional crosses often results in
a great amount of variation, but it also leads to instability of
character, and is therefore very little employed in the pro
duction of fixed and well-marked races. For this purpose, in
fact, it has to be carefully avoided, as it is only by isolation and
pure breeding that any specially desired i|iialilies can be in-
creased by selection. It is for this reason that among savage
peoples, whose animals run half wild, little improvement takes
place ; and the difficulty of isolation also explains why distinct
and pure breeds of cats are so rarely met \vith. The wide dis-
tribution of useful animals and plants from a very remote
epoch has, no doubt, been a powerful cause of modification,
because the particular breed first introduced into each country
has often been kept pure for many years, and has also been
subjected to slight differences of conditions. It will also
usually have been selected for a somewhat different purpose
in each locality, and thus very distinct races would soon
originate.

The important physiological effects of crossing breeds or
strains, and the part this plays in the economy of nature, will
be explained in a future chapter.

Concluding Remark*.

The examples of variation now adduced and these might
have been almost indefinitely increased will suffice to show
that there is hardly an organ or a quality in plants or animals
which has not been observed to vary ; and further, that when-
ever any of these variations have been useful to man he has

100 DARWINISM CHAP.

been able to increase them to a marvellous extent by the
simple process of always preserving the best varieties to breed
from. Along with these larger variations others of smaller
amount occasionally appear, sometimes in external, sometimes
in internal characters, the very bones of the skeleton often
changing slightly in form, size, or number ; but as these
secondary characters have been of no use to man, and have
not been specially selected by him, they have, usually, not
been developed to any great amount except when they have
been closely dependent on those external characters which he
has largely modified.

As man has considered only utility to himself, or the
satisfaction of his love of beauty, of novelty, or merely of
something strange or amusing, the variations he has thus pro-
duced have something of the character of monstrosities. Not
only are they often of no use to the animals or plants them-
selves, but they are not unfrequently injurious to them. In
the Tumbler pigeons, for instance, the habit of tumbling is
sometimes so excessive as to injure or kill the bird ; and many
of our highly-bred animals have such delicate constitutions
that they are very liable to disease, while their extreme
peculiarities of form or structure would often render them
quite unfit to live in a wild state. In plants, many of our
double flowers, and some fruits, have lost the power of pro-
ducing seed, and the race can thus be continued only by means
of cuttings or grafts. This peculiar character of domestic
productions distinguishes them broadly from wild species and
varieties, which, as will be seen by and by, are necessarily
adapted in every part of their organisation to the conditions
under which they have to live. Their importance for our
present inquiry depends on their demonstrating the occurrence
of incessant slight variations in all parts of an organism, with
the transmission to the offspring of the special characteristics
of the parents ; and also, that all such slight variations are
capable of being accumulated by selection till they present
very large and important divergencies from the ancestral
stock.

We thus see, that the evidence as to variation afforded
by animals and plants under domestication strikingly accords
with that which we have proved to exist in a state of nature.

iv VARIATION UNDER DOMESTICATION 101

And it is not at all surprising that it should be so, since all
the species were in a state of nature when first domesticated
or cultivated by man, and whatever variations occur must be
due to purely natural causes. Moreover, on comparing the
variations which occur in any one generation of domesticated
animals with those which we know to occur in wild animals,
we find no evidence of greater individual variation in the
former than in the latter. The results of man's selection are
more striking to us because we have always considered the
varieties of each domestic animal to be essentially identical,
while those which we observe in a wild state are held to be
essentially diverse. The greyhound and the spaniel seem
wonderful, as varieties of one animal produced by man's
selection ; while we think little of the diversities of the fox
and the wolf, or the horse and the zebra, because we have
been accustomed to look upon them as radically distinct
animals, not as the results of nature's selection of the
varieties of a common ancestor.

CHAPTER V

NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE

FITTEST

Effect of struggle for existence under unchanged conditions The effect
under change of conditions Divergence of character In insects In
birds In mammalia Divergence leads to a maximum of life in each
area Closely allied species inhabit distinct areas Adaptation to
conditions at various periods of life The continued existence of low
forms of life Extinction of low types among the higher animals-
Circumstances favourable to the origin of new species Probable origin
of the dippers The importance of isolation On the advance of organi-
sation by natural selection Summary of the first five chapters.

IN the preceding chapters we have accumulated a body of
facts and arguments which Avill enable us now to deal with the
very core of our subject the formation of species by means
of natural selection. We have seen how tremendous is the
struggle for existence always going on in nature owing to the
great powers of increase of all organisms ; we have ascertained
the fact of variability extending to every part and organ, each
of which varies simultaneously and for the most part independ-
ently ; and we have seen that this variability is both large in
its amount in proportion to the size of each part, and usually
affects a considerable proportion of the individuals in the large
and dominant species. And, lastly, we have seen. how similar
variations, occurring in cultivated plants and domestic animals,
are capable of being perpetuated and accumulated by artificial
selection, till they have resulted in all the wonderful varieties
of our fruits, flowers, and vegetables, our domestic animals and
household pets, many of which differ from each other far more
in external characters, habits, and instincts than do species in

CHAP, v NATURAL SELECTION 103

a state of nature. We have now to inquire whether there is
any analogous process in nature, by which wild animals and
plants can le permanently modified and new races or new
species produced.

A'//-,/ ,>f Xfnit/!//i' f,,r K.i-ixti m-e ujiJ, r Unchanged Conditions.

Let us first consider what will lie the effect of the struggle
for existence upon the animals and plants which we see around
US, under conditions which do not perceptibly vary from year
to year or from century to century. We have seen that every
>peeies is exposed to numerous and varied dangers throughout
its entire existence, and that it is only liy means of the exacl
adaptation of its organisation including its instincts and habits
to its surroundings that it is enabled to li\e till it produces
offspring which may take its place when it ceases to exist.
We ha\e seen also that, of the whole annual increase only a
very small fraction survives; and thou-h the survival in indi-
vidual cases ma\ sometimes be due rather to accident than
to any real superiority, yet we cannot doubt that, in the long
run, those survive which arebe.-t fitted by their perfect organisa-
tion to escape the dangers that surround them. This "survival
of the fittest" is what Darwin termed "natural selection,"
because it leads to the same results in nature as are produced
by man's selection among domestic animals and cultivated
plants. Its primary effect will, clearly, be to keep each species
in the most perfect health and vigour, with even part of its
organisation in full harmony with the conditions of its existence.
It prevents any possible deterioration in the organic world, and
produces that appearance of exuberant life and enjoyment, of
health and beauty, that affords us so much pleasure, and Avhich
might lead a superficial observer to suppose that peace and
quietude reigned throughout nature.

The. Effect under changed Conditions.

But the very same process which, so long as conditions re-
main substantially the same, secures the continuance of each
species of animal or plant in its full perfection, will usually,
under changed conditions, bring about whatever change of
structure or habits may be necessitated by them. The changed
conditions to which we refer are such as we know have occurred

104 DARWINISM CHAP.

throughout all geological time and in every part of the world.
Land and water have been continually shifting their positions ;
some regions are undergoing subsidence with diminution of
area, others elevation with extension of area ; dry land has
been converted into marshes, while marshes have been drained
or have even been elevated into plateaux. Climate too has
changed again and again, either through the elevation of
mountains in high latitudes leading to the accumulation of
snow and ice, or by a change in the direction of winds and
ocean currents produced by the subsidence or elevation of lands
which connected continents and divided oceans. Again, along
with all these changes have come not less important changes
in the distribution of species. Vegetation has been greatly
modified by changes of climate and of altitude ; while every
union of lands before separated has led to extensive migrations
of animals into new countries, disturbing the balance that
before existed among its forms of life, leading to the extermina-
tion of some species and the increase of others.

When such physical changes as these have taken place, it is
evident that many species must either become modified or
cease to exist. When the vegetation has changed in character
the herbivorous animals must become able to live on new and
perhaps less nutritious food ; while the change from a damp
to a dry climate may necessitate migration at certain periods
to escape destruction by drought. This will expose the species
to new dangers, and require special modifications of structure
to meet them. Greater swiftness, increased cunning, nocturnal
habits, change of colour, or the power of climbing trees and
living for a time on their foliage or fruit, may be the means
adopted by different species to bring themselves into harmony
with the new conditions ; and by the continued survival of
those individuals, ojily, which varied sufficiently in the right
direction, the necessary modifications of structure or of func-
tion would be brought about, just as surely as man has been
able to breed the greyhound to hunt by sight and the fox-
hound by scent, or has produced from the same wild plant
such distinct forms as the cauliflower and the brussels sprouts.

We will now consider the special characteristics of the
changes in species that are likely to be effected, and how far
they agree with what we observe in nature.

NATURAL SELECTION 105

Divergence of

In species which have a wide range the struggle for exist-
ence will often cause some individuals or groups of individuals

to adopt new habits in order to seize upon vacant places in
nature win-re the struggle is less severe. Some, living among
extensive marshes, may adopt a more aquatic mode of life ;
others, living where forests abound, may become more arboreal.
In either case we cannot doubt that the changes of structure
needed to adapt them to their new habits would soon be
brought about, because we know that variations in all the

O

external organs and all their separate parts are very abundant
and are also considerable in amount. That such divergence of
character has actually occurred we have some direct evidence.
Mr. Darwin informs us that in the Catskill Mountains in the
United States there are two varieties of wolves, one with a
light greyhound-like form which pursues deer, the other more
bulky with shorter legs, which more frequently attacks sheep. 1
Another good example is that of the insects in the island of
Madeira, many of which have either lost their wings or have
had them so much reduced as to be useless for flight, while the
very same species on the continent of Europe possess fully
developed wings. In other cases the wingless Madeira species
are distinct from, but closely allied to, winged species of Europe.
The explanation of this change is, that Madeira, like many
oceanic islands in the temperate /.one, is much exposed to
sudden gales of wind, and as most of the fertile land is on the
coast, insects which flew much would be very liable to bo
blown out to sea and lost. Year after year, therefore, those
individuals which had shorter wings, or which used them least,
were preserved ; and thus, in time, terrestrial, wingless, or im-
perfectly winged races or species have been produced. That
this is the true explanation of this singular fact is proved by
much corroborate evidence. There are some few flower-
frequenting insects in Madeira to whom wings are essential,
and in these the wings are somewhat larger than in the same
species on the mainland. AVe thus see that there is no general
tendency to the abortion of wings in Madeira, but that it is
simply a case of adaptation to new conditions. Those insectn

1 Ori'tiu ft' Species, \<. 71.

106 DARAVINISM CHAP.

to whom wings were not absolutely essential escaped a serious
danger by not using them, and the wings therefore became
reduced or were completely lost. But when they were essential
they were enlarged and strengthened, so that the insect could
battle against the winds and save itself from destruction at
sea. Many flying insects, not varying fast enough, would be
destroyed before they could establish themselves, and thus we
may explain the total absence from Madeira of several whole
families of winged insects which must have had many oppor-
tunities of reaching the islands. Such are the large groups of
the tiger-beetles (Cicindelidse), the chafers (Melolonthidse), the
click-beetles (Elateridre), and many others.

But the most curious and striking confirmation of this
portion of Mr. Darwin's theory is afforded by the case of
Kerguelen Island. This island was visited by the Transit of
Venus expedition. It is one of the stormiest places on the
globe, being subject to almost perpetual gales, while, there
being no wood, it is almost entirely without shelter. The
Rev. A. E. Eaton, an experienced entomologist, was naturalist
to the expedition, and he assiduously collected the few insects
that were to be found. All were incapable of flight, and most
of them entirely without wings. They included a moth,
several flies, and numerous beetles. As these insects could
hardly have reached the islands in a wingless state, even if
there were any other known land inhabited by them which
there is not we must assume that, like the Madeiran insects,
they were originally winged, and lost their power of flight
because its possession was injurious to them.

It is no doubt due to the same cause that some butterflies
on small and exposed islands have their wings reduced in size,
as is strikingly the case with the small tortoise-shell butterfly
(Vanessa urticse) inhabiting the Isle of Man, Avhich is only
about half the size of the same species in England or Ireland ;
and Mr. Wollaston notes that Vanessa callirhoe a closely allied
South European form of our red-admiral butterfly is perma-
nently smaller in the small and bare island of Porto Santo
than in the larger and more wooded adjacent island of Madeira.

A very good example of comparatively recent divergence
of character, in accordance with new conditions of life, is
afforded by our red grouse. This bird, the Lagopus scoticus of

NATURAL SELECTION 107

naturalists, is entirely confined to the British Isles. It is,
however, \ery closely allied to the willow Arouse (Lagopus
alhus), a lu'rd which ranges all over Europe, Northern Asia,
and North America, but which, unlike our species, change- t,,
wliite in winter. Xo difference in form or structure can be
detected between the two birds, but as they differ so decidedly
in colour our species being usually rather darker in winter
than in summer, while there are also slight differences in the
call note and in habits, the two species are generally con-
sidered to be distinct. The differences, however, are so
clearly adaptations to changed conditions that we can hardly
doubt that, during the early part of the glacial period, when
our islands were united to the continent, our -roii>e was
identical with that of the resl of Knropc. lint when the cold
passed away and our islands became permanently separated
from the mainland, with a mild and equable climate and very
little snow in winter, the change to white, at that season
became hurtful, rendering the birds more conspicuous instead
of serving as a mean- of .-oncralment. The colour was, there-
fore, madually changed by the process of variation and natural
selection; and as the birds obtained ample shelter among the
heather which clothes so many of our moorlands, it became
useful for them to assimilate with its brown and dusky stems
and withered flowers rather than with the snow of the higher
mountains. An interesting confirmation of this change having
really occurred is afforded by the occasional occurrence in
Scotland of birds with a considerable amount of white in the
\\inter plumage. This is considered to be a case of reversion
to the ancestral type, just as the slaty colours and banded
wings of the wild rock-pigeon sometimes reappear in our fancy
breeds of domestic pigeons. 1

The principle of "divergence of character" pervades all
nature from the lowest groups to the highest, as may be
well seen in the class of birds. Among our native species we
see it well marked in the different species of titmice, pipits,
and chats. The great titmouse (Parus major) by its larger
size and stronger bill is adapted to feed on larger insects, and
is even said sometimes to kill small and weak birds. The
smaller and weaker coal titmouse (Parus ater) has adopted a

L Yarrell's British Birds, fourth edition, vol. iii. p. 77.

108 DARWINISM CHAP.

more vegetarian diet, eating seeds as well as insects, and
feeding on the ground as well as among trees. The delicate
little bine titmouse (Parus coeruleus), with its very small bill,
feeds on the minutest insects and grubs which it extracts
from crevices of bark and from the buds of fruit-trees. The
marsh titmouse, again (Parus palustris), has received its name
from the low and marshy localities it frequents ; while the
crested titmouse (Parus cristatus) is a northern bird frequenting
especially pine forests, on the seeds of which trees it partially
feeds. Then, again, our three common pipits the tree-pipit
( Anthus arboreus), the meadow-pipit (Anthus pratensis), and the
rock-pipit or sea-lark (Anthus obscurus) have each occupied a
distinct place in nature to which they have become specially
adapted, as indicated by the different form and size of the
hind toe and claw in each species. So, the stone-chat (Saxicola
rubicola), the whin-chat (S. rubetra), and the wheat-ear (S.
osnanthe) are all slightly divergent forms of one type, with
modifications in the shape of the wing, feet, and bill adapting
them to slightly different modes of life. The whin-chat is the
smallest, and frequents furzy commons, fields, and lowlands,
feeding on worms, insects, small molluscs, and berries ; the
stone-chat is next in size, and is especially active and lively,
frequenting heaths and uplands, and is a permanent resident
with us, the two other species being migrants ; while the
larger and more conspicuous wheat-ear, besides feeding on
grubs, beetles, etc., is able to capture flying insects on the
wing, something after the manner of true flycatchers.

These examples sufficiently indicate how divergence of
chai'acter has acted, and has led to the adaptation of numerous
allied species, each to a more or less special mode of life, with
the variety of food, of habits, and of enemies which must
necessarily accompany such diversity. And when we extend
our inquiries to higher groups we find the same indications of
divergence and special adaptation, often to a still more marked
extent. Thus we have the larger falcons, which prey upon
birds, while some of the smaller species, like the hobby
(Falco subbuteo), live largely on insects. The true falcons
capture their prey in the air, while the hawks usiiaily seize it
on or near the ground, feeding on hares, rabbits, squirrels,
grouse, pigeons, and poultry. Kites and buzzards, on the

v NATURAL SELECTION 109

other hand, seize their prey upon the ground, and the former
feed largely on reptiles and otlal as well as on liirds and
quadrupeds. Others have adopted tisli as their chief food,
and the osprey snatches its |>ivy from the \\ater with as much
facility as a gull or a petrel ; while the South American
caracaras (Polyborus) have adopted the habits of vultures and
live altogether on carrion. In every great group there is the
same divergence of habits. There are ground-pigeons, rock-
pigeons, and wood-pigeons, seed-eating pigeons and fruit
eating pigeons; there are carrion eating, insect-eating, and
fruit-eating crows. Even kingfishers are, some aquatic, some
terrestrial in their habits ; sonic live on fish, some on insects,
some on reptiles. Lastly, among the primary divisions of liirds
we find a purely terrestrial group the Katita-, including the
ostriches, cassowaries, etc.; other great groii] s, including the
ducks, cormorants, gulls, penguins, etc., are aquatic ; while tin-
bulk of the Passerine birds are aerial and arboreal. The
same general facts can be detected in all other classes of
animals. In the mammalia, for example, we have in the common
rat a fish-eater and flesh cater as well as a grain eater, which
has no doubt helped to give it the power of spreading over
the world and driving away the native rats of other countries.
Throughout the Kodent tribe we find everywhere aquatic,
terrestrial, and arboreal forms. In the weasel and cat tribes
some live more in trees, others on the ground ; squirrels have
diverged into terrestrial, arboreal, and flying species ; and
finally, in the bats we have a truly aerial, and in the whales
a truly aquatic order of mammals. \Ve thus see that,
beginning with different varieties of the same species, we
have allied species, genera, families, and orders, with similarly
divergent habits, and adaptations to different modes of life,
indicating some general principle in nature which has been
operative in the development of the organic world. But in
order to be thus operative it must be a generally useful
principle, and Mr. Darwin has very clearly shown us in what
this utility consists.

Divergence leads to a Maximum of Organic Forms in each Area.

Divergence of character has a double purpose and use. In
the first place it enables a species which is being overcome

110 DARWINISM CHAV.

by rivals, or is in process of extinction by enemies, to save
itself by adopting new habits or by occupying vacant places
in nature. This is the immediate and obvious efl'ect of all
the numerous examples of divergence of character which we
have pointed out. But there is another and less obvious
result, which is, that the greater the diversity in the organisms
inhabiting a country or district the greater will be the total
amount of life that can be supported there. Hence the
continued action of the struggle for existence will tend to
bring about more and more diversity in each area, which may
be shown to be the case by several kinds of evidence. As an
example, a piece of turf, three feet by four in size, was found
by Mr. Darwin to contain twenty species of plants, and the>e
twenty species belonged to eighteen genera and to eight
orders, showing how greatly they differed from each other.
Farmers find that a greater quantity of hay is obtained from
ground sown with a variety of genera of grasses, clover,
etc., than from similar land sown with one or two species
only ; and the same principle applies to rotation of crops,
plants differing very widely from each other giving the
best results. So, in small and uniform islands, and in
small ponds of fresh water, the plants and insects, though
few in number, are found to be wonderfully varied in
character.

The same principle is seen in the naturalisation of plants
and animals by man's agency in distant lands, for the species
that thrive best and establish themselves permanently are
not only very varied among themselves but differ greatly from
the native inhabitants. Thus, in the Northern United States
there are, according to Dr. Asa Gray, 260 naturalised flower-
ing plants which belong to no less than 162 genera; and of
these, 100 genera are not natives of the United States. So, in
Australia, the rabbit, though totally unlike any native animal,
has increased so much that it probably outnumbers in in-
dividuals all the native mammals of the country ; and in
New Zealand the rabbit and the pig have equally multiplied.
Darwin remarks that this "advantage of diversification of
structure in the inhabitants of the same region is, in fact, the.
same as that of the physiological division of labour in the
organs of the same body. No physiologist doubts that a

V NATUKAI. SELECTION 111

stomach adapted to digest vegetable matter alone, or flesh
alone, draws more nutriment from these substances. So, in
the general economy of any land, the more widely and
perfectly the animals and plant.- are diversified for ditl'erent
habits of life, so will a greater number of individuals be
capable uf there supporting themselves." 1

mutt <-l, *<lii a II 'm/ Species /////'</<// <1!*t'n><i

One of the curious results of the general action of this
principle in nature is, that the most closely allied species
those whose differences though often real and important are
hardly perceptible to any one lnit a naturalist are usually
not found in the same lnit in widely separated countries.
Thus, the m-arcst allies to our Kuropean golden plover are
found in North America and Kast Asia ; the nearest ally
of our Kuropean jay is found in -lapan, although there art-
several othei- species of jays in Western Asia and North
Africa ; and though we have several species of titmice in
England they are not very closely allied to each other.
The form most akin to our blue tit is the azure tit of
Central Asia (I'arus a/ureus) ; the 1'arus ledouci of Algeria
is very near our coal tit, and the 1'aius Insulins of South-
Ka stern Europe and Asia Minor is nearest to our marsh tit.
So, our four species of wild pigeons the ringdove, stock-
dove, rock-pigeon, and turtle-dove are not closely allied to
each other, but each of them belongs, according to some
ornithologists, to a separate genus or subgenus, and has its
nearest relatives in distant parts of Asia and Africa, In
mammalia the same thing occurs. Each mountain region of
Europe and Asia has usually its own species of wild sheep
and goat, and sometimes of antelope and deer ; so that in
each region there is found the greatest diversity in this
class of animals, while the closest allies inhabit quite distinct
and often distant areas. In plants we find the same
phenomenon prevalent. Distinct species of columbine are
found in Central Europe (Aguilegia vulgaris), in Eastern
Europe, and Siberia (A. glandulosa), in the Alps (A. Alpina),
in the Pyrenees (A. pyrenaiea), in the Greek mountains (A.
ottonis), and in Corsica (A. Bernard!), but rarely are two

1 Origin of Species, p. 89.

112 DARWINISM CHAP.

species found in the same area. So, each part of the
world has its own peculiar forms of pines, firs, and cedars,
but the closely allied species or varieties are in almost
every case inhabitants of distinct areas. Examples are the
deodar of the Himalayas, the cedar of Lebanon, and that of
North Africa, all very closely allied but confined to distinct
areas ; and the numerous closely allied species of true pine
(genus Pinus), which almost always inhabit different countries
or occupy different stations. We Avill now consider some
other modes in which natural selection will act, to adapt
organisms to changed conditions.

Adaptation to Conditions at J r arlous Periods of Life.

It is found, that, in domestic animals and cultivated plants,
variations occurring at any one period of life reappear in the
offspring at the same period, and can be perpetuated and
increased by selection without modifying other parts of the
organisation. Thus, variations in the caterpillar or the cocoon
of the silkworm, in the eggs of poultry, and in the seeds
or young shoots of many culinary vegetables, have been
accumulated till those parts have become greatly modified and,
for man's purposes, improved. Owing to this fact it is easy
for organisms to become so modified as to avoid dangers that
occur at any one period of life. Thus it is that so many
seeds have become adapted to various modes of dissemination
or protection. Some are winged, or have down or hairs
attached to them, so as to enable them to be carried long
distances in the air ; others have curious hooks and prickles,
which cause them to be attached firmly to the fur of mammals
or the feathers of birds ; while others are buried within sweet
or juicy and brightly coloured fruits, which are seen and
devoured by birds, the hard smooth seeds passing through
their bodies in a fit state for germination. In the struggle
for existence it must benefit a plant to have increased means
of dispersing its seeds, and of thus having young plants pro-
duced in a greater variety of soils, aspects, and surroundings,
with a greater chance of some of them escaping their numerous
enemies and arriving at maturity. The various differences
referred to would, therefore, be brought about by variation and
survival of the fittest, just as surely as the length and quality

v NATURAL SELECTION 113

of cotton on the seed of the cotton-plant have been increased
by man's selection.

The larva) of insects have thus been wonderfully modified
in order to escape the numerous enemies to whose attacks
tliry arc exposed at this period of their existence. Their
colours and markings have become marvellously adapted to
conceal them among the foliage of the plant they live upon,
and this colour often changes completely after the last moult,
when the creature has to descend to the ground for its change
to the pupa state, during which period a In-own instead of a
green colour is protective. Others have acquired curious
attitudes and large ocelli, which cause them to resemble the
hrad of some reptile, or they have curious horns or coloured
ejectile processes which frighten away enemies ; while a great
number have acquired secretions which render them offensive
to the taste of their enemies, and these are always adorned
with very conspicuous markings or brilliant colours, which
serve as a sign of inedibility and prevent their being needlessly
attacked. This, however, is a portion of the very large sub-
ject of organic colour and marking, which will be fully dis-
cussed and illustrated in a separate chapter.

In this way every possible modification of an animal or
plant, whether in colour, form, structure, or habits, which
would be serviceable to it or to its progeny at any period of
its existence, may be readily brought about. There are some
curious organs which are used only once in a creature's life,
but which are yet essential to its existence, and thus have
very much the appearance of design by an intelligent designer.
Such are, the great, jaws possessed by some insects, used ex-
clusively for opening the cocoon, and the hard tip to the beak
of unhatched birds used for breaking the eggshell. The
increase in thickness or hardness of the cocoons or the e^_-
being useful for protection against enemies or to avoid
accidents, it is probable that the change has been very
gradual, because it would be constantly checked by the
necessity for a corresponding change in the young insects or
birds enabling them to overcome the additional obstacle of a
tougher cocoon or a harder eggshell. As we have seen,
however, that every part of the organism appears to be
varying independently, at the same time, though to different

I

114 DARWINISM CHAP.

amounts, there seems no reason to believe that the necessity
for two or more coincident variations would prevent the
required change from taking place.

The Continued Existence of Low Forms of Life.

Since species are continually undergoing modifications
giving them some superiority over other species or enabling
them to occupy fresh places in nature, it may be asked Why
do any low forms continue to exist ? Why have they not long-
since been improved and developed into higher forms 1 The
answer, probably, is, that these low forms occupy places in
nature which cannot be filled by higher forms, and that they
have few or no competitors ; they therefore continue to
exist. Thus, earthworms are adapted to their mode of life
better than they would be if more highly organised. So, in
the ocean, the minute foraminifera and infusoria, and the
larger sponges and corals, occupy places which more highly
developed creatures could not fill. They form, as it were, the
base of the great structure of animal life, on which the next
higher forms rest ; and though in the course of ages they
may undergo some changes, and diversification of form
and structure, in accordance with changed conditions, their
essential nature has probably remained the same from the
very dawn of life on the earth. The low aquatic diatomacese
and confervas, together with the lowest fungi and lichens,
occupy a similar position in the vegetable kingdom, filling
places in nature which would be left vacant if only highly
organised plants existed. There is, therefore, no motive
power to destroy or seriously to modify them ; and they have
thus probably persisted, under slightly varying forms, through
all geological time.

Extinction of Lower Types among the Higher Animals.

So soon, however, as we approach the higher and more
fully developed groups, we see indications of the often re-
peated extinction of lower by higher forms. This is shown
by the great gaps that separate the mammalia, birds, reptiles,
and fishes from each other ; while the lowest forms of each are
always few in number and confined to limited areas. Such

NATURAL SELECTION 315

are the lowest mammals the echidna and ornithorhynehus of
Australia; the lowest birds the apteryx of New Zealand
and the cassowaries of the New (iuinea region; while the
lowest fish the amphioxus or lancelet, is completely isolated,
and has apparently survived only by its habit of burro w-
ing in the sand. The great distinctness of the carnivora,
ruminants, rodents, whales, bats, and other orders of
mammalia; of the accipitres, pigeons, and parrots, among
birds ; and of the beetles, bees, flies, and moths, among insects,
all indicate an enormous amount of extinction among the
comparatively low forms by which, on any theory of evolution,
these higher and more specialised groups must have been
preceded.

Circumstances favourable to the Origin of New Species by
.\<i/nral Selection.

We have already seen that, when there is no change in
the physical or organic conditions of a country, the effect of
natural selection is to keep all the species inhabiting it in a
state of perfect health and full development, and to preserve
the balance that already exists between the different groups
of organisms. But, whenever the physical or organic condi
tions change, to however small an extent, some correspond-
ing change will be produced in the flora and fauna, sin<r,
considering the severe struggle for existence and the complex
relations of the various organisms, it is hardly possible that
the change should not be beneficial to some species and
hurtful to others. The most common effect, therefore, AM' 11
be that some species Avill increase and others Avill diminish :
and in cases Avhere a species was already small in numbers a
further diminution might lead to extinction. This would
afford room for the increase of other species, and thus a
considerable readjustment of the proportions of the several
species might take place. When, hoAvever, the change was of
a more important character, directly affecting the existence of
many species so as to render it difficult for them to maintain
themselves Avithout some considerable change in structure or
habits, that change would, in some cases, be brought about by
variation and natural selection, and thus IICAV varieties or HCAV
species might be formed. We have to consider, then, which

116 DARWINISM CHAP.

arc the species that would be most likely to be so modified,
while others, not becoming modified, would succumb to the
changed conditions and become extinct.

The most important condition of all is, undoubtedly, that
variations should occur of sufficient amount, of a sufficiently
diverse character, and in a large number of individuals, so as
to afford ample materials for natural selection to act upon ;
and this, we have seen, does occur in most, if not in all, large,
wide-ranging, and dominant species. From some of these,
therefore, the new species adapted to the changed conditions
would usually be derived ; and this would especially be the
case when the change of conditions was rather rapid, and when
a correspondingly rapid modification could alone save some
species from extinction. But when the change was very
gradual, then even less abundant and less widely distributed
species might become modified into new forms, more especially
if the extinction of many of the rarer species left vacant
places in the economy of nature.

Probable Origin of the Dippers.

An excellent example of how a limited group of species
has been able to maintain itself by adaptation to one of
these "vacant places" in nature, is afforded by the curious
little birds called dippers or water- ouzels, forming the genus
Cinclus and the family Cinclidse of naturalists. These birds
are something like small thrushes, with very short wings and
tail, and very dense plumage. They frequent, exclusively,
mountain torrents in the northern hemisphere, and obtain
their food entirely in the water, consisting, as it does, of water-
beetles, caddis -worms and other insect- larvse, as well as
numerous small fresh-water shells. These birds, although not
far removed in structure from thrushes and wrens, have the
extraordinary power of flying under water ; for such, ac-
cording to the best observers, is their process of diving in
search of their prey, their dense and somewhat fibrous
plumage retaining so much air that the water is prevented
from touching their bodies or even from wetting their feathers
to any great extent. Their powerful feet and long curved
claws enable them to hold on to stones at the bottom, and
thus to retain their position while picking up insects, shells,

v NATURAL SELECTION 117

etc. As they frequent chiefly the most rapid and boisterous
torrents, among rocks, waterfalls, and huge boulders, the
water is never frozen over, and they are thus able to live
during the severest winters. Only a very few species of
dipper are known, all those of the old world being so closely
allied to our British bird that some ornithologists consider
them to be merely local races of one species ; while in North
America and the northern Andes there are two other
species.

Here then we have a bird, which, in its whole structure,
shows a close affinity to the smaller typical perching birds,
but which has departed from all its allies in its habits and
mode of life, and has secured for itself a place in nature
where it has few competitors and few enemies. We may
well suppose, that, at some remote period, a bird which was
perhaps the common and more generalised aneotor of most
of our thrushes, warblers, wrens, etc., had spread widely over
the great northern continent, and had given rise to numerous
varieties adapted to special conditions of life. Among these
some took to feeding on the borders of clear streams, picking
out such larva? and molluscs as they could reach in shallow
water. When food became scarce they would attempt to
pick them out of deeper and deeper water, and while doing
this in cold weather many would become frozen and starved.
But any which possessed denser and more hairy plumage
than usual, which was able to keep out the water, would
survive ; and thus a race would be formed which would depend
more and more on this kind of food. Then, following up the
frozen streams into the mountains, they would be able to live
there during the winter; and as such places afforded them much
protection from enemies and ample shelter for their nests and
young, further adaptations would occur, till the wonderful
power of diving and flying under water was acquired by a
true land-bird.

That such habits might be acquired under stress of need
is rendered highly probable by the facts stated by the well-
known American naturalist, Dr. Abbott. He says that " the
water-thrushes (Seiurus sp.) all wade in water, and often,
seeing minute mollusca on the bottom of the stream, plunge
both head and neck beneath the surface, so that often, for

118 DARWINISM CHAP.

several seconds, a large part of the body is submerged. Now
these birds still have the plumage pervious to water, and so
are liable to be drenched and sodden ; but they have also the
faculty of giving these drenched feathers such a good shaking
that night is practicable a moment after leaving the water.
Certainly the water -thrushes (Seiurus ludovicianus, S. aurica-
pillus, and S. noveboracensis) have taken many preliminary
steps to becoming as aquatic as the dipper ; and the winter-
wren, and even the Maryland yellow -throat are not far
behind." 1

Another curious example of the way in which species have
been modified to occupy new places in nature, is afforded by
the various animals which inhabit the water-vessels formed
by the leaves of many epiphytal species of Bromelia. Fritz
Miiller has described a caddis-fly larva which lives among these
leaves, and Avhich has been modified in the pupa state in
accordance with its surroundings. The pupa? of caddis-flies
inhabiting streams have fringes of hair on the tarsi to enable
them to reach the surface on leaving their cases. But in the
species inhabiting bromelia leaves there is no need for swimming,
and accordingly we find the tarsi entirely bare. In the same
plants are found curious little Entomostraca, very abundant
there but found nowhere else. These form a new genus, but
are most nearly allied to Cy there, a marine type. It is believed
that the transmission of this species from one tree to another
must be effected by the young Crustacea, which are very
minute, clinging to beetles, many of which, both terrestrial and
aquatic, also inhabit the bromelia leaves ; and as some water-
beetles are known to frequent the sea, it is perhaps by these
means that the first emigrants established themselves in this
strange new abode. Bromelise are often very abundant on trees
growing on the water's edge, and this would facilitate the tran-
sition from a marine to an arboreal habitat. Fritz Miiller has
also found, among the bromelia leaves, a small frog bearing
its eggs on its back, and having some other peculiarities of
structure. Several beautiful little aquatic plants of the genus
Utricularia or bladder- wort also inhabit bromelia leaves ; and
these send runners out to neighbouring plants and thus spread
themselves with great rapidity.

1 Nature, vol. xxx. p. 30.

NATURAL SELECTION 119

The Lujmrfiiinr o

Isolation is no doubt an important aid to natural selection,
as shown by the fact that islands so often present a number
of peculiar species ; and the same thing is seen on the two
sides of a great mountain range or on opposite coasts of a
continent. The importance of isolation is twofold. In the
first place, it leads to a body of individuals of each species being
limited in their range and thus subjected to uniform condi-
tions for long spaces of time. Both the direct action of the
environment and the natural selection of such varieties only
as are suited to the conditions, will, therefore, be able to
produce their full effect. In the second place, the process of
change will not be interfered with by intercrossing with other
individuals which are becoming adapted to somewhat different
conditions in an adjacent area. I Jut this question of the
swamping effects of intercrossing will be considered in another
chapter.

Mr. Darwin was of opinion that, on the whole, the largeness
of the area occupied ly a species was of more importance than
isolation, as a factor in the production of new species, and in
this I quite agree with him. It must, too, be remembered,
that isolation Avill often be produced in a continuous area
whenever a species becomes modified in accordance with varied
conditions or diverging habits. For example, a wide-ranging
species may in the northern and colder part of its area become
modified in one direction, and in the southern part in another
direction ; and though for a long time an intermediate form
may continue to exist in the intervening area, this will be
likely soon to die out, both because its numbers will be small,
and it will be more or less pressed upon in varying seasons by
the modified varieties, each better able to endure extremes of
climate. So, when one portion of a terrestrial species takes to
a more arboreal or to a more aquatic mode of life, the change
of habit itself leads to the isolation of each portion. Again,
as will be more fully explained in a future chapter, any
difference of habits or of haunts usually leads to some modi-
fication of colour or marking, as a means of concealment from
enemies ; and there is reason to believe that this difference will
be intensified by natural selection as a means of identification

120 DARWINISM CHAP.

and recognition by members of the same variety or incipient
species. It has also been observed that each differently
coloured variety of Avild animals, or of domesticated animals
which have run wild, keep together, and refuse to pair with
individuals of the other colours ; and this must of itself act to
keep the races separate as completely as physical isolation.

On the Advance of Organisation by Natural Selection.

As natural selection acts solely by the preservation of use-
ful variations, or those which are beneficial to the organism
under the conditions to which it is exposed, the result must
necessarily be that each species or group tends to become more
and more improved in relation to its conditions. Hence we
should expect that the larger groups in each class of animals
and plants those which have persisted and have been abundant
throughout geological ages would, almost necessarily, have
arrived at a high degree of organisation, both physical and
mental. Illustrations of this are to be seen everywhere.
Among mammalia we have the carnivora, which from Eocene
times have been becoming more and more specialised, till they
have culminated in the cat and dog tribes, which have reached
a degree of perfection both in structure and intelligence fully
equal to that of any other animals. In another line of
development, the herbivora have been specialised for living
solely on vegetable food till they have culminated in the sheep,
the cattle, the deer, and the antelopes. The horse tribe,
commencing with an early four-toed ancestor in the Eocene
age, has increased in size and in perfect adaptation of feet and
teeth to a life on open plains, and has reached its highest per-
fection in the horse, the ass, and the zebra. In birds, also, we
see an advance from the imperfect tooth-billed and reptile-
tailed birds of the secondary epoch, to the wonderfully
developed falcons, crows, and swallows of our time. So, the
ferns, lycopods, conifers, and monocotyledons of the palaeozoic
and mesozoic rocks, have developed into the marvellous wealth
of forms of the higher dicotyledons that now adorn the earth.

But this remarkable advance in the higher and larger groups
does not imply any universal law of progress in organisation,
because we have at the same time numerous examples (as has
been already pointed out) of the persistence of lowly organised

v NA'ITKAL SELECTION 121

forms, and also of absolute degradation or degeneration. Ser-
pents, for example, have been developed from some li/ard-like
t \ pe which has lost its limbs ; and though this loss has enabled
them to occupy fresh places in nature and to increase and
flourish to a marvellous extent, yet it must be considered to be
a retrogression rather than an advance in organisation. The
si me remark will apply to the whale tribe among mammals;
to the blind amphibia and insects of the great caverns; and
among plants to the numerous cases in which flowers, once
>pecially adapted to be fertilised by insects, have lost their
gay corollas and their special adaptations, and have become
degraded into wind-fertilised forms. Such are our plantains,
our meadow biirnct. and even, as some botanists maintain, our
rushes, sedges, and grasses. The causes which have led to
this degeneration will he discussed in a future, chapter: but
the facts are undisputed, and they show us that although
variation and the struggle for existence may lead, on the
whole, to a continued advance of organisation; yet they also
lead in many cases to a ret n>greion, when such retrogression
may aid in the preservation of any form under new conditions.
They also lead to the persistence, with slight modifications, of
numerous lowly organised forms which are suited to places
which higher forms could not fully occupy, or to conditions
under which they could not exist. Such are the ocean
depths, the soil of the earth, the mud of rivers, deep caverns,
subterranean waters, etc. ; and it is in such places as these, as
well as in some oceanic islands which competing higher forms
have not been able to reach, that we find many curious relics
of an earlier world, which, in the free air and sunlight and in
the great continents, have long since been driven out or exter-
minated by higher types.

Summary of the first Five Chapters.

We have now passed in review, in more or less detail, the
main facts on which the theory of " the origin of species by
means of natural selection " is founded. In future chapters
we shall have to deal mainly with the application of the theory
to explain the varied and complex phenomena presented by the
organic world ; and, also, to discuss some of the theories put
forth by modern writers, either as being more fundamental than

122 DARWINISM CHAP.

that of Darwin or as supplementary to it. Before doing this,
however, it will be well briefly to summarise the facts and
arguments already set forth, because it is only by a clear
comprehension of these that the full importance of the theory
can be appreciated and its further applications understood.

The theory itself is exceedingly simple, and the facts on
which it rests though excessively numerous individually, and
coextensive with the entire organic world yet come under a
few simple and easily understood classes. These facts are,
first, the enormous powers of increase in geometrical progres-
sion possessed by all organisms, and the inevitable struggle for
existence among them ; and, in the second place, the occurrence
of much individual variation combined with the hereditary
transmission of such variations. From these two great classes
of facts, which are universal and indisputable, there necessarily
arises, as Darwin termed it, the " preservation of favoured races
in the struggle for life," the continuous action of which, under
the ever-changing conditions both of the inorganic and organic
universe, necessarily leads to the formation or development of
new species.

But, although this general statement is complete and indis-
putable, yet to see its applications under all the complex
conditions that actually occur in nature, it is necessary always
to bear in mind the tremendous power and universality of the
agencies at work. We must never for an instant lose sight
of the fact of the enormously rapid increase of all organisms,
which has been illustrated by actual cases, given in our second
chapter, no less than by calculations of the results of un-
checked increase for a few years. Then, never forgetting
that the animal and plant population of any country is, on
the whole, stationary, we must be always trying to realise the
ever-recurring destruction of the enormous annual increase,
and asking ourselves what determines, in each individual case,
the death of the many, the survival of the few. We must
think over all the causes of destruction to each organism, to
the seed, the young shoot, the growing plant, the full-grown
tree, or shrub, or herb, and again the fruit and seed ; and among
animals, to the egg or new-born young, to the youthful, and
to the adults. Then, we must always bear in mind that what
goes on in the case of the individual or family group we may

v NATURAL SELECTION 123

observe or think of, goes on also among the millions and
scores of millions of individuals which are comprised in almost
every species ; and must get rid of the idea that clun/r,'
determines which shall live and which die. For, although in
many individual cases death may be due to chance rather
than to any inferiority in those which die first, yet we cannot
possibly believe that this can be the case on the large scale
on which nature w r orks. A plant, for instance, cannot be in-
creased unless there are suitable vacant places its seeds can
grow in, or stations where it can overcome other less vigorous
and healthy plants. The seeds of all plants, by their varied
modes of dispersal, may be said to be seeking out such places
in which to grow; and we cannot doubt that, in the long run,
those individuals whose seeds are the most numerous, have the
greatest powers of dispersal, and the greatest vigour of growth,
will leave more descendants than the individuals of the same
species which are inferior in all these respects, although now
and then some seed of an inferior individual may chance to be
carried to a spot where it can grow and survive. The same
rule will apply to every period of life and to every danger to
which plants or animals are exposed. The best organised, or
the most healthy, or the most active, or the best protected, or
the most intelligent, will inevitably, in the long run, gain an
advantage over those which are inferior in these qualities;
that is, tin jiff*'*/ mil x/irrive, the fittest being, in each particular
case, those which are superior in the special qualities on
which safety depends. At one period of life, or to escape one
kind of danger, concealment may be necessary ; at another
time, to escape another danger, swiftness ; at another, intel-
ligence or cunning ; at another, the power to endure rain or
cold or hunger ; and those which possess all these faculties in
the fullest perfection will generally survive.

Having fully grasped these facts in all their fulness and
in their endless and complex results, we have next to consider
the phenomena of variation, discussed in the third and fourth
chapters ; and it is here that perhaps the greatest difficulty will
be felt in appreciating the full importance of the evidence as set
forth. It has been so generally the practice to speak of
\ ariation as something exceptional and comparatively rare as
an abnormal deviation from the uniformity and stability of the

124 DARWINISM CHAP.

characters of a species and so few even among naturalists
have ever compared, accurately, considerable numbers of
individuals, that the conception of variability as a general
characteristic of all dominant and widespread species, large in
its amount and affecting, not a few, but considerable masses of
the individuals which make up the species, will be to many
entirely new. Equally important is the fact that the vari-
ability extends to every organ and every structure, external
and internal ; while perhaps most important of all is the
independent variability of these several parts, each one vary-
ing without any constant or even usual dependence on, or
correlation with, other parts. No doubt there is some such
correlation in the differences that exist between species and
species more developed wings usually accompanying smaller
feet and vice versa but this is, generally, a useful adaptation
which has been brought about by natural selection, and does
not apply to the individual variability which occurs within
the species.

It is because these facts of variation are so important and
so little understood, that they have been discussed in what
will seem to some readers wearisome and unnecessary detail.
Many naturalists, however, will hold that even more evidence
is required ; and more, to almost any amount, could easily
have been given. The character and variety of that already
adduced will, however, I trust, convince most readers that
the facts are as stated ; while they have been drawn from
a sufficiently wide area to indicate a general principle through-
out nature.

If, now, we fully realise these facts of variation, along with
those of rapid multiplication and the struggle for existence,
most of the difficulties in the way of comprehending how species
have originated through natural selection will disappear. For
whenever, through changes of climate, or of altitude, or of
the nature of the soil, or of the area of the country, any
species are exposed to new dangers, and have to maintain
themselves and provide for the safety of their offspring under
new and more arduous conditions, then, in the variability of
all parts, organs, and structures, no less than of habits and
intelligence, we have the means of producing modifications
which will certainly bring the species into harmony with its

v NATURAL SELECTION 125

new conditions. And if we remember that all such physical
changes are slow and gradual in their operation, we shall see
that the amount of variation which we know occurs in every
new generation will be quite sufficient to enable modification
and adaptation to go on at the same rate. Mr. l>ar\vin
was rather inclined to exaggerate the necessary slowness of
the action of natural selection ; but with the knowledge we
now possess of the great amount and range of individual
variation, there seems no difficulty in an amount of change,
quite equivalent to that which usually distinguishes allied
species, sometimes taking place in less than a century, should
any rapid change of conditions necessitate an equally rapid
adaptation. This may often have occurred, either to im-
migrants into a new land, or to residents whose country has
been cut off by subsidence from a larger and more varied
area over which they had formerly roamed. When no change
of conditions occurs, species may remain unchanged for very long
periods, and thus produce that appearance of stability of species
which is even now often adduced as an argument against
evolution by natural selection, but which is really quite in
harmony with it.

On the principles, and by the light of the facts, now briefly
summarised, we have been able, in the present chapter, to
indicate how natural selection acts, how divergence of char-
acter is set up, how adaptation to conditions at various periods
of life has been effected, how it is that low forms of life
continue to exist, what kind of circumstances are most
favourable to the formation of new species, and, lastly, to
what extent the advance of organisation to higher types is
produced by natural selection. We will now pass on to con-
sider some of the more important objections and difficulties
which have been advanced by eminent naturalists,

CHAPTER VI

DIFFICULTIES AND OBJECTIONS

Difficulty as to smallness of variations As to the right variations occur-
ring when required The beginnings of important organs The mam-
mary glands The eyes of flatfish Origin of the eye Useless or
non-adaptive characters Recent extension of the region of utility in
plants The same in animals Uses of tails Of the horns of deer
Of the scale-ornamentation of reptiles Instability of non-adaptive
characters Delboeuf's law No "specific" character proved to be
useless The swamping effects of intercrossing Isolation as prevent-
ing intercrossing Gulick on the effects of isolation Cases in which
isolation is ineffective.

IN the present chapter I propose to discuss the more obvious
and often repeated objections to Darwin's theory, and to show
how far they affect its character as a true and sufficient
explanation of the origin of species. The more recondite
difficulties, affecting such fundamental questions as the causes
and laws of variability, will be left for a future chapter, after
we have become better acquainted with the applications of the
theory to the more important adaptations and correlations of
animal and plant life.

One of the earliest and most often repeated objections was,
that it was difficult " to imagine a reason why variations tend-
ing in an infinitesimal degree in any special direction should
be preserved," or to believe that the complex adaptation of
living organisms could have been produced " by infinitesimal
beginnings." Now this term " infinitesimal," used by a well-
known early critic of the Origin of Specie-; was never made use
of by Darwin himself, Avho spoke only of variations being
"slight," and of the "small amount" of the variations that might
be selected. Even in using these terms he undoubtedly afforded

CHAP, vi DIFFICULTIES AXD OBJECTIONS 127

grounds for the objection above made, that such small and
slight variations could be of no real use, and would not
determine the survival of the individuals possessing them. AYe
have seen, however, in our third chapter, that even Darwin's
terms were hardly justified ; and that the variability of many im-
portant species is of considerable amount, and may very often
be properly described as large. As this is found to be the
case both in animals and plants, and in all their chief groups
and subdivisions, and also to apply to all the separate parts
and organs that have been compared, we must take it as
proved that the average "//////</ of variability presents no
difficulty whatever in the way of the action of natural selection.
It may be here mentioned that, up to the time of the prepara-
tion of the last edition of Thf Or'ui/n (</'>///</'>, Darwin had
not seen the work of Mr. J. A. Allen of Harvard University
(then only just published), which gave us the first body of accu-
rate comparisons and measurements demonstrating this large
amount of variability. Since then evidence of this nature
has been accumulating, and we are, therefore, now in a far
better position to appreciate the facilities for natural selection,
in this respect, than was Mr. harwin himself.

Another objection of a similar nature is, that the chances
are immensely against the right variation or combination of
variations occurring just when required; and further, that no
variation can be perpetuated that is not accompanied by
several concomitant variations of dependent parts greater
length of a wing in a bird, for example, would be of little use
if unaccompanied by increased volume or contractility of the
muscles which move it. This objection seemed a very strong
one so long as it was supposed that variations occurred singly
and at considerable intervals ; but it ceases to have any weight
now we know that they occur simultaneously in various parts
of the organism, and also in a large proportion of the in-
dividuals which make up the species. A considerable number
of individuals will, therefore, every year possess the required
combination of characters ; and it may also be considered
probable that when the two characters are such that they
always act together, there will be such a correlation between
them that they will frequently vary together. But there is
another consideration that seems to show that this coincident

128 DARWINISM CHAP.

variation is not essential. All animals in a state of nature
are kept, by the constant struggle for existence and the
survival of the fittest, in such a state of perfect health and
usually superabundant vigour, that in all ordinary circumstances
they possess a surplus power in every important organ a
surplus only drawn upon in cases of the direst necessity when
their very existence is at stake. It follows, therefore, that
any additional power given to one of the component parts of
an organ must be useful an increase, for example, either in
the wing muscles or in the form or length of the wing might give
some increased powers of flight ; and thus alternate variations
in one generation in the muscles, in another generation in the
wing itself might be as effective in permanently improving the
powers of flight as coincident variations at longer intervals.
On either supposition, however, this objection appears to have
little weight if we take into consideration the large amount of
coincident variability that has been shown to exist.

The Beginnings of Important Organs.

We now come to an objection Avhich has perhaps been
more frequently urged than any other, and which Darwin
himself felt to have much weight the first beginnings of im-
portant organs, such, for example, as wings, eyes, mammary
glands, and numerous other structures. It is urged, that it
is almost impossible to conceive how the first rudiments of
these could have been of any use, and, if not of use they could
not have been preserved and further developed by natural
selection.

Now, the first remark to be made on objections of this
nature is, that they are really outside the question of the
origin of all existing species from allied species not very far
removed from them, which is all that Darwin -undertook to
prove by means of his theory. Organs and structures such as
those above mentioned all date back to a very remote past,
Avhen the world and its inhabitants were both very different
from what they are now. To ask of a new theory that it
shall reveal to us exactly what took place in remote geological
epochs, and how it took place, is unreasonable. The most
that should be asked is, that some probable or possible mode of
origination should be pointed out in some at least of these

vr niFFlrn.TIKS AND OBJECTIONS 129

difficult cases, and thi- Mi. D;ir\viii has done. One or two of
these may he briefly given here, hut the whole series should
be carefully read by any one who wishes to see how many
curious facts and ohsenat i<>ns have been required in order to
elucidate them ; whence \ve may conclude that further know-
ledge will probably throw light on any difficulties that still

remain. 1

In the case of the mammary glands Mr. Darwin remarks
that it is admitted that the ancestral mammals were allied to
the marsupials. Now in the very earliest mammals, almost
before they really deserved that name, the young may have
been nourished by a fluid secreted by the interior surface of
the marsupial sack, as is helie\ed to be the case with the
fish (Hippocampus) whose eggs are hatched within a some-
what similar sack. This being the case, those individual-
which secreted a more nutritious fluid, and those who-e
young were able to obtain and swallow a more constant supply
by suction, would be more likely to live and come to a health)
maturity, and would therefore be preserved by natural selec-
tion.

In another case which has been adduced as one of special
difficulty, a more complete explanation is given. Soles,
turbots, and other flatfish are, as is well known, unsym-
metrical. They live and move on their sides, the under side
being usually differently coloured from that which is kept
uppermost. Now the eyes of these fish are curiously distorted
in order that both eyes may be on the upper side, where alone
they would be of any use. It was objected by Mr. Mivart
that a sudden transformation of the eye from one side to the
other was inconceivable, while, if the transit were gradual
the first step could be of no use, since this would not remove
the eye from the lower side. But, as Mr. Danvin shows by
reference to the researches of Malm and others, the young of
these fish are quite symmetrical, and during their growth
exhibit to us the whole process of change. This begins by
the fish (owing to the increasing depth of the body) being un-
able to maintain the vertical position, so that it falls on one side.
It then twists the lower eye as much as possible towards the
upper side ; and, the whole bony structure of the head being at

1 See Origin of Species, pp. 176-198.
K

130 DARWINISM CHAP.

this time soft and flexible, the constant repetition of this effort
causes the eye gradually to move round the head till it comes
to the upper side. Now if we suppose this process, which in
the young is completed in a few days or weeks, to have been
spread over thousands of generations during the development of
these fish, those usually surviving whose eyes retained more
and more of the position into which the young fish tried to
twist them, the change becomes intelligible ; though it still
remains one of the most extraordinary cases of degeneration, by
which symmetry which is so universal a characteristic of
the higher animals is lost, in order that the creature may be
adapted to a new mode of life, whereby it is enabled the better
to escape danger and continue its existence.

The most difficult case of all, that of the eye the thought
of which even to the last, Mr. Darwin says, " gave him a cold
shiver " is nevertheless shown to be not unintelligible ;
granting of course the sensitiveness to light of some forms of
nervous tissue. For he shows that there are, in several of the
lower animals, rudiments of eyes, consisting merely of pigment
cells covered with a translucent skin, which may possibly
serve to distinguish light from darkness, but nothing more.
Then we have an optic nerve and pigment cells ; then we
find a hollow filled with gelatinous substance of a convex
f orm the first rudiment of a lens. Many of the succeeding
steps are lost, as would necessarily be the case, owing to the
great advantage of each modification which gave increased
distinctness of vision, the creatures possessing it inevitably
surviving, while those below them became extinct. But we
can well understand how, after the first step was taken, every
variation tending to more complete vision would be preserved
till we reached the perfect eye of birds and mammals. Even
this, as we know, is not absolutely, but only relatively, perfect.
Neither the chromatic nor the spherical aberration is absolutely
corrected ; while long- and short- sightedness, and the various
diseases and imperfections to which the eye is liable, may be
looked upon as relics of the imperfect condition from which
the eye has been raised by variation and natural selection.

These few examples of difficulties as to the origin of remark-
able or complex organs must suffice here ; but the reader who
wishes further information on the matter may study carefully

vi DIFFICULTIES AM) OBJECTIONS 131

the whole of the sixth and seventh chapters of the last edition
of Tit? OrJijiii of Species^ in which these and many other cases
are discussed in considerable detail.

I'xeless or non-<i<Ij>tir<' Clmnicfi />.

Many naturalists seem to lie of opinion that a considerable
number of the diameters which distinguish species are of no
service whatever to their possessors, and therefore eanimt ha\e
been produced or increased by natural selection. Professors
Bronu and Uroca have urged this objection on the continent.
In America, I>r. Cope, the well-known palaeontologist, hashing
since put forth the same objection, declaring that non-adaptive
characters are as numerous as those which are adaptive ; but
he dillers completely from most who hold the same general
opinion in considering that they occur chiefly "in tin-
characters of the classes, orders, families, and other higher
groups;" and the objection, therefore, is quite distinct from
that in which it is urged that ''specific characters " are mostly
useless. More recently, Professor (1. -I. Romanes has urged this
difficulty in his paper on "Physiological Selection " (Junrn.
Li n n. ,S'<'., vol. xix. pp. 338, 344). He says that the characters
"which serve to distinguish allied species are frequently, if
not usually, of a kind with which natural selection can have
had nothing to do," being without any utilitarian significance.
Again he speaks of "the enormous number," and further on of
" the innumerable multitude " of specific peculiarities which
are useless ; and he finally declares that the question needs no
further arguing, " because in the later editions of his works
Mr. Darwin freely acknowledges that a large proportion of
specific distinctions must be conceded to be useless to the
species presenting them."

I have looked in vain in Mr. Darwin's works to find any
such acknowledgment, and I think Mr. Romanes has not
sufficiently distinguished between " useless characters " and
"useless specific distinctions." On referring to all the passages
indicated by him I find that, in regard to specific characters,
Mr. Darwin is very cautious in admitting inutility. His
most pronounced " admissions" on this question are the follow-
ing : " But when, from the nature of the organism and of
the conditions, modifications have been induced which are

132 DARWINISM CHAP.

unimportant for the welfare of the species, they may be, and
apparently often have been, transmitted in nearly the same
state to numerous, otherwise modified, descendants" (Origin, p.
175). The words I have here italicised clearly show that
such characters are usually not " specific," in the sense that
they are such as distinguish species from each other, but are
found in numerous allied species. Again : " Thus a large
yet undefined extension may safely be given to the direct and
indirect results of natural selection ; but I now admit, after
reading the essay of Nageli on plants, and the remarks by
various authors with respect to animals, more especially those
recently made by Professor Broca, that in the earlier editions
of my Origin of Species I perhaps attributed too much to the
action of natural selection or the survival of the fittest. I
have altered the fifth edition of the Origin so as to confine my
remarks to adaptive changes of structure, but I am convinced,
from the light gained during even the last few years, that very
many structures which now appear to us useless, will hereafter be
2'i'i>t'ed to be useful, and will therefore come within the range of
natural selection. Nevertheless I did not formerly consider
sufficiently the existence of structures which, as far as we can at
present judge, are neither beneficial nor injurious ; and this I
believe to be one of the greatest oversights as yet detected in
my work." Now it is to be remarked that neither in these
passages nor in any of the other less distinct expressions of
opinion on this question, does Darwin ever admit that "specific
characters " that is, the particular characters Avhich serve to
distinguish one species from another are ever useless, much
less that " a large proportion of them " are so, as Mr. Romanes
makes him "freely acknowledge." On the other hand, in
the passage which I have italicised he strongly expresses his
view that much of what we suppose to be useless is due to
our ignorance ; and as I hold myself that, as regards many of
the supposed useless characters, this is the true explanation,
it may be well to give a brief sketch of the progress of know-
ledge in transferring characters from the one category to
the other.

We have only to go back a single generation, and not even
the most acute botanist could have suggested a reasonable use,
for each species of plant, of the infinitely varied forms, sizes,

vr DIFFICULTIES AXD OBJECTIONS 133

;iiil colours of tlie Mowers, the shapes and arrangement of the
leaves, and the numerous other external characters of the
whole plant. But since Mr. Darwin showed that plants
gained both in vigour and in fertility by being crossed with
other individuals of the same species, and that this crossing
\\;is usually effected by insects which, in search of nectar or
pollen, carried the pollen from one plant to the flowers of
another plant, almost every detail is found to have a purpose
and a use. The shape, the size, and the colour of the petals,
e\en the streaks and spots with which they are adorned, the
position in which they stand, the movements of the stamens
and pistil at various times, especially at the period of, and
just after, fertilisation, have been proved to be strictly
adaptive in so many cases that botanists now believe that all
the external characters of flowers either are or have been of
use to the species.

It has also been shown, by Kerner and other botanists,
that another set of characteristics have relation to the pre-
vention of ants, slugs, and other animals from reaching the
flowers, because these creatures would devour or injure
them without effecting fertilisation. The spines, hairs,
or sticky glands on the stem or flower-stalk, the curious
hairs or processes shutting up the flower, or sometimes
even the extreme smoothness and polish of the outside of
the petals so that few insects can hang to the part, have
Keen shown to be related to the possible intrusion of
these "unbidden guests." 1 And, still more recently, attempts
have been made by Grant Allen and Sir John Lubbock
to account for the innumerable forms, textures, and groupings
of leaves, by their relation to the needs of the plants
themselves ; and there can be little doubt that these
attempts will be ultimately successful. Again, just as flowers
have been adapted to secure fertilisation or cross-fertilisation,
fruits have been developed to assist in the dispersal of seeds ;
and their forms, sizes, juices, and colours can be shown to be
specially adapted to secure such dispersal by the agency of
birds and mammals ; while the same end is secured in other

1 See Kerner's Flowers and their Unbidden Guests for numerous other
structures and peculiarities of plants which are shown to be adaptive and
useful.

134 DARWINISM CHAP.

cases by downy seeds to be wafted through the air, or by
hooked or sticky seed-vessels to be carried away, attached to
skin, wool, or feathers.

Here, then, we have an enormous extension of the region of
utility in the vegetable kingdom, and one, moreover, .which
includes almost all the specific characters of plants. For the
species of plants are usually characterised either by differences
in the form, size, and colour of the flowers, or of the fruits ;
or. by peculiarities in the shape, size, dentation, or arrange-
ment of the leaves ; or by peculiarities in the spines, hairs, or
down with which various parts of the plant are clothed. In
the case of plants it must certainly be admitted that " specific "
characters are pre-eminently adaptive ; and though there may
be some which are not so, yet all those referred to by Darwin
as having been adduced by various botanists as useless, either
pertain to genera or higher groups, or are found in some
plants of a species only that is, are individual variations not
specific characters.

In the case of animals, the most recent wide extension of
the sphere of utility has been in the matter of their colours
and markings. It was of course always known that certain
creatures gained protection by their resemblance to their
normal surroundings, as in the case of white arctic animals,
the yellow or brown tints of those living in deserts, and the
green hues of many birds and insects surrounded by tropical
vegetation. But of late years these cases have been greatly
increased both in number and variety, especially in regard to
those which closely imitate special objects among which they
live ; and there are other kinds of coloration which long
appeared to have no use. Large numbers of animals, more
especially insects, are gaudily coloured, either with vivid hues
or with striking patterns, so as to be very easily seen. Now
it has been found, that in almost all these cases the creatures
possess some special quality which prevents their being
attacked by the enemies of their kind whenever the
peculiarity is known ; and the brilliant or conspicuous colours
or markings serve as a warning or signal flag against attack.
Large numbers of insects thus coloured are nauseous and
inedible ; others, like wasps and bees, have stings ; others are
too hard to be eaten by small birds ; while snakes with

vi DIFFICULTIES AND <>P,.IK<TIONS 135

poisonous fangs usually have some characteristic either of
rattle, hood, or unusual colour, which indicates that they had
better be left alone.

But there is yet another form of coloration, which
consists in special markings Viands, spots, or patches of white,
or of bright colour, which vary in every species, and are often
concealed when the creature is at rest Itut displayed when in
motion, as in the case of the bands and spots so frequent on
the wings and tails of birds. Now these specific markings
are believed, with good reason, to serve the purpose of enabling
each species to be i[iiickly recognised, even at a distance, 1>\
its fellows, especially the parents by their young and the t \\ o
sexes by each other ; and this recognition mu>t often be an
important factor in securing the safety of individuals, and
therefore the wellbeing and continuance of the species.
These interesting peculiarities will be more fully described in
a future chapter, but they are briefly referred to here in
order to show that the most common of all the characters by
which species are distinguished from each other their colours
and markings can be shown to be adaptive or utilitarian in
their nature.

But besides colour there are almost always some structural
characters which distinguish species from species, and, as re-
gards many of these also, an adaptive character can be often
discerned. In birds, for instance, we have differences in the
size or shape of the bill or the feet, in the length of the wing
or the tail, and in the proportions of the several feathers of
which these organs are composed. All these differences in
the organs on which the very existence of birds depends,
which determine the character of flight, facility for running
or climbing, for inhabiting chiefly the ground or trees, and
the kind of food that can be most easily obtained for
themselves and their offspring, must surely be in the highest
degree utilitarian ; although in each individual case we, in our
ignorance of the minutiae of their life-history, may be quite
unable to see the use. In mammalia specific differences other
than colour usually consist in the length or shape of the ears
and tail, in the proportions of the limbs, or in the length
and quality of the hair on different parts of the body. As
regards the ears and tail, one of the objections by Professor

136 DARWINISM CHAP.

Bronn relates to this very point. He states that the length of
these organs differ in the various species of hares and of mice,
and he considers that this difference can be of no service
whatever to their possessors. But to this objection Darwin
replies, that it has been shown by Dr. Schobl that the ears of
mice " are supplied in an extraordinary manner with nerves,
so that they no doubt serve as tactile organs." Hence, when
we consider the life of mice, either nocturnal or seeking their
food in dark and confined places, the length of the ears
may be in each case adapted to the particular habits and
surroundings of the species. Again, the tail, in the larger
mammals, often serves the purpose of driving off flies and
other insects from the body ; and when we consider in how
many parts of the world flies are injurious or even fatal to
large mammals, we see that the peculiar characteristics of this
organ may in each case have been adapted to its requirements
in the particular area where the species was developed. The
tail is also believed to have some use as a balancing organ,
which assists an animal to turn easily and rapidly, much as
our arms are used when running ; while in whole groups it is
a prehensile organ, and has become modified in accordance
with the habits and needs of each species. In the case of
mice it is thus used by the young. Darwin informs us that
the late Professor Henslow kept some harvest- mice in con-
finement, and observed that they frequently curled their tails
round the branches of a bush placed in the cage, and thus
aided themselves in climbing ; while Dr. Giinther has actually
seen a mouse suspend itself by the tail (Origin, p. 189).

Again, Mr. Lawson Tait has called attention to the use of
the tail in the cat, squirrel, yak, and many other animals as
a means of preserving the heat of the body during the
nocturnal and the winter sleep. He says, that in cold weather
animals with long or bushy tails will be found lying curled up,
with their tails carefully laid over their feet like a rug, and
with their noses buried in the fur of the tail, which is thus
used exactly in the same way and for the same purpose as we
use respirators. 1

Another illustration is furnished by .the horns of deer
which, especially when very large, have been supposed to be
1 Nature, vol. xx. p. 603.

vi DIFFICULTIES AND >M.I K< TIOXS 137

a danger to the animal in passing rapidly through dense
thickets. But Sir James Hector states, that the wapiti, in
North America, throws back its head, thus placing the horns
along the sides of the back, and is then enabled to rush
through the thickest forest with great rapidity. The brow-
antlers protect the face and eyes, while the widely spreading
horns prevent injury to the neck or Hanks. Thus an organ
which was certainly developed as a sexual weapon, has been
so guided and modified during its increase in size as to be of
use in other ways. A similar use of the antlers of deer
has been observed in India. 1

The various classes of facts now referred to serve to show
us that, in the case of the two higher groups mammalia
and birds almost all the characters by which species are
distinguished from each other are, or may be, adaptive. It is
these two classes of animals which have been most studied
and whose life-histories are supposed, to be most fully known,
yet even here the assertion of inutility, by an eminent
naturalist, in the case of two important organs, has been
sufficiently met by minute details either in the anatomy or in
the habits of the groups referred to. Such a fact as this,
together with the extensive series of characters already
enumerated which have been of late years transferred from
the "useless" to the "useful" class, should convince us, that
the assertion of " inutility ' in the case of any organ or
peculiarity which is not a rudiment or a correlation, is not,
and can never be, the statement of a fact, but merely an
expression of our ignorance of its purpose or origin. 2

re, vol. xxxviii. p. 328.

2 A very remarkable illustration of function in an apparently useless
ornament is given liy Semper. He says, "It is known that the skin of
reptiles endoses the body with scales. These scales are distinguished by
v.-ry various sculpturing*, highly characteristic of the different species.
Irrespective of their systematic significance they appear to be of no value in
the life of the animal ; indeed, they are viewed as ornamental without regard
to the fact that they are microscopic and much too delicate to be visible to
other animals of their own species. It might, therefore, seem hopeless to show
the necessity for their existence on Darwinian principles, and to prove that
they are physiologically active organs. Nevertheless, recent investigations on
this point have furnished evidence that this is possible.

" It is known that many reptiles, and above all the snakes, cast off the
whole skin at once, whereas human beings do so by degrees. If by any
accident they are prevented doing so, they infallibly die, because the old

138 DARWINISM CHAP.

Instability of Non-adaptive Characters.

One very weighty objection to the theory that specific
characters can ever be Avholly useless (or wholly uncon-
nected with useful organs by correlation of growth) appears
to have been overlooked by those who have maintained
the frequency of such characters, and that is, their almost
necessary instability. Darwin has remarked on the extreme
variability of secondary sexual characters such as the horns,
crests, plumes, etc., which are found in males only,- the
reason being, that, although of some use, they are not
of such direct and vital importance as those adaptive
characters on which the wellbeing and very existence of the
animals depend. But in the case of wholly useless structures,

skin has grown so tough and hard that it hinders the increase in volume
which is inseparable from the growth of the animal. The casting of the
skin is induced by the formation on the surface of the inner epidermis, of a
layer of very fine and equally distributed hairs, which evidently serve the
purpose of mechanically raising the old skin by their rigidity and position.
These hairs then may be designated as casting hairs. That they are destined
and calculated for this end is evident to me from the fact established by Dr.
Braun, that the casting of the shells of the river cray-fish is induced in exactly
the same manner by the formation of a coating of hairs which mechanically
loosens the old skin or shell from the new. Now the researches of Braun and
Cartier have shown that these casting hairs which serve the same purpose in
two groups of animals so far apart in the systematic scale after the castiug,
are partly transformed into the concentric stripes, sharp spikes, ridges, or
warts which ornament the outer edges of the skin-scales of reptiles or the
carapace of crabs." 1 Professor Semper adds that this example, with many
others that might be quoted, shows that we need not abandon the hope of
explaining morphological characters on Darwinian principles, although their
nature is often difficult to understand.

During a recent discussion of this question in the pages of Nature, Mr.
St. George Mivart adduces several examples of what he deems useless specific
characters. Among them are the aborted index finger of the lemurine Potto,
and the thumbless hands of Colobus and Ateles, the "life-saving action" of
either of which he thinks incredible. These cases suggest two remarks. In
the first place, they involve generic, not specific, characters ; and the three
genera adduced are somewhat isolated, implying considerable antiquity and
tin; extinction of many allied forms. This is important, because it affords
ample time for great changes of conditions since the structures in question
originated ; and without a knowledge of these changes we can never safely
assert that any detail of structure could not have been useful. In the second
place, all three are cases of aborted or rudimentary organs ; and these are
admitted to be explained by non-use, leading to diminution of size, a further
reduction being brought about by the action of the principle of economy

1 The Natural Conditions of Existence as they affect Animal Life, p. 19.

vi DIFFICULTIES AND OBJECTIONS 139

which are not rudiments of once useful organs, we cannot see
what there is to ensure any amount of constancy or stability.
One of the cases on which Mr. Romanes lays great stress in
his paper on "Physiological Selection" (Jomn.Linn.Soc.,\o\. xi.v.
}>. 384) is that of the fleshy appendages on the corners of the
jaw of Normandy pigs and of some other breeds. But it is
expressly stated that they are not constant; they appear
"frequently," or "occasionally," they are "not strictly
inherited, for they occur or fail in animals of the same litter ;"
and they are not always symmetrical, sometimes appearing
on one side of the face alone. Now whatever may be the
cause or explanation of these anomalous appendages they
cannot be classed with " specific characters," the most
essential features of which are, that they are symmetrical,

of growth. Mnt, when so reduced, the rudiment might be inconvenient or even
hurtful. and then natural selection would aid in its complete abortion ; in
other words the abortion of the part would be iimfn/, and would therefore be
subject to the law of survival of the fittest. The genera Ateles and Colobus
are two of the most puiely arboreal types of monkeys, and it is not difficult
to convive that the constant use of the elongated fingers for climbing from
tree to tree, and catching on to branches while making great leaps, might
ivi[iiire all the nervous energy and muscular growth to be directed to the
lingers, the small thumb remaining useless. The case of the Potto is more
ditlicult, both because it is, presumably, a more ancient type, and its actual life-
history and habits are completely unknown. These cases are, therefore, not
at all to the point as proving that positive specific characters not mere
rudiments characterising whole genera are in any case useless.

Mr. Mivart further objects to the alleged rigidity of the action of natural
selection, because wounded or malformed animals have been found which had
cxidciitly lived a considerable time in their imperfect condition. But this
simply proves that they were living under a temporarily favourable environ-
ment, and that the real struggle for existence, in their case, had not yet
taken place. We must surely admit that, when the pinch came, and when
perfectly formed stoats were dying for want of food, the one-footed animal,
referred to by Mr. Mivart, would be among the first to succumb ; and the
same remark will apply to his abnormally toothed hares and rheumatic
monkeys, which might, nevertheless, get on very well under favourable
conditions. Tin: struggle for existence, under which all animals and plants
have been developed, is intermittent, and exceedingly irregular in its incidence
and severity. It is most severe and fatal to the young ; but when an animal
has once reached maturity, and especially when it has gained experience by
several years of an eventful existence, it may be able to maintain itself under
conditions which would be fatal to a young and inexperienced creature of the
same species. The examples adduced by Mr. Mivart do not, therefore, in
any way impugn the hardness of nature as a taskmaster, or the extreme
severity of the recurring struggle for existence. 1

1 See Nature, vol. xxxix. p. 127.

140 DARWINISM CHAP.

that they are inherited, and that they are constant. Ad-
mitting that this peculiar appendage is (as Mr. Romanes says
rather confidently, " we happen to know it to be ") wholly
useless and meaningless, the fact would be rather an argument
against specific characters being also meaningless, because the
latter never have the characteristics which this particular
variation possesses.

These useless or non-adaptive characters are, apparently, of
the same nature as the " sports " that arise in our domestic
productions, but which, as Mr. Darwin says, without the aid
of selection would soon disappear ; while some of them may
be correlations with other characters which are or have been
useful. Some of these correlations are very curious. Mr.
Tegetmeier informed Mr. Darwin that the young of white, yellow,
or dun-coloured pigeons are born almost naked, whereas other
coloured pigeons are born well clothed with down. Now, if
this difference occurred between wild species of different colours,
it might be said that the nakedness of the young could not be
of any use. But the colour with which it is correlated might,
as has been shown, be useful in many ways. The skin and its
various appendages, as horns, hoofs, hair, feathers, and teeth,
are homologous parts, and are subject to very strange correla-
tions of growth. In Paraguay, horses with curled hair occur,
and these always have hoofs exactly like those of a mule, while
the hair of the mane and tail is much shorter than usual.
Now, if any one of these characters were useful, the others
correlated with it might be themselves useless, but Avould still
be tolerably constant because dependent on a useful organ.
So the tusks and the bristles of the boar are correlated and
vary in development together, and the former only may be
useful, or both may be useful in unequal degrees.

The difficulty as to how individual differences or sports can
become fixed and perpetuated, if altogether useless, is evaded
by those who hold that such characters are exceedingly common.
Mr. Romanes says that, upon his theory of physiological selec-
tion, "it is quite intelligible that when a varietal form is
differentiated from its parent form by the bar of sterility, any
little meaningless peculiarities of structure or of instinct should
at first be allowed to arise, and that they should then be allowed
to perpetuate themselves by heredity," until they are finally

vi DIFFICULTIES AND OBJECTIONS 141

eliminated by disuse. But this is entirely begging the ques-
tion. Do meaningless peculiarities, which we admit often arise
as spontaneous variations, ever perpetuate themselves in all
the individuals constituting a variety or race, without selec-
tion either human or natural ? Such characters present them-
selves as unstable variations, and as such they remain, unless
preserved and accumulated by selection ; and they can there-
fore never become " specific " characters unless they are strictly
correlated with some useful and important peculiarities.

As bearing upon this question we may refer to what is
termed Delba-uf's law, which has been thus briefly stated by
Mr. Murphv in his work on I/iil>it mn/ Lif'//i;/nn't', p.
241.

" If, in any species, a number of individuals, bearing a
ratio not infinitely small to the entire number of births, are- in
every gem-ration born with a particular variation which is
neither beneficial nor injurious, and if it is not counteracted by
reversion, then tin- proportion of the new variety to the original
form will increase till it approaches indefinitely near to
equality."

It is not impossible that some definite varieties, such as the
melanic form of the jaguar and the bridled variety of the guille-
mot are due to this cause ; but from their very nature such
varieties are unstable, and are continually reproduced in
varying proportions from the parent forms. They can,
therefore, never constitute species unless the variation in
question becomes beneficial, when it will be fixed by natural
sele'ction. Darwin, it is true, says "There can be little
doubt that the tendency to vary in the same manner has often
been so strong that all the individuals of the same specie-,
have been similarly modified without the aid of any form of
selection." l But no proof whatever is offered of this state-
ment, and it is so entirely opposed to all we know of the facts
of variation as given by Darwin himself, that the important
word " all " is probably an oversight.

On the whole, then, I submit, not only has it not been
proved that an " enormous number of specific peculiarities "
are useless, and that, as a logical result, natural selection is
" not a theory of the origin of species," but only of the origin

1 Origin of Sjxcies, p. 72.

142 DARWINISM CHAP.

of adaptations which are usually common to many species, or,
more commonly, to genera and families ; but, I urge further,
it has not even been proved that any truly " specific "
characters those which either singly or in combination dis-
tinguish each species from its nearest allies are entirely un-
adaptive, useless, and meaningless ; while a great body of facts
on the one hand, and some weighty arguments on the other,
alike prove that specific characters have been, and could only
have been, developed and fixed by natural selection because of
their utility. We may admit, that among the great number of
variations and sports which continually arise many are altogether
useless without being hurtful ; but no cause or influence has
been adduced adequate to render such characters fixed and
constant throughout the vast number of individuals which con-
stitute any of the more dominant species. 1

The Swamping Effects of Intercrossing.

This supposed insuperable difficulty was first advanced in
an article in the North British Review in 1867, and much
attention has been. attracted to it by the acknowledgment of
Mr. Darwin that it proved to him that " single variations,"
or what are usually termed "sports," could very rarely, if ever,
be perpetuated in a state of nature, as he had at first thought
might occasionally be the case. But he had always considered
that the chief part, and latterly the whole, of the materials
with which natural selection works, was afforded by individual
variations, or that amount of ever fluctuating variability Avhich
exists in all organisms and in all their parts. Other writers
have urged the same objection, even as against individual
variability, apparently in total ignorance of its amount and
range ; and quite recently Professor G. J. Komanes has adduced

1 Darwin's latest expression of opinion on this question is interesting, since
it shows that he was inclined to return to his earlier view of the general, or
universal, utility of specific characters. In a letter to Semper (30th Nov.
1878) he writes: "As our knowledge advances, very slight differences, con-
sidered by systematists as of no importance in structure, are continually
found to be functionally important ; and I have been especially struck with
this fact in the case of plants, to which my observations have, of late years,
been confined. Therefore it seems to me rather rash to consider slight
differences between representative species, for instance, those inhabiting the
different islands of the same archipelago, as of no functional importance, ami
as not in any way due to natural selection " (Life of Darwin, vol. iii. p. 161).

vi DIFFICULTIES AND OBJECTIONS 143

it as one of the difficulties which can alone he overcome by his
theory of physiological selection. He urges, that the same
variation does not occur simultaneously in a number of
individuals inhabiting the same area, and that it is mere
assumption to say it does; Avhile he admits that "if the
assumption were -ranted there would lie an end of the present
difficulty ; for if a sufficient number of individuals were thus
simultaneously and similarly modified, there need be no longer
any danger of the variety becoming swamped by intercrossing."
I must again refer my readers to my third chapter for the
proof that such simultaneous variability is not an assumption
but a fact ; but, even admitting this to be proved, the problem
is not altogether solved, and there is so much misconception
regarding variation, and the actual process of the origin of
new species is so obscure, that >ome further discussion and
elucidation of the subject are desirable.

In one of the preliminary chapters of Mr. Seebohm's recent
work on the t'lnir<n1rii<lir, lie discusses the differentiation of
species; and he expresses a rather widespread view among
naturalists when, speaking of the swamping effects of inter-
crossing, he adds: "This is unquestionably a very grave
difficulty, to my mind an absolutely fatal one, to the theory of
accidental variation." And in another passage he says : " The
simultaneous appearance, and its repetition in successive genera-
tions, of a beneficial variation, in a large number of individuals in
the same locality, cannot possibly be ascribed to chance." These
remarks appear to me to exhibit an entire misconception of the
facts of variation as they actually occur, and as they have been
utilised by natural selection in the modification of species. I
have already shown that every part of the organism, in common
species, does vary to a very considerable amount, in a large
number of individuals, and in the same locality ; the only point
that remains to be discussed is, whether any or most of these
variations are "beneficial." But every one of these variations
consists either in increase or diminution of size or power of the
organ or faculty that varies ; they can all be divided into a
more effective and a less effective group that is, into one that
is more beneficial or less beneficial. If less size of body would
be beneficial, then, as half the variations in size are above and
half below the mean or existing standard of the species, there

144 DARWINISM CHAP.

would be ample beneficial variations ; if a darker colour or
a longer beak or wing were required, there are always a con-
siderable number of individuals darker and lighter in colour
than the average, with longer or with shorter beaks and wings,
and thus the beneficial variation must always be present. And
so with every other part, organ, function, or habit ; because, as
variation, so far as we know, is and always must be in the two
directions of excess and defect in relation to the mean amount,
whichever kind of variation is wanted is always present in some
degree, and thus the difficulty as to " beneficial " variations
occurring, as if they were a special and rare class, falls to the
ground. No doubt some organs may vary in three or perhaps
more directions, as in the length, breadth, thickness, or curva-
ture of the bill. But these may be taken as separate varia-
tions, each of which again occurs as " more " or " less "; and thus
the " right " or " beneficial " or " useful " variation must always
be present so long as any variation at all occurs ; and it has not
yet been proved that in any large or dominant species, or in
any part, organ, or faculty of such species, there is no variation.
And even were such a case found it would prove nothing, so
long as in numerous other species variation was shown to exist ;
because we know that great numbers of species and groups
throughout all geological time have died out, leaving no
descendants ; and the obvious and sufficient explanation of this
fact is, that they did not vary enough at the time when varia-
tion was required to bring them into harmony with changed
conditions. The objection as to the "right" or "beneficial"
variation occurring when required, seems therefore to have no
weight in view of the actual facts of variation.

Isolation to prevent Intercrossing.

Most writers on the subject consider the isolation of a
portion of a species a very important factor in the formation
of new species, while others maintain it to be absolutely
essential. This latter view has arisen from an exasperated

OO

opinion as to the power of intercrossing to keep down any
variety or incipient species, and merge it in the parent stock.
But it is evident that this can only occur with varieties which
are not useful, or which, if useful, occur in very small
numbers; and from this kind of variations it is clear that

vi DIFFICULTIES AND OBJECTIONS 145

new species do not arise. Complete isolation, as in an oceanic
island, will no doubt enable natural selection to act more
rapidly, for several reasons. In the first place, the absence of
competition \vill for some time allow the new immigrants to
increase rapidly till they reach the limits of subsistence.
They will then struggle among themselves, and by survival of
the fittest will quickly become adapted to the new conditions
of their environment. Organs which they formerly needed,
to defend themselves against, or to escape from, enemies,
being no longer required, would be encumbrances to be got
rid of, while the power of appropriating and digesting new
and varied food would rise in importance. Thus we mav
explain the origin of so many flightless and rather bulky birds
in oceanic islands, as the dodo, the cassowary, and the extinct
moas. Again, while; this process was going on, the complete
isolation would prevent its being checked by the immigration
of new competitors or enemies, which would be very likely to
occur in a continuous area ; while, of course, any intercrossing
with the original unmodified stock would be absolutely pre-
vented. If, now, before this change has gone very far, the
variety spreads into adjacent but rather distant islands, the
somewhat different conditions in each may lead to the
development of distinct forms constituting what are termed
representative species; and these we find in the separate
islands of the ( ialapagos, the West Indies, and other ancient
groups of islands.

But such cases as these will only lead to the production of
a few peculiar species, descended from the original settlers
which happened to reach the islands ; whereas, in wide areas,
and in continents, we have variation and adaptation on a much
larger scale ; and, whenever important physical changes de-
mand them, with even greater rapidity. The far greater
complexity of the environment, together with the occurrence of
variations in constitution and habits, will often allow of
effective isolation, even here, producing all the results of actual
physical isolation. As we have already explained, one of the
most frequent modes in which natural selection acts is by
adapting some individuals of a species to a somewhat different
mode of life, whereby they are able to seize upon unappropriated
places in nature, and in so doing they become practically

L

146 DARWINISM CHAP.

isolated from their parent form. Let us suppose, for example,
that one portion of a species usually living in forests ranges
into the open plains, and finding abundance of food remains
there permanently. So long as the struggle for existence is
not exceptionally severe, these two portions of the species may
remain almost unchanged ; but suppose some fresh enemies are
attracted to the plains by the presence of these new immi-
grants, then variation and natural selection would lead to the
preservation of those individuals best able to cope with the
difficulty, and thus the open country form would become
modified into a marked variety or into a distinct species ;
and there would evidently be little chance of this modifica-
tion being checked by intercrossing with the parent form
which remained in the forest.

Another mode of isolation is brought about by the variety
either owing to habits, climate, or constitutional change-
breeding at a slightly different time from the parent species.
This is known to produce complete isolation in the case of
many varieties of plants. Yet another mode of isolation is
brought about by changes of colour, and by the fact that in a
wild state animals of similar colours prefer to keep together
and refuse to pair with individuals of another colour. The
probable reason and utility of this habit will be explained
in another chapter, but the fact is well illustrated by the
cattle which have run wild in the Falkland Islands. These
are of several different colours, but each colour keeps in a
separate herd, often restricted to one part of the island ;
and one of these varieties the mouse-coloured is said to
breed a month earlier than the others ; so that if this
variety inhabited a larger area it might very soon be estab-
lished as a distinct race or species. 1 Of course where the
change of habits or of station is still greater, as when a ter-
restrial animal becomes sub-aquatic, or when aquatic animals
come to live in tree- tops, as with the frogs and Crustacea
described at p. 118, the danger of intercrossing is reduced to
a minimum.

Several writers, however, not content with the indirect
effects of isolation here indicated, maintain that it is in itself
a cause of modification, and ultimately of the origination of
1 See Variation of Animals and Plants, vol. i. p. 86.

vi DIFFICULTIES AND OBJECTIONS 147

new species. This WMS the keynote of Mr. Vernon Wollaston's
ess;iy on " Yariat i.m of Species," published in 1856, and it is
adopted by the Rev. J. G. Guliek in liis paper on " 1 >i \ersity
of Evolution under one Set of External Conditions " (Jot/rn.
Linn. Soc. /""/., vol. xi. p. 496). The idea seems to be
that there is an inherent tendency to variation in certain
divergent lines, and that when one portion of a species is
isolated, even though under identical conditions, that tendency
sets up a divergence which carries that portion farther and
farther away from the original species. This view is held to
be supported by the case of the land shells of the Sandwich
Islands, which certainly present some very remarkable
phenomena. In this comparatively small area there are
about 300 species of land shells, almost all of which belong
to one family (or sub-family), the Achatinelliche, found
nowhere else in the world. The interesting point is the
extreme restriction of the species ;i nd varieties. The
average range of each species is only five or six miles,
while some ;ire restricted to l>nt one or two square miles,
and only a very few range over a whole island. The forest
region that extends over one of the mountain -ranges of the
island of Oahu, is about forty miles in length and five or six
miles in breadth; and this small territory furnishes about
17.") species, represented by 700 or 800 varieties. Mr.
Guliek states, that the \egetation of the different valleys
on the same side of this range is much the same, yet each
has a molluscan fauna differing in some degree from that
of any other. "We frequently find a genus represented
in several successive valleys by allied species, sometimes
feeding on the same, sometimes on different plants. In
every such case the valleys that are nearest to each other
furnish the most nearly allied forms ; and a full set of the
varieties of each species presents a minute gradation of forms
between the more divergent types found in the more widely
separated localities." He urges, that these constant differences
cannot be attributed to natural selection, because they occur
in different valleys on the same side of the mountain, where
food, climate, and enemies are the same ; and also, because
there is no greater difference in passing from the rainy to the
dry side of the mountains than in passing from one valley to

148 DARWINISM CHAP.

another on the same side an equal distance apart. In a very
lengthy paper, presented to the Linnean Society last year, on
"Divergent Evolution through Cumulative Segregation," Mr.
Gtilick endeavours to work out his views into a complete
theory, the main point of which may perhaps be indicated by
the following passage : " No two portions of a species possess
exactly the same average character, and the initial differences
are for ever reacting on the environment and on each other
in such a way as to ensure increasing divergence in each
successive generation as long as the individuals of the two
groups are kept from intercrossing." 1

It need hardly be said that the views of Mr. Darwin and
myself are inconsistent with the notion that, if the environment
were absolutely similar for the two isolated portions of the
species, any such necessary and constant divergence would
take place. It is an error to assume that what seem to
us identical conditioiis are really identical to such small
and delicate organisms as these land molluscs, of whose
needs and difficulties at each successive stage of their existence,
from the freshly-laid egg up to the adult animal, we are so
profoundly ignorant. The exact proportions of the various
species of plants, the numbers of each kind of insect or of
bird, the peculiarities of more or less exposure to sunshine
or to wind at certain critical epochs, and other slight
differences which to us are absolutely immaterial and un-
recognisable, may be of the highest significance to these
humble creatures, and be quite sufficient to require some
slight adjustments of size, form, or colour, which natural
selection will bring about. All we know of the facts of
variation leads us to believe that, without this action of
natural selection, there would be produced over the whole area
a series of inconstant varieties mingled together, not a distinct
segregation of forms each confined to its own limited area.

Mr. Darwin has shown that, in the distribution and
modification of species, the biological is of more importance
than the physical environment, the struggle with other
organisms being often more severe than that with the forces
of nature. This is particularly evident in the case of plants,
many of which, when protected from competition, thrive in a
1 Journal of the Linnean Society, Zoology, vol. xx. ]>. 215.

vi DIFFICULTIES AND OBJECTIONS 149

soil, climate, and atmosphere widely different from those of
their native habitat. Thus, many alpine plants only found
near perpetual snow thrive well in our gardens at the level of
the sea ; as do the tritomas from the sultry plains of South
Africa, the yuccas from the arid hills of Texas and Mexico, and
the fuchsias from the damp and dreary shores of the Straits of
Magellan. It has been well said that plants do not live
where they like, but where they can ; and the same remark Avill
apply to the animal world. Horses and cattle run wild and
thrive both in North and South America; rabbits, once con-
fined to the south of Europe, have established themselves in
our own country and in Australia; while the domestic fowl, a
native of tropical India, thrives well in every part of the
temperate zone.

If, then, we admit that when one portion of a species is
separated from the rest, there will necessarily be a slight
difference in the average characters of the two portions, it
does not follow that this difference has much if any effect
upon the characteristics that are developed by a long period
of isolation. In the first place, the difference itself will
necessarily be very slight unless there is an exceptional
amount of variability in the species ; and in the next place,
if the average characters of the species are the expression of
its exact adaptation to its whole environment^ then, given
a precisely similar environment, and the isolated portion will
inevitably be brought back to the same average of characters.
But, as a matter of fact, it is impossible that the environment
of the isolated portion can be exactly like that of the bulk of
the species. It cannot be so physically, since no two separated
areas can be absolutely alike in climate and soil ; and even if
these are the same, the geographical features, size, contour, and
relation to winds, seas, and rivers, would certainly differ.
Biologically, the differences are sure to be considerable. The
isolated portion of a species will almost always be in a much
smaller area than that occupied by the species as a whole, hence
it is at once in a different position as regards its own kind.
The proportions of all the other species of animals and plants
are also sure to differ in the two areas, and some species will
almost always be absent in the smaller which are present in
the larger country. These differences will act and react on

150 DARWINISM CHAP

the isolated portion of the species. The struggle for existence
will differ in its severity and in its incidence from that which
affects the bulk of the species. The absence of some one
insect or other creature inimical to the young animal or plant
may cause a vast difference in its conditions of existence, and
may necessitate a modification of its external or internal
characters in quite a different direction from that which
happened to be present in the average of the individuals
which were first isolated.

On the whole, then, we conclude that, while isolation is an
important factor in effecting some modification of species, it is
so, not on account of any effect produced, or influence exerted
by isolation per se, but because it is always and necessarily
accompanied by a change of environment, both physical and
biological. Natural selection will then begin to act in
adapting the isolated portion to its new conditions, and will
do this the more quickly and the more effectually because of
the isolation. We have, however, seen reason to believe that
geographical or local isolation is by no means essential to the
differentiation of species, because the same result is brought
about by the incipient species acquiring different habits or
frequenting a different station ; and also by the fact that
different varieties of the same species are known to prefer to
pair with their like, and thus to bring about a physiological
isolation of the most effective kind. This part of the subject
will be again referred to when the very difficult problems
presented by hybridity are discussed. 1

Cases in which Isolation is Ineffective.

One objection to the views of those who, like Mr. Gulick,
believe isolation itself to be a cause of modification of species
deserves attention, namely, the entire absence of change where,

1 In Mr. Gulick's last paper (Journal of Linn. Soc. Zool.,\o\. xx. pp. 189-
274) he discusses the various forms of isolation above referred to, under no
less than thirty-eight different divisions and subdivisions, with an elaborate
terminology, and he argues that these will frequently bring about divergent
evolution without any change in the environment or any action of natural
selection. The discussion of the problem here given will, I believe, sufficiently
expose the fallacy of his contention ; but his illustration of the varied and
often recondite modes by which practical isolation may be brought about,
may help to remove one of the popular difficulties iu the way of the action
of natural selection in the origination of species.

vi DIFFICULTIES AND OBJECTIONS 151

if this were a mm c<>u.<u, Ave should expect to find it. In
Ireland we have an excellent test case, for we know that it
has been separated from Britain since the end of the glacial
epoch, certainly many thousand years. Yet hardly one of
its mammals, reptiles, or land molluscs has undergone the
slightest change, even although there is certainly a distinct
difference in the environment both inorganic and organic.
That changes have not occurred through natural selection, is
perhaps due to the less severe struggle for existence owing to
the smaller number of competing species; but, if isolation
itself were an efficient cause, acting continuously and cumula-
tively, it is incredible that a decided change should not have
been produced in thousands of years. That no such change has
occurred in tin'-, and many other cases of isolation, seems to
prove that it is not in itself a cause of modification.

There yet remain a number of ditliculties and objections
relating to the question of hybridity, which are so important
as to require a separate chapter for their adequate discussion.

CHAPTER VII

ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES
AND THE USUAL STERILITY OF THEIR HYBRID OFFSPRING

Statement of the problom Extreme susceptibility of the reproductive
functions Reciprocal crosses Individual differences in respect to
cross -fertilisation Dimorphism and trimorphism among plants
Cases of the fertility of hybrids and of the infertility of mongrels
-The effects of close inter-breeding Mr. Huth's objections Fertile
hybrids among animals Fertility of hybrids among plants Cases of
sterility of mongrels Parallelism between crossing and change of
conditions Remarks on the facts of hybridity Sterility due to
changed conditions and usually correlated with other characters
Correlation of colour with constitutional peculiarities The isolation
of varieties by selective association The influence of natural selection
upon sterility and fertility Physiological selection Summary and
concluding remarks.

ONE of the greatest, or perhaps we may say the greatest, of
all the difficulties in the way of accepting the theory of
natural selection as a complete explanation of the origin of
species, has been the remarkable difference between varieties
and species in respect of fertility when crossed. Generally
speaking, it may be said that the varieties of any one species,
however different they may be in external appearance, are
perfectly fertile when crossed, and their mongrel offspring ore
equally fertile when bred among themselves ; while distinct
species, on the other hand, however closely they may resemble
each other externally, are usually infertile when crossed, and
their hybrid offspring absolutely sterile. This used to be
considered a fixed law of nature, constituting the absolute test
and criterion of a species as distinct from a variety ; and so
long as it was believed that species were separate creations, or

CHAP, vii OX TMK INFERTILITY OF CROSSES 153

at nil e\ents had an origin quite distinct from that of varieties,
this law could have no exception-. because, if any two species
had been found to lie fertile when crossed and their hybrid
offspring to be also fertile, thi> fact would h.ive been held to
prove them to be not ,</>/>> but varieties. On the other hand,
if two varietie> had been found to be infertile, or their mongrel
offspring to be sterile, then it would have been said : These
are not varieties but true species. Thus the old theory led
to inevitable reasoning in a circle ; and what might be only a
rather common fact was elevated into a law which had no

exception-.

The elaborate and careful examination of the whole subject
by Mr. Ihnwin, who has brought together a vast mass of
evidence from the experience of agriculturists and horti-
culturists, as well as from scientific experimenters, has demon-
strated that there is no such fixed law in nature as was
formerly supposed. He shows us that crosses between some
varieties are infertile or even sterile, while crosses between
some species are quite fertile; and that there are besides a
number of curious phenomena connected with the subject
which render it impossible to believe that sterility is anything
more than an incidental property of species, due to the
extreme delicacy and susceptibility of the reproductive powers,
and dependent on ph\ -iolo^ical causes we have not yet been
able to trace. Nevertheless, the fact remains that most species
which have hitherto been crossed produce sterile hybrids, as
in the well-known case of the mule ; while almost all domestic
varieties, when crossed, produce offspring which are perfectly
fertile among themselves. I will now endeavour to give such
a sketch of the subject as may enable the reader to see some-
thing of the complexity of the problem, referring him to Mr.
Darwin's works for fuller details.

E.dreme Susceptibility tf the Reproductive Functions.

One of the most interesting facts, as showing how sus-
ceptible to changed conditions or to slight constitutional
changes are the reproductive powers of animals, is the very
general difficulty of getting those which are kept in confine-
ment to breed ; and this is frequently the only bar to
domesticating wild species. Thus, elephants, bears, foxes,

154 DARWINISM CHAP.

and numbers of species of rodents, very rarely breed in
confinement; while other species do so more or less freely.
Hawks, vultures, and owls hardly ever breed in confinement ;
neither did the falcons kept for hawking ever breed. Of the
numerous small seed- eating birds kept in aviaries, hardly
any breed, neither do parrots. Gallinaceous birds usually
breed freely in confinement, but some do not ; and even
the guans and curassows, kept tame by the South American
Indians, never breed. This shows that change of climate has
nothing to do with the phenomenon ; and, in fact, the same
species that refuse to breed in Europe do so, in almost every
case, when tamed or confined in their native countries. This
inability to reproduce is not due to ill -health, since many
of these creatures are perfectly vigorous and live very long.

With our true domestic animals, on the other hand,
fertility is perfect, and is very little affected by changed
conditions. Thus, we see the common fowl, a native of
tropical India, living and multiplying in almost every part of
the world ; and the same is the case with our cattle, sheep,
and goats, our dogs and horses, and especially with domestic
pigeons. It therefore seems probable, that this facility for
breeding under changed conditions was an original property
of the species which man has domesticated a property
which, more than any other, enabled him to domesticate them.
Yet, even with these, there is evidence that great changes of
conditions affect the fertility. In the hot valleys of the
Andes sheep are less fertile ; while geese taken to the high
plateau of Bogota were a-t first almost sterile, but after some
generations recovered their fertility. These and many
other facts seem to show that, with the majority of animals,
even a slight change of conditions may produce infertility or
sterility ; and also that after a time, when the animal has
become thoroughly acclimatised, as it were, to the new
conditions, the infertility is in some cases diminished or
altogether ceases. It is stated by Bechsteiri that the canary
was long infertile, and it is only of late years that good
breeding birds have become common ; but in this case no
doubt selection has aided the change.

As showing that these phenomena depend on deep-seated
causes and are of a very general nature, it is interesting

vii ON THE INFERTILITY OF CROSSES 155

to note that they occur also in the vegetable kingdom.
Allowing for all the circumstances which are known to
prevent the production of seed, such as too great luxuriance
of foliage, too little or too much heat, or the absence of
insects to cross-fertilise the flowers, Mr. Darwin shows that
many species which grow and flower with us, apparently in
perfect health, yet never produce seed. Other plants are
affected by very slight changes of conditions, producing seed
freely in one soil and not in another, though apparently
growing equally well in both ; while, in some cases, a
difference of position even in the same garden produces a
similar result. 1

Reciprocal Crosses.

Another indication of the extreme delicacy of the
adjustment between the sexes, which is necessary to produce
fertility, is afforded by the behaviour of many species and
varieties when reciprocally crossed. This will be best
illustrated by a few of the examples furnished us by Mr.
ha nvin. The two distinct species of plants, Mirabilis jalapa
and M. longiflora, can be easily crossed, and will produce
healthy and fertile hybrids when the pollen of the latter is
applied to the stigma of the former plant. But the same
experimenter, Kolreuter, tried in vain, more than two hundred
times during eight years, to cross them by applying the pollen
of M. jalapa to the stigma of M. longiflora. In other cases two
plants are so closely allied that some botanists class them as
varieties (as with Matthiola annua and M. glabra), and yet
there is the same great difference in the result when they are
reciprocally crossed.

Differences in respect to Cross-Fertilisation.

A still more remarkable illustration of the delicate
balance of organisation needful for reproduction, is afforded
by the individual differences of animals and plants, as regards
both their power of intercrossing with other individuals or
other species, and the fertility of the offspring thus produced.
Among domestic animals, Darwin states that it is by no means
rare to find certain males and females which will not breed

1 Darwin's Animals and Plants under Domestication, vol. ii. pp. 163-170.

156 DARWINISM CHAP.

together, though both are known to be perfectly fertile with
other males and females. Cases of this kind have occurred
among horses, cattle, pigs, dogs, and pigeons ; and the
experiment has been tried so frequently that there can be no
doubt of the fact. Professor G. J. Romanes states that he
has a number of additional cases of this individual incom-
patibility, or of absolute sterility, between two individuals,
each of which is perfectly fertile with other individuals.

During the numerous experiments that have been made
on the hybridisation of plants similar peculiarities have been
noticed, some individuals being capable, others incapable, of
being crossed with a distinct species. The same individual
peculiarities are found in varieties, species, and genera.
Kolreuter crossed five varieties of the common tobacco
(Nicotiana tabacum) with a distinct species, Nicotiana
glutinosa, and they all yielded very sterile hybrids ; but
those raised from one variety were less sterile, in all the
experiments, than the hybrids from the four other varieties.
Again, most of the species of the genus Nicotiana have been
crossed, and freely produce hybrids ; but one species, N.
acuminata, not particularly distinct from the others, could
neither fertilise, nor be fertilised by, any of the eight other
species experimented on. Among genera we find some-
such as Hippeastrum, Crinum, Calceolaria, Dianthus almost
all the species of which will fertilise other species and produce
hybrid offspring ; while other allied genera, as Zephyranthes
and Silene, notwithstanding the most persevering efforts, have
not produced a single hybrid even between the most closely
allied species.

Dimorphism and Trimorphism.

Peculiarities in the reproductive system affecting indi-
viduals of the same species reach their maximum in what are
called heterostyled, or dimorphic and trimorphic flowers,
the phenomena presented by which form one of the most
remarkable of Mr. Darwin's many discoveries. Our common
cowslip and primrose, as well as many other species of the
genus Primula, have two kinds of flowers in about equal
proportions. In one kind the stamens are short, being
situated about the middle of the tube of the corolln, while the

VII

ON THE INFERTILITY OF CROSSES

157

style is long, the globular stigma appearing just in the centre
of the open flower. In the other kind the stamens are long,
appearing in the centre or throat of the flower, while the
style is short, the stigma being situated halfway down the
tube at the same level as the stamens in the other form.
These two forms have long been known to florists as the
"pin-eyed" and the "thrum -eyed," but they are called by
Darwin the long-styled and short-styled forms (see woodcut).

Long-styled form. Short-styled form.

Fio. 17. Primula veris (Cowslip).

The meaning and use of these different forms was quite
unknown till Darwin discovered, first, that cowslips and
primroses are absolutely barren if insects are prevented from
visiting them, and then, what is still more extraordinary, that
each form is almost sterile when fertilised by its own pollen,
and comparatively infertile when crossed with any other
plant of its own form, but is perfectly fertile when the pollen
of a long-styled is carried to the stigma of a short- styled
plant, or vice versa. It will be seen, by the figures, that the
arrangement is such that a bee visiting the flowers will carry
the pollen from the long anthers of the short -sty led form to
the stigma of the long -styled form, Avhile it would never
reach the stigma of another plant of the short- styled form.

158 DARWINISM CHAP.

But an insect visiting, first, a long-styled plant, would deposit
the pollen on the stigma of another plant of the same kind if
it were next visited ; and this is probably the reason why the
wild short-styled plants were found to be almost always most
productive of seed, since they must be all fertilised by the
other form, whereas the long- styled plants might often be
fertilised by their own form. The whole arrangement,
however, ensures cross-fertilisation ; and this, as Mr. Darwin
has shown by copious experiments, adds both to the vigour
and fertility of almost all plants as well as animals.

Besides the primrose family, many other plants of several
distinct natural orders pr-esent similar phenomena, one or
two of the most curious of which must be referred to. The
beautiful crimson flax (Linum grandiflormn) has also two
forms, the styles only differing in length ; and in this case Mr.
Darwin found by numerous experiments, Avhich have since
been repeated and confirmed by other observers, that each
form is absolutely sterile with pollen from another plant of
its own form, but abundantly fertile when crossed with any
plant of the other form. In this case the pollen of the two
forms cannot be distinguished under the microscope (when-; is
that of the two forms of Primula differs in size and .shape),
yet it has the remarkable property of being absolutely
powerless on the stigmas of half the plants of its own species.
The crosses between the opposite forms, which are fertile, are
termed by Mr. Darwin "legitimate," and those between
similar forms, which are sterile, "illegitimate"; and he
remarks that we have here, within the limits of the same
species, a degree of sterility which rarely occurs except
between plants or animals not only of different species but of
different genera.

But there is another set of plants, the trimorphic, in which
the styles and stamens have each three forms long, medium,
and short, and in these it is possible to have eighteen different
crosses. By an elaborate series of experiments it was .shown
that the six legitimate unions that is, when a plant was
fertilised by pollen from stamens of length corresponding to
that of its style in the two other forms were all abundantly
fertile; while the twelve illegitimate unions, when a plant was
fertilised by pollen from stamens of a different length from its

vn ON THE INFERTILITY OF CROSSES 159

own style, in any of the three forms, were either comparatively
or wholly sterile. 1

We have here a wonderful amount of constitutional
difference of the reproductive organs within a single species,
greater than usually occurs within the numerous distinct
species of a genus or group of genera ; and all this diversity
appears to have arisen for a purpose which has been obtained
by many other, and apparently simpler, changes of structure
or of function, in other plants. This seems to show us, in the
first place, that variations in the mutual relations of the repro-
ductive organs of different individuals must be as frequent as
structural variations have been shown to be ; and, also, that
sterility in itself can be no test of specific distinctness. But
this point will be better considered when we have further
illustrated and discussed the complex phenomena of hybridity.

Cases of the Fertility of Hybrids, and of the Infertility of Mongrels.

I now propose to adduce a few cases in which it has been
proved, by experiment, that hybrids between two distinct
species are fertile inter se; and then to consider why it is that
such cases are so few in number.

The common domestic goose (Anser ferus) and the Chinese
goose (A. cygnoides) are very distinct species, so distinct that
some naturalists have placed them in different genera ; yet they
have bred together, and Mr. Eyton raised from a pair of these
hybrids a brood of eight. This fact was confirmed by Mr.
Darwin himself, who raised several fine birds from a pair of
hybrids which were sent him. 2 In India, according to
Mr. Blyth and Captain Hutton, whole flocks of these hybrid
geese are kept in various parts of the country where neither
of the pure parent species exists, and as they are kept for
profit they must certainly be fully fertile.

Another equally striking case is that of the Indian humped
and the common cattle, species which differ osteologically, and
also in habits, form, voice, and constitution, so that they are
by no means closely allied ; yet Mr. Darwin assures us that he

1 For a full account of these interesting facts and of the various problems
to which they give rise, the reader must consult Darwin's volume on The
Different Forms of Flowers in Plants of the same Species, chaps, i.-iv.

" See Nature, vol. xxi. p. 207.

160 DARWINISM CHAP.

has received decisive evidence that the hybrids between these
are perfectly fertile infer se.

Dogs have been frequently crossed with wolves and with
jackals, and their hybrid offspring have been found to be fertile
ii/fi'f se to the third or fourth generation, and then usually to
show some signs of sterility or of deterioration. The wolf
and dog may be originally the same species, but the jackal is
certainly distinct ; and the appearance of infertility or of weak-
ness is probably due to the fact that, in almost all these experi-
ments, the offspring of a single pair themselves usually from
the same litter were bred in-and-in, and this alone sometimes
produces the most deleterious effects. Thus, Mr. Low in his
great work on the Domesticated Animals of Great IJrifuhi,
says : "If we shall breed a pair of dogs from the same litter,
and unite again the offspring of this pair, we shall produce at
once a feeble race of creatures ; and the process being repeated
for one or two generations more, the family will die out, or be
incapable of propagating their race. A gentleman of Scotland
made the experiment on a large scale with certain foxhounds,
and he found that the race actually became monstrous
and perished utterly." The same writer tells us that hogs
have been made the subject of similar experiments : " After a
few generations the victims manifest the change induced in the
system. They become of diminished size ; the bristles are
changed into hairs ; the limbs become feeble and short ; the
litters diminish in frequency, and in the number of the young
produced ; the mother becomes unable to nourish them, and,
if the experiment be carried as far as the case will allow, the
feeble, and frequently monstrous offspring, will be incapable of
being reared up, and the miserable race will utterly perish." 1

These precise statements, by one of the greatest authorities
on our domesticated animals, are sufficient to show that the
fact of infertility or degeneracy appearing in the offspring of
hybrids after a few generations need not be imputed to the
fact of the first parents being distinct species, since exactly the
same phenomena appear when individuals of the sunic species
are bred under similar adverse conditions. But in almost all
the experiments that have hitherto been made in crossing
distinct species, no care has been taken to avoid close inter-

1 Low's Domesticated Animals of Great llritin. Introduction. ]>. Ixiv.

vii ON THE INFERTILITY OF CROSSES 161

breeding by securing several hybrids from quite distinct
stocks to start with, 'and by having two or more sets of experi-
ments carried on at once, so that crosses between the hybrids
produced may be occasionally made. Till this is done no
experiments, such as those hitherto tried, can be held to prove
that hybrids are in all cases infertile inter se.

It has, however, been denied by Mr. A. H. Huth, in his
interesting work on The Marriage of Near Kin, that any
amount of breeding in-and-in is in itself hurtful ; and he quotes
the evidence of numerous breeders whose choicest stocks have
always been so bred, as well as cases like the Porto Santo
rabbits, the goats of Juan Fernandez, and other cases in which
animals allowed to run wild have increased prodigiously and
continued in perfect health and vigour, although all derived
from a single pair. But in all these cases there has been
rigid selection by which the weak or the infertile have been
eliminated, and with such selection there is no doubt that the
ill effects of close interbreeding can be prevented for a long
time ; but this by no means proves that no ill effects are pro-
duced. Mr. Huth himself quotes M. Allie, M. Aube, Stephens,
Giblett, Sir John Sebright, Youatt, Druce, Lord Weston, and
other eminent breeders, as finding from experience that close
interbreeding does produce bad effects ; and it cannot be
supposed that there would be such a consensus of opinion
on this point if the evil were altogether imaginary. Mr.
Huth argues, that the evil results which do occur do not
depend on the close interbreeding itself, but on the tendency
it has to perpetuate any constitutional weakness or other
hereditary taints ; and he attempts to prove this by the argu-
ment that " if crosses act by virtue of being a cross, and not
by virtue of removing an hereditary taint, then the greater the
difference between the two animals crossed the more beneficial
will that act be." He then shows that, the wider the difference
the less is the benefit, and concludes that a cross, as such, has
no beneficial effect. A parallel argument would be, that change
of air, as from inland to the sea-coast, or from a low to an
elevated site, is not beneficial in itself, because, if so, a change
to the tropics or to the polar regions should be more beneficial.
In both these cases it may well be that no benefit would
accrue to a person in perfect health ; but then there is no

M

162 DARWINISM CHAP.

such thing as " perfect health " in man, and probably no such
thing as absolute freedom from constitutional taint in animals.
The experiments of Mr. Danvin, showing the great and
immediate good effects of a cross between distinct strains in
plants, cannot be explained away ; neither can the innumerable
arrangements to secure cross-fertilisation by insects, the real
use and purport of which will be discussed in our eleventh
chapter. On the whole, then, the evidence at our command
proves that, whatever may be its ultimate cause, close inter-
breeding does usually produce bad results ; and it is only by
the most rigid selection, whether natural or artificial, that
the danger can be altogether obviated.

Fertile Hybrids among Animals.

One or two more cases of fertile hybrids may be given
before we pass on to the corresponding experiments in plants.
Professor Alfred Newton received from a friend a pair of
hybrid ducks, bred from a common duck (Anas boschas), and a
pintail (I)afila acuta). From these he obtained four ducklings,
but these latter, when grown up, proved infertile, and did not
breed again. In this case we have the results of close inter-
breeding, with too great a difference between the original
species, combining to produce infertility, yet the fact of a
hybrid from such a pair producing healthy offspring is itself
noteworthy.

Still more extraordinary is the following statement of Mr.
Low : " It has been long known to shepherds, though ques-
tioned by naturalists, that the progeny of the cross between the
sheep and goat is fertile. Breeds of this mixed race are
numerous in the north of Europe." * Nothing appears to be
known of such hybrids either in Scandinavia or in Italy ; but
Professor Giglioli of Florence has kindly given me some useful
references to works in which they are described. The following
extract from his letter is very interesting : " I need not tell
you that there being such hybrids is now generally accepted as
a fact. Buffon (Supplements, torn. iii. p. 7, 1756) obtained one
such hybrid in 1751 and eight in 1752. Sanson (La Culture,
vol. vi. p. 372, 1865) mentions a case observed in the Vosges,
France. Geoff. St. Hilaire (Hist. Nat. G6n. des reg. or<j., vol. iii. p.
1 Low's Duinesticated Aninmls, p. 28.

vil ON THE INFERTILITY OF CROSSES 163

163) was the first to mention, I believe, that in different parts
of South America the ram is more usually crossed with the
she-goat than the sheep with the he-goat. The well-known
'pellones' of Chile are produced by the second and third
generation of such hybrids (Gay, ' Hist, de Chile/ vol. i. p. 466,
Agriculture, 1862). Hybrids bred from goat and sheep are
called ' chabin ' in French, and ' cabruno ' in Spanish. In
Chile such hybrids are called 'carneros lanudos'; their breed-
ing inter se appears to be not always successful, and often the
original cross has to be recommenced to obtain the proportion of
three-eighths of he-goat and five-eighths of sheep, or of three-
eighths of ram and five-eighths of she-goat ; such being the
reputed best hybrids."

With these numerous facts recorded by competent observers
we can hardly doubt that races of hybrids between these very
distinct species have been produced, and that such hybrids are
fairly fertile intir se; and the analogous facts already given lead
us to believe that whatever amount of infertility may at first
exist could be eliminated by careful selection, if the crossed
races were bred in large numbers and over a considerable area
of country. This case is especially valuable, as showing how
careful we should be in assuming .the infertility of hybrids
when experiments have been made with the progeny of a single
pair, and have been continued only for one or two generations.

Among insects one case only appears to have been recorded.
The hybrids of two moths (Bombyx cynthia and B. arrindia)
were proved in Paris, according to M. Quatrefages, to be fertile
inter se for eight generations.

Fertility of Hybrids among Plants.

Among plants the cases of fertile hybrids are more numerous,
owing, in part, to the large scale on which they are grown by
gardeners and nurserymen, and to the greater facility with
which experiments can be made. Darwin tells us that Kolreuter
found ten cases in which two plants considered by botanists
to be distinct species were quite fertile together, and he there-
fore ranked them all as varieties of each other. In some
cases these were grown for six to ten successive generations, but
after a time the fertility decreased, as we saw to be the case in

164 DARWINISM CHAP.

animals, and presumably from the same cause, too close inter-
breeding.

Dean Herbert, who carried on experiments with great care
and skill for many years, found numerous cases of hybrids
which were perfectly fertile inter se. Crinum capense, fertilised
by three other species C. pedunculatum, C. canaliculatum, or
C. defixum all very distinct from it, produced perfectly
fertile hybrids ; Avhile other species less different in appearance
were quite sterile with the same C. capense.

All the species of the genus Hippeastrum produce hybrid
offspring which are invariably fertile. Lobelia syphylitica and
L. fulgens, two very distinct species, have produced a hybrid
which has been named Lobelia speciosa, and which reproduces
itself abundantly. Many of the beautiful pelargoniums of
our greenhouses are hybrids, such as P. ignescens from a cross
between P. citrinodorum and P. fulgidum, which is quite
fertile, and has become the parent of innumerable varieties of
beautiful plants. All the varied species of Calceolaria, how-
ever different in appearance, intermix with the greatest readi-
ness, and the hybrids are all more or less fertile. But the
most remarkable case is that of two species of Petunia, of which
Dean Herbert says : " It is very remarkable that, although
there is a great difference in the form of the flower, especially
of the tube, of P. nyctanigena'flora and P. phcenicea the
mules between them are not only fertile, but I have found
them seed much more freely with me than either parent.
.... From a pod of the above-mentioned mule, to which
no pollen but its own had access, I had a large batch of seed-
lings in which there was no variability or difference from
itself ; and it is evident that the mule planted by itself, in a
congenial climate, would reproduce itself as a species ; at least
as much deserving to be so considered as the various Calceo-
larias of different districts of South America." 1

Darwin was informed by Mr. C. Noble that he raises stocks
for grafting from a hybrid between Rhododendron ponticum
and R. catawbiense, and that this hybrid seeds as freely as it
is possible to imagine. He adds that horticulturists raise
large beds of the same hybrid, and such alone are fairly
treated ; for, by insect agency, the several individuals arc freely
1 Aiufiri/llt'daceoc, by the Hoii. and Rev. William Herbert, p. 379.

vn ON THE INFERTILITY OF CROSSES 165

crossed with each other, and the injurious influence of close
interbreeding is thus prevented. Had hybrids, when fairly
treated, always gone on decreasing in fertility in each suc-
cessive generation, as Gartner believed to be the case, the fact
would have been notorious to nurserymen. 1

Cases of Sterility of Mongrels.

The reverse phenomenon to the fertility of hybrids, the
sterility of mongrels or of the crosses between varieties of the
same species, is a comparatively rare one, yet some undoubted
cases have occurred, (iartner, who believed in the absolute
distinctness of species and varieties, had two varieties of
maize one dwarf with yellow seeds, the other taller with red
seeds ; yet they never naturally crossed, and, when fertilised
artificially, only a single head produced any seeds, and this one
only five grains. Yet these few seeds were fertile ; so that in
this case the first cross was almost sterile, though the hybrid
when at length produced was fertile. In like manner, dis-
similarly coloured varieties of Verbascum or mullein have been
found by two distinct observers to be comparatively infertile.
The two pimpernels (Anagallis arvensis and A. cnerulea), classed
by most botanists as varieties of one species, have been found,
after repeated trials, to be perfectly sterile when crossed.

No cases of this kind are recorded among animals ; but
this is not to be wondered at, when we consider how very few
experiments have been made with natural varieties ; while
there is good reason for believing that domestic varieties are
exceptionally fertile, partly because' one of the conditions of
domestication was fertility under changed conditions, and also
because long continued domestication is believed to have the
effect of increasing fertility and eliminating whatever sterility
may exist. This is shown by the fact that, in many cases,
domestic animals are descended from two or more distinct
species. This is almost certainly the case Avith the dog, and
probably with the hog, the ox, and the sheep ; yet the various
breeds are now all perfectly fertile, although we have every
reason to suppose that there would be some degree of infer-
tility if the several aboriginal species were crossed together
for the first time.

1 Origin of .tyecies, p. 239.

166 DARWINISM CHAP.

Ictween Crossing <ni<l Change of Conditions.

In the whole series of these phenomena, from the beneficial
effects of the crossing of different stocks and the evil effects of
close interbreeding, up to the partial or complete sterility
induced by crosses between species belonging to different
genera, we have, as Mr. Darwin points out, a curious parallelism
with the effects produced by change of physical conditions.
It is well known that slight changes in the conditions of life
are beneficial to all living things. Plants, if constantly grown
in one soil and locality from their own seeds, are greatly
benefited by the importation of seed from some other locality.
The same thing happens with animals ; and the benefit we our-
selves experience from " change of air " is an illustration of
the same phenomenon. But the amount of the change which
is beneficial has its limits, and then a greater amount is
injurious. A change to a climate a few degrees warmer or
colder mny be good, while a change to the tropics or to the
arctic regions might be injurious.

Thus we see that, both slight changes of conditions and
a slight amount of crossing, are beneficial ; while extreme
changes, and crosses between individuals too far removed in
structure or constitution, are injurious. And there is not
only a parallelism but an actual connection between the two
classes of facts, for, as we have already shown, many species
of animals and plants are rendered infertile, or altogether
sterile, by the change from their natural conditions which
occurs in confinement or in cultivation ; while, on the other
hand, the increased vigour or fertility which is invariably pro-
duced by a judicious cross may be also effected by a judicious
change of climate and surroundings. We shall see in a subse-
quent chapter, that this interchangeability of thebeneficial effects
of crossing and of new conditions, serves to explain some very
puzzling phenomena in the forms and economy of Mowers.

rks on the Facts of Ili/hridif//.

The facts that have now been adduced, though not very
numerous, arc sufficiently conclusive to prove that the old
belief, of the universal sterility of hybrids and fertility of
mongrels, is incorrect. The doctrine that such a universal

vii ON THE INFERTILITY OF CROSSES 167

law existed was never more than a plausible generalisa-
tion, founded on a few inconclusive facts derived from
domesticated animals and cultivated plants. The facts were,
and still are, inconclusive for several reasons. They are
founded, primarily, on what occurs among animals in
domestication ; and it has been shown that domestication
both tends to increase fertility, and was itself rendered
possible by the fertility of those particular species being little
affected by changed conditions. The exceptional fertility of
all the varieties of domesticated animals does not prove that
a similar fertility exists among natural varieties. In the next
place, the generalisation is founded on too remote crosses, as in
the case of the horse and the ass, the tAvo most distinct and
Avidely separated species of the genus Equus, so distinct indeed
that they have been held by some naturalists to form distinct
genera. Crosses between the two species of zebra, or even
between the zebra and the quagga, or the quagga and the ass,
might have led to a very different result. Again, in pre-
Darwinian times it was so universally the practice to argue in
a circle, and declare that the fertility of the offspring of a
cross proved the identity of species of the parents, that experi-
ments in hybridity were usually made between very remote
species and even between species of different genera, to avoid
the possibility of the reply : " They are both really the same
species ;" and the sterility of the hybrid offspring of such
remote crosses of course served to strengthen the popular
belief.

Now that we have arrived at a different standpoint, and
look upon a species, not as a distinct entity due to special
creation, but as an assemblage of individuals which have become
somewhat modified in structure, form, and constitution so as
to adapt them to slightly different conditions of life ; which
can be differentiated from other allied assemblages ; which
reproduce their like, and which usually breed together we
require a fresh set of experiments calculated to determine the
matter of fact, whether such species crossed with their near
allies do always produce offspring which are more or less
sterile inter se. Ample materials for such experiments exist,
in the numerous "representative species " inhabiting distinct
areas on a continent or different islands of a group ; or even

168 DARWINISM CHAP.

in those found in the same area but frequenting somewhat
different stations.

To carry out these experiments with any satisfactory result,
it will be necessary to avoid the evil effects of confinement
and of too close interbreeding. If birds are experimented
with, they should be allowed as much liberty as possible, a
plot of ground with trees and bushes being enclosed with
wire netting overhead so as to form a large open aviary.
The species experimented with should be obtained in con-
siderable numbers, and by two separate persons, each making
the opposite reciprocal cross, as explained at p. 155. In the
second generation these two stocks might be themselves crossed
to prevent the evil effects of too close interbreeding. By such
experiments, carefully carried out with different groups of
animals and plants, we should obtain a body of facts of a
character now sadly wanting, and without which it is hopeless
to expect to arrive at a complete solution of this difficult
problem. There are, however, some other aspects of the
question that need to be considered, and some theoretical
views which require to be carefully examined, having done
which we shall be in a condition to state the general con-
clusions to which the facts and reasonings at our command
seem to point.

Sterility due to changed Conditions and usually correlated with
other Characters, especially with Colour.

The evidence already adduced as to the extreme suscep-
tibility of the reproductive system, and the curious irrc-u
larity with which infertility or sterility appears in the crosses
between some varieties or species while quite absent in those
between others, seem to indicate that sterility is a charac-
teristic which has a constant tendency to appear, either by
itself or in correlation with other characters. It is kiio\vn
to lie especially liable to occur under changed conditions of
life; and, as such change is usually the starting-point and
cause of the development of new species, we have already
found a reason why it should so often appear when specie^
become fully differentiated.

In almost all the cases of infertility or sterility between
varieties or species, we have some external differences with

vii ON THE INFERTILITY OF CROSSES 169

which it is correlated ; and though these differences are
sometimes slight, and the amount of the infertility is not
always, or even usually, proportionate to the external dif-
ference between the two forms crossed, we must believe that
there is some connection between the two classes of facts.
This is especially the case as regards colour ; and Mr. Darwin
has collected a body of facts which go far to prove that
colour, instead of being an altogether trifling and un-
important character, as was supposed by the older natural-
ists, is really one of great significance, since it is un-
doubtedly often correlated with important constitutional
differences. Now colour is one of the characters that most
usually distinguishes closely allied species; and when we
hear that the most closely allied species of plants are
infertile together, while those more remote are fertile, the
meaning usually is that the former differ chiefly in the colour
of their flowers, while the latter differ in the form of the
flowers or foliage, in habit, or in other structural characters.

It is therefore a most curious and suggestive fact, that in
all the recorded cases, in which a decided infertility occurs
between varieties of the same species, those varieties are
distinguished by a difference of colour. The infertile
varieties of Verbascum were white and yellow flowered
respectively ; the infertile varieties of maize were red and
yellow seeded ; while the infertile pimpernels were the red
and the blue flowered varieties. So, the differently coloured
varieties of hollyhocks, though grown close together, each
reproduce their own colour from seed, showing that they are
not capable of freely intercrossing. Yet Mr. Darwin assures
us that the agency of bees is necessary to carry the pollen
from one plant to another, because in each flower the pollen
is shed before the stigma is ready to receive it. We have
here, therefore, either almost complete sterility between
varieties of different colours, or a prepotent effect of pollen
from a flower of the same colour, bringing about the same
result.

Similar phenomena have not been recorded among
animals ; but this is not to be wondered at when we consider
that most of our pure and valued domestic breeds are
characterised by definite colours which constitute one of their

170 DARWINISM CHAP.

distinctive marks, and they are, therefore, seldom crossed with
these of another colour ; and even when they are so crossed, no
notice would be taken of any slight diminution of fertility, since
this is liable to occur from many causes. We have also reason
to believe that fertility has been increased by long domestica-
tion, in addition to the fact of the original stocks being
exceptionally fertile ; and no experiments have been made on
the differently coloured varieties of wild animals. There are,
however, a number of very curious facts showing that colour
in animals, as in plants, is often correlated with constitutional
differences of a remarkable kind, and as these have a close
relation to the subject we are discussing, a brief summary of
them will be here given.

Correlation of Colour with Constitutional Peculiarities.

The correlation of a white colour and blue eyes in male
cats with deafness, and of the tortoise-shell marking with the
female sex of the same animal, are two well-known but most
extraordinary cases. Equally remarkable is the fact, com-
municated to Darwin by Mr. Tegetmeier, that white, yellow,
pale blue, or dun pigeons, of all breeds, have the young birds
born naked, while in all other colours they are well covered
with down. Here we have a case in which colour seems of
more physiological importance than all the varied structural
differences between the varieties and breeds of pigeons.
In Virginia there is a plant called the paint-root (Lachnanthes
tinctoria), which, when eaten by pigs, colours their bones
pink, and causes the hoofs of all but the black varieties to
drop off; so that black pigs only can be kept in the district. 1
Buckwheat in flower is also said to be injurious to white
pigs but not to black. In the Tarentino, black shivp
are not injured by eating the Hypericum crispum a species
of St. John's-wort which kills white sheep. White terriers
suffer most from distemper; white chickens from the gapes.
White-haired horses or cattle are subject to rutaneoii*
diseases from which the dark coloured are free ; while, both in
Thuringia and the West Indies, it has been noticed that white
or pale coloured cattle are much more troubled by tlies than are
those which are brown or black. The same law even extends
1 Orii/i)t of Species, sixth edition, p. 9.

vii ON THE INFERTILITY OF CROSSES 171

to insects, for it is found that silkworms which produce white
cocoons resist the fungus disease much better than do those
which produce yellow cocoons. 1 Among plants, we have in
North America green and yellow-fruited plums not affected by
a disease that attacked the purple-fruited varieties. Yellow-
fleshed peaches suffer more from disease than white-fleshed
kinds. In Mauritius, white sugar-canes were attacked by a
disease from which the red canes were free. White onions
and verbenas are most liable to mildew ; and red-flowered
hyacinths were more injured by the cold during a severe
winter in Holland than any other kinds. 2

These curious and inexplicable correlations of colour with
constitutional peculiarities, both in animals and plants, render
it probable that the correlation of colour with infertility,
which has been detected in several cases in plants, may also
extend to animals in a state of nature ; and if so, the fact
is of the highest importance as throwing light on the origin
of the infertility of many allied species. This will be better
understood after considering the facts which will be now
described.

The Isolation of Varieties by Selective Association.

In the last chapter I have shown that the importance of
geographical isolation for the formation of new species by
natural selection has been greatly exaggerated, because the

1 In the Medico-Chirurgical Transactions, vol. liii. (1870), Dr. Ogle has
adduced some curious physiological facts bearing on the presence or absence
of white colours in the higher animals. He states that a dark pigment in the
olfactory region of the nostrils is essential to perfect smell, and that this
pigment is rarely deficient except when the whole animal is pure white, and
the creature is then almost without smell or taste. He observes that there is
no proof that, in any of the cases given above, the black animals actually eat
the poisonous root or plant ; and that the facts are readily understood if the
senses of smell and taste are dependent on a pigment which is absent in the
white animals, who therefore eat what those gifted with normal senses avoid.
This explanation however hardly seems to cover the facts. We cannot sup-
pose that almost all the sheep in the world (which are mostly white) are
without smell or taste. The cutaneous disease on the white patches of hair
on horses, the special liability of white terriers to distemper, of white chickens
to the gapes, and of silkworms which produce yellow silk to the fungus, are
not explained by it. The analogous facts in plants also indicate a real con-
stitutional relation with colour, not an affection of the sense of smell and
taste only.

2 For all these facts, see Animals and Plants under Domestication, vol. ii
pp. 335-338.

172 DARWINISM CHAP.

very change of conditions, which is the initial power in
starting such new forms, leads also to a local or stational
segregation of the forms acted upon. But there is also a very
powerful cause of isolation in the mental nature the likes
and dislikes of animals ; and to this is probably due the fact
of the comparative rarity of hybrids in a state of nature.
The differently coloured herds of cattle in the Falkland
Islands, each of which keeps separate, have been already
mentioned ; and it may be added, that the white variety seem
to have already developed a physiological peculiarity in breed-
ing three months earlier than the others. Similar facts occur,
however, among our domestic animals and are well known to
breeders. Professor Low, one of the greatest authorities on
our domesticated animals, says : " The female of the dog, when
not under restraint, makes selection of her mate, the mastiff*
selecting the mastiff", the terrier the terrier, and so on." And
again : " The Merino sheep and Heath sheep of Scotland, if
two flocks are mixed together, each will breed with its own
variety." Mr. Darwin has collected many facts illustrating
this point. One of the chief pigeon-fanciers in England
informed him that, if free to choose, each breed would prefer
pairing with its own kind. Among the wild horses in Para-
guay those of the same colour and size associate together;
Avhile in Circassia there are three races of horses which have
received special names, and which, when living a free life,
almost always refuse to mingle and cross, and will even
attack one another. On one of the Faroe Islands, not nmiv
than half a mile in diameter, the half-wild native black sheep
do not readily mix with imported white sheep. In the
Forest of Dean, and in the New Forest, the dark and pale
coloured herds of fallow deer have never been known to
mingle; and even the curious Ancon sheep of quite modern
origin have been observed to keep together, separating them-
selves from the rest of the flock when put into enclosures
with other sheep. The same rule applies to birds, for Darwin
was informed by the liev. W. I). Fox that his flocks of white
and Chinese geese kept distinct. 1

This constant preference of animals for their like, even in the
ease of slightly different varieties of the same speeie-s is e\ idem 1\
1 Animals and Plants under Domestication, \\. ii. pp. lU'j, In".

vii ON THE INFERTILITY OF CROSSES 173

a fact of great importance in considering the origin of species
by natural selection, since it shows us that, so soon as a slight
differentiation of form or colour has been effected, isolation
will at once arise by the selective association of the animals
themselves; and thus the great stumbling-block of "the
swamping effects of intercrossing," which has been so pro-
minently brought forward by many naturalists, will be com-
pletely obviated.

If now we combine with this fact the correlation of colour
with important constitutional peculiarities, and, in some cases,
with infertility ; and consider, further, the curious parallelism
that has been shown to exist between the effects of changed
conditions and the intercrossing of varieties in producing
either an increase or a decrease of fertility, we shall have
obtained, at all events, a starting-point for the production of
that infertility which is so characteristic a feature of distinct
species when intercrossed. All we need, now, is some means
of increasing or accumulating this initial tendency ; and to a
discussion of this problem we will therefore address ourselves.

The Influence of Natural Selection upon Sterility and Fertility.

It will occur to many persons that, as the infertility or
sterility of incipient species would be useful to them when
occupying the same or adjacent areas, by neutralising the
effects of intercrossing, this infertility might have been in-
creased by the action of natural selection ; and this will be
thought the more probable if we admit, as we have seen
reason to do, that variations in fertility occur, perhaps as
frequently as other variations. Mr. Darwin tells us that, at
one time, this appeared to him probable, but he found the
problem to be one of extreme complexity ; and he was also
influenced against the view by many considerations which
seemed to render such an origin of the sterility or infertility
of species when intercrossed very improbable. The fact that
species which occupy distinct areas, and which nowhere come
in contact with each other, are often sterile when crossed, is one
of the difficulties ; but this may perhaps be overcome by the
consideration that, though now isolated, they may, and often
must, have been in contact at their origination. More
important is the objection that natural selection could not

174 DARWINISM CHAP.

possibly have produced the difference that often occurs
between reciprocal crosses, one of these being sometimes
fertile, while the other is sterile. The extremely different
amounts of infertility or sterility between different species
of the same genus, the infertility often bearing no proportion
to the difference between the species crossed, is also an
important objection. But none of these objections would
have much weight if it could be clearly shown that natural
selection is able to increase the infertility variations of in-
cipient species, as it is certainly able to increase and develop
all useful variations of form, structure, instincts, or habits.
Ample causes of infertility have been shown to exist, in the
nature of the organism and the laws of correlation ; the
agency of natural selection is only needed to accumulate
the effects produced by these causes, and to render their final
results more uniform and more in accordance with the facts
that exist.

About twenty years ago I had much correspondence and
discussion with Mr. Darwin on this question. I then believed
that I was able to demonstrate the action of natural selection in
accumulating infertility ; but I could not convince him, owing
to the extreme complexity of the process under the conditions
which he thought most probable. I have recently returned
to the question ; and, with the fuller knowledge of the facts of
variation we now possess, I think it may be shown that
natural selection is, in some probable cases at all events, able
to accumulate variations in infertility between incipient species.

The simplest case to consider, Avill be that in which two
forms or varieties of a species, occupying an extensive area, are
in process of adaptation to somewhat different modes of life
Avithin the same area. If these two forms freely intercross
with each other, and produce mongrel offspring which are
quite fertile inter se, then the further differentiation of the
forms into two distinct species Avill be retarded, or perhaps
entirely prevented ; for the offspring of the crossed unions
Avill be, perhaps, more vigorous on account of the cross,
although less perfectly adapted to the conditions of existence
than either of the pure breeds ; and this would certain]) estab-
lish a powerful antagonistic influence to the further differentia-
tion of the two forms.

vii ON THE INFERTILITY OF CROSSES 175

Now, let us suppose that a partial sterility of the hybrids
between the two forms arises, in correlation with the different
modes of life and the slight external or internal peculiarities
that exist between them, both of which we have seen to be
real causes of infertility. The result Avill be that, even if the
hybrids between the two forms are still freely produced, these
hybrids will not themselves increase so rapidly as the two
pure forms ; and as these latter are, by the terms of the
problem, better suited to their conditions of life than are
the hybrids between them, they Avill not only increase more
rapidly, but will also tend to supplant the hybrids altogether
whenever the struggle for existence becomes exceptionally
severe. Thus, the more complete the sterility of the hybrids
the more rapidly will they die out and leave the two parent
forms pure. Hence it Avill follow that, if there is greater
infertility between the two forms in one part of the area than
the other, these forms will be kept more pure wherever
this greater infertility prevails, will therefore have an
advantage at each recurring period of severe struggle for
existence, and will thus ultimately supplant the less infertile
or completely fertile forms that may exist in other portions
of the area. It thus appears that, in such a case as here
supposed, natural selection would preserve those portions of
the two breeds which were most infertile with each other, or
whose hybrid offspring Avere most infertile ; and would,
therefore, if variations in fertility continued to arise, tend to
increase that infertility. It must particularly be noted that
this effect would result, not by the preservation of the
infertile variations on account of their infertility, but by the
inferiority of the hybrid offspring, both as being fewer in
numbers, less able to continue their race, and less adapted to
the conditions of existence than either of the pure forms. It
is this inferiority of the hybrid offspring that is the essential
point ; and as the number of these hybrids will be per-
manently less where the infertility is greatest, therefore those
portions of the two forms in which infertility is greatest will
have the advantage, and will ultimately survive in the struggle
for existence.

The differentiation of the two forms into distinct species,
with the increase of infertility between them, would be

170 DARWINISM CHAP.

greatly assisted by two other important factors in the
problem. It has already been shown that, with each
modification of form and habits, and especially with modifica-
tions of colour, there arises a disinclination of the two forms
to pair together ; and this would produce an amount, of
isolation which would greatly assist the specialisation of tin-
forms in adaptation to their different conditions of life.
Again, evidence has been adduced that change of condition.
or of mode of life is a potent cause of disturbance of tin-
reproductive system, and, consequently, of infertility. We
may therefore assume that, as the two forms adopted more
and more different modes of life, and perhaps acquired al.-o
decided peculiarities of form and coloration, the infertility
between them would increase or become more general ; and as
we have seen that every such increase of infertility would
give that portion of the species in which it arose an advantage
over the remaining portions in which the two varieties were,
more fertile together, all this induced infertility would main-
tain itself, and still further increase the general infertility be-
tween the two forms of the species.

It follows, then, that specialisation to separate conditions
of life, differentiation of external characters, disinclination to
cross-unions, and the infertility of the hybrid produce of these
unions, would all proceed pari pami, and would ultimately
lead to the production of two distinct forms having all the
characteristics, physiological as Avell as structural, of true
species.

In the case now discussed it has been supposed, that some
amount of general infertility might arise in correlation with
the different modes of life of two varieties or incipient.
species. A considerable body of facts already adduced
renders it probable that this is the mode in which any
widespread infertility would arise; and, if so, it lias been
shown that, by the influence of natural selection and the
known laws which affect varieties, the infertility would be
gradually increased. But, if we suppose the infertility to
arise sporadically within the two forms, and to affect only a
small proportion of the individuals in any area, it will be
difficult, if not impossible, to show that such infertility \\onld
have any tendency to increase, or would produce any but a

vii ON THE INFERTILITY OF CROSSES 177

prejudicial effect. If, for example, five per cent of each
form thus varied so as to be infertile with the other form,
the result would be hardly perceptible, because the individuals
which formed cross-unions and produced hybrids would con-
stitute a very small portion of the whole species ; and the
hybrid offspring, being at a disadvantage in the struggle for
existence and being themselves infertile, Avould soon die out,
whils the much more numerous fertile portion of the two
forms would increase rapidly, and furnish a sufficient number
of pure-bred offspring of each form to take the place of the
somewhat inferior hybrids between them whenever the
struggle for existence became severe. We must suppose that
the normal fertile forms would transmit their fertility to their
progeny, and the few infertile forms their infertility ; but
the latter would necessarily lose half their proper increase
by the sterility of their hybrid offspring whenever they
crossed with the other form, and when they bred with their
own form the tendency to sterility would die out except in
the very minute proportion of the five per cent (one-twentieth)
that chance would lead to pair together. Under these
circumstances the incipient sterility between the two forms
would rapidly be eliminated, and could never rise much above
the numbers which were produced by sporadic variation each
year.

It was, probably, by a consideration of some such case as
this that Mr. Darwin came to the conclusion that infertility
arising between incipient species could not be increased by
natural selection ; and this is the more likely, as he was
always disposed to minimise both the frequency and the
amount even of structural variations.

We have yet to notice another mode of action of natural
selection in favouring and perpetuating any infertility that
may arise between two incipient species. If several distinct
species are undergoing modification at the same time and in
the same area, to adapt them to some new conditions that
have arisen there, then any species in which the structural or
colour differences that have arisen between it and its varieties
or close allies were correlated with infertility of the crosses
between them, would have an advantage over the corre-
sponding varieties of other species in which there was no such

N

178 DARWINISM

physiological peculiarity. Thus, incipient species which were
infertile together would have an advantage over other
incipient species which were fertile, and, whenever the
struggle for existence became severe, would prevail over them
and take their place. Such infertility, being correlated with
constitutional or structural differences, would probably, as
already suggested, go on increasing as these differences
increased ; and thus, by the time the new species became
fully differentiated from its parent form (or brother variety)
the infertility might have become as well marked as we
usually find it to be between distinct species.

This discussion has led us to some conclusions of the greatest
importance as bearing on the difficult problem of the cause of
the sterility of the hybrids between distinct species. Accept-
ing, as highly probable, the fact of variations in fertility
occurring in correlation with variations in habits, colour, or
structure, we see, that so long as such variations occurred only
sporadically, and affected but a small proportion of the in-
dividuals in any area, the infertility could not be increased by
natural selection, but would tend to die out almost as fast as
it was produced. If, however, it was so closely correlated
with physical variations or diverse modes of life as to
affect, even in a small degree, a considerable proportion of
the individuals of the two forms in definite areas, it would
be preserved by natural selection, and the portion of the
varying species thus affected would increase at the expense of
those portions which were more fertile when crossed. Kadi
further variation towards infertility between the two forms
would be again preserved, and thus the incipient infertility
of the hybrid offspring might be increased till it bet-aim- so
great as almost to amount to sterility. Yet further, we have
seen that if several competing species in the same area were
being simultaneously modified, those between whose varieties
infertility arose would have an advantage over those whose
varieties remained fertile inter .sv, and would ultimately sup-
plant them.

The preceding argument, it will be seen, depends entirely
upon the assumption that some amount of infertility char-
acterises the distinct varieties which are in process of
differentiation into species; and it may be objected that of

vn ON THE INFERTILITY OF CROSSES 179

such infertility there is no proof. This is admitted ; but it is
urged that facts have been adduced which render such
infertility probable, at least in some cases, and this is all
that is required. It is by no means necessary that all varieties
should exhibit incipient infertility, but only some varieties ;
for we know that, of the innumerable varieties that occur
but few become developed into distinct species, and it may be
that the absence of infertility, to obviate the effects of inter-
crossing, is one of the usual causes of their failure. All I
have attempted to show is, that when incipient infertility does
occur in correlation with other varietal differences, that in-
fertility can be, and in fact must be, increased by natural
selection ; and this, it appears to me, is a decided step in
advance in the solution of the problem. 1

1 As this argument is a rather difficult one to follow, while its theoretical
importance is very great, I add here the following briefer exposition of it, in a
series of propositions ; being, with a few verbal alterations, a copy of what I
wrote on the subject about twenty years back. Some readers may find this
easier to follow than the fuller discussion in the text :

Can Sterility of Hybrids have been Produced by Natural Selection ?

1. Let there be a species which has varied into two forms each adapted to
certain existing conditions better than the parent form, which they soon
supplant.

2. If these two forms, which are supposed to coexist in the same
district, do not intercross, natural selection will accumulate all favourable
variations till they become well suited to their conditions of life, and form
two slightly differing species.

3. But if these two fo'rms freely intercross with each other, and produce
hybrids, which are also quite fertile inter se, then the formation of the two
distinct races or species will be retarded, or perhaps entirely prevented ; for
the offspring of the crossed unions will be more vigorous owing to the cross,
although less adapted to their conditions of life than either of the pure
breeds.

4. Now, let a partial sterility of the hybrids of some considerable propor-
tion of these two forms arise ; and, as this would probably be due to some
special conditions of life, we may fairly suppose it to arise in some definite
portion of the area occupied by the two forms.

5. The result will be that, in that area, the hybrids (although continually
produced by first crosses almost as freely as before) will not themselves
increase so rapidly as the two pure forms ; and as the two pure forms are, by
the terms of the problem, better suited to their several conditions of life than
the hybrids, they will inevitably increase more rapidly, and will continually
tend to supplant the hybrids altogether at every recurrent severe struggle for
existence.

6. We may fairly suppose, also, that as soon as any sterility appears some
disinclination to cross unions will appear, and this will further tend to the
diuuuution of the production of hybrids.

180 DARWINISM CHAP.

Physiological Selection.

Another form of infertility has been suggested by Professor
G. J. Romanes as having aided in bringing about the char-
acteristic infertility or sterility of hybrids. It is founded on
the fact, already noticed, that certain individuals of some
species possess what may be termed selective sterility that is,
while fertile with some individuals of the species they are
sterile with others, and this altogether independently of any
differences of form, colour, or structure. The phenomenon,
in the only form in which it has been observed, is that of " in-
fertility or absolute sterility between two individuals, each of
which is perfectly fertile with all other individuals;" but Mr.
Romanes thinks that "it would not be nearly so remarkable, or
physiologically improbable, that such incompatibility should run
through a whole race or strain." 1 Admitting that this may be

7. In the other part of the area, however, where hybridism occurs with
perfect freedom, hybrids of various degrees may increase till they equal or
even exceed in number the pure species that is, the incipient species will
be liable to be swamped by intercrossing.

8. The first result, then, of a partial sterility of crosses appearing in one
part of the area occupied by the two forms, will be that the great majority
of the individuals will there consist of the two pure forms only, while in the
remaining part these will be in a minority, which is the same as saying that
the new physiological variety of the two forms will be better suited to the
conditions of existence than the remaining portion which has not varied
physiologically.

9. But when the struggle for existence becomes severe, that variety which
is best adapted to the conditions of existence always supplants that which is
in i perfectly adapted ; therefore, by natural selection the varieties which are
n/i'i'ile when crossed will become established as the only ones.

10. Now let variations in the amount of sterility and in the ilinincliiifitiini
In I'/'nfised unions continue to occur also in certain parts of the area : exactly
the same result must recur, and the progeny of this new physiological variety
will in time occupy the whole area.

11. There is yet another consideration that would facilitate the process.
It seems probable that the sterility variations would, to some extent, concur
with, and perhaps depend upon, the specific varintimix ; so that, just in propor-
tion as the two forms diverged and became better adapted to the conditions of
existence, they would become more sterile \\heii intercrossed. If this were
the case, then natural selection would act with double strength ; and those
which were better adapted to survive both structurally and physiologically
would certainly do so.

1 Cases of this kind are referred to at p. lf>f>. It must, however, be noted,
that, such sterility in first crosses appears to he equally rare l>et \\ccn ditl'erent

Species of the same genus as between individuals of tile same species. Mules

and other hybrids arc freely produced between very distinct species, but are

vii ON THE INFERTILITY OF CROSSES 181

so, though we have at present no evidence whatever in
support of it, it remains to be considered whether such physio-
logical varieties could maintain themselves, or whether, as in
the cases of sporadic infertility already discussed, they would
necessarily die out unless correlated with useful characters.
Mr. Romanes thinks that they would persist, and urges that
" whenever this one kind of variation occurs it cannot escape
the preserving agency of physiological selection. Hence, even
if it be granted that the variation which affects the re-
productive system in this particular way is a variation of
comparatively rare occurrence, still, as it /////>/ always be
preserved whenever it does occur, its influence in the manu-
facture of specific types muxt In- cumulative." The very positive
statements which I have italicised would lead most readers to
believe that the alleged fact had been demonstrated by a
careful working out of the process in some definite supposed
cases. This, however, has nowhere been done in Mr. Eomanes'
paper ; and as it is the vital theoretical point on which any
possible value of the new theory rests, and as it appears so
opposed to the self-destructive effects of simple infertility,
which we have already demonstrated when it occurs between
the intermingled portion of two varieties, it must be carefully
examined. In doing so, I will suppose that the required
variation is not of "rare occurrence," but of considerable
amount, and that it appears afresh each year to about the
same extent, thus giving the theory every possible advantage.
Let us then suppose that a given species consists of 100,000
individuals of each sex, with only the usual amount of
fluctuating external variability. Let a physiological variation
arise, so that 10 per cent of the whole number 10,000
individuals of each sex while remaining fertile inter se
become quite sterile with the remaining 90,000. This
peculiarity is not correlated with any external differences of

themselves infertile or quite sterile ; and it is this infertility or sterility of the
hybrids that is the characteristic and was once thought to be the criterion
of species, not the sterility of their first crosses. Hence we should not
expect to find any constant infertility in the first crosses between the distinct
strains or varieties that formed the starting-point of new species, but only a
slight amount of infertility in their mongrel offspring. It follows, that Mr.
Romanes' theory of Physiological Selection which assumes sterility or in-
fertility between first crosses as the fundamental fact in the origin of species
does not accord with the general phenomena of hybridism in nature.

182 DARWINISM CHAP.

form or colour, or with inherent peculiarities of likes or
dislikes leading to any choice as to the pairing of the t\vo sets
of individuals. AVe have now to inquire, What would be the
result ?

Taking, first, the 10,000 pairs of the physiological or
abnormal variety, we find that each male of these might
pair with any one of the whole 100,000 of the opposite
sex. If, therefore, there was nothing to limit their choice
to particular individuals of either variety, the probabilities
are that 9000 of them Avould pair with the opposite variety,
and only 1000 with their own variety that is, that 9000
would form sterile unions, and only one thousand would form
fertile unions.

Taking, next, the 90,000 normal individuals of either sex,
we find, that each male of these has also a choice of 100,000
to pair with. The probabilities are, therefore, that nine-
tenths of them that is, 81,000 would pair with their
normal fellows, while 9000 would pair with the opposite
abnormal variety forming the above-mentioned sterile unions.

Now, as the number of individuals forming a species
remains constant, generally speaking, from year to year, we
shall have next year also 100,000 pairs, of which the two
physiological varieties will be in the proportion of eighty-one
to one, or 98,780 pairs of the normal variety to 1220 1 of
the abnormal, that being the proportion of the fertile unions
of each. In this year we shall find, by the same rule of
probabilities, that only 15 males of the abnormal variety will
pair with their like and be fertile, the remaining 1205 forming
sterile unions with some of the normal variety. The follow-
ing year the total 100,000 pairs will consist of 99, ( .)S4 of the
normal, and only 16 of the abnormal variety; and the prob
abilities, of course, are, that the whole of these latter will
pair with some of the enormous preponderance of normal
individuals, and, their unions being sterile, the physiological
variety will become extinct in the third year.

If now in the second and each succeeding year a similar

proportion as at first (10 per cent) of the physiological variety

is produced afresh from the ranks of the normal variety, the

same rate of diminution will go on, and it will be found that,

1 The exact number is 1219 '51, Imt the fractions arc omitted for clearness.

vn ON THE INFERTILITY OF CROSSES 183

on the most favourable estimate, the physiological variety can
never exceed 12,000 to the 88,000 of the normal form of the
species, as shown by the following table :

1st Year. 10,000 of physiological variety to 90,000 of normal variety.
2d 1,220 + 10,000 again produced.

3d 16 + 1,220 + 10,000 do. = 11,236

4th 0+ 16+ 1,220+10,000 do. =11,236

6th 0+16+ 1,220 + 10,000 = 11,236

and so on for any number of generations.

In the preceding discussion we have given the theory the
advantage of the large proportion of 10 per cent of this very
exceptional variety arising in its midst year by year, and we
have seen that, even under these favourable conditions, it is
unable to increase its numbers much above its starting-point,
and that it remains wholly dependent on the continued
renewal of the variety for its existence beyond a few years.
It appears, then, that this form of inter -specific sterility
cannot be increased by natural or any other known form of
selection, but that it contains within itself its own principle
of destruction. If it is proposed to get over the difficulty by
postulating a larger percentage of the variety annually arising
within the species, we shall not affect the law of decrease until
we approach equality in the numbers of the two varieties.
But with any such increase of the physiological variety the
species itself would inevitably suffer by the large propor-
tion of sterile unions in its midst, and would thus be at a
great disadvantage in competition with other species which
were fertile throughout. Thus, natural selection will always
tend to weed out any species with too great a tendency to
sterility among its own members, and will therefore prevent
such sterility from becoming the general characteristic of vary-
ing species, which this theory demands should be the case.

On the whole, then, it appears clear that no form of
infertility or sterility between the individuals of a species,
can be increased by natural selection unless correlated with
some useful variation, while all infertility not so correlated
has a constant tendency to effect its own elimination. But
the opposite property, fertility, is of vital importance to every
species, and gives the offspring of the individuals which
possess it, in consequence of their superior numbers, a greater

184 DARWINISM CHAP.

chance of survival in the battle of life. It is, therefore,
directly under the control of natural selection, which acts
both by the self-preservation of fertile and the self-destruction
of infertile stocks except always Avhere correlated as above,
when they become useful, and therefore subject to be increased
by natural selection.

Summary and Concluding Remarks on Hylridity.

The facts which are of the greatest importance to a com-
prehension of this very difficult subject are those which show
the extreme susceptibility of the reproductive system both in
plants and animals. We have seen how both these classes of
organisms may be rendered infertile, by a change of conditions
Avhich does not affect their general health, by captivity, or
by too close interbreeding. We have seen, also, that infertility
is frequently correlated with a difference of colour, or with other
characters ; that it is not proportionate to divergence of
structure ; that it varies in reciprocal crosses between pairs of
the same species ; while in the cases of dimorphic and tri-
morphic plants the different crosses between the same pair
of individuals may be fertile or sterile at the same time. It
appears as if fertility depended on such a delicate adjustment
of the male and female elements to each other, that, unless
constantly kept up by the preservation of the most fertile
individuals, sterility is always liable to arise. This preservation
always occurs within the limits of each species, both because
fertility is of the highest importance to the continuance of the
race, and also because sterility (and to a less extent infertility)
is self-destructive as well as injurious to the species.

So long therefore as a species remains undivided, and in
occupation of a continuous area, its fertility is kept up by
natural selection ; but the moment it becomes separated,
either by geographical or selective isolation, or by diveisit \
of station or of habits, then, while each portion must lie kept
fertile inter sc, there is nothing to prevent infertility arising
between the two separated portions. As the two portions
will necessarily exist under somewhat different conditions of
life, and will usually have acquired some diversity of form and
coloiir both which circumstance's we know to be cither tin*
cause of infertility or to be correlated with it, the fact of

vii ON THE INFERTILITY OF CROSSES 185

some degree of infertility usually appearing between closely
allied but locally or physiologically segregated species is exactly
what we should expect.

The reason why varieties do not usually exhibit a similar
amount of infertility is not difficult to explain. The popular
conclusions on this matter have been drawn chiefly from what
occurs among domestic animals, and we have seen that the
very first essential to their becoming domesticated was that
they should continue fertile under changed conditions of life.
During the slow process of the formation of new varieties by
conscious or unconscious selection, fertility has always been
an essential character, and has thus been invariably preserved
or increased ; while there is some evidence to show that
domestication itself tends to increase fertility.

Among plants, wild species and varieties have been more
frequently experimented on than among animals, and we
accordingly find numerous cases in which distinct species of
plants are perfectly fertile when crossed, their hybrid offspring
being also fertile inter se. We also find some few examples of
the converse fact varieties of the same species which, when
crossed are infertile or even sterile.

The idea that either infertility or geographical isolation is
absolutely essential to the formation of new species, in order
to prevent the swamping effects of intercrossing, has been
shown to be unsound, because the varieties or incipient
species will, in most cases, be sufficiently isolated by
having adopted different habits or by frequenting different
stations ; while selective association, which is known to be
general among distinct varieties or breeds of the same species,
will produce an effective isolation even when the two forms
occupy the same area.

From the various considerations now adverted to, Mr.
Darwin arrived at the conclusion that the sterility or in-
fertility of species with each other, whether manifested in the
difficulty of obtaining first crosses between them or in the
sterility of the hybrids thus obtained, is not a constant or
necessary result of specific difference, but is incidental on
unknown peculiarities of the reproductive system. These
peculiarities constantly tend to arise under changed conditions
owing to the extreme susceptibility of that system, and they

186 DARWINISM CHAP, vn

are usually correlated with variations of form or of colour.
Hence, as fixed differences of form and colour, slowly gained
by natural selection in adaptation to changed conditions, are
what essentially characterise distinct species, some amount of
infertility between species is the usual result.

Here the problem was left by Mr. Darwin ; but we have
shown that its solution may be carried a step further. If we
accept the association of some degree of infertility, however
slight, as a not unfrequent accompaniment of the external
differences which always arise in a state of nature between
varieties and incipient species, it has been shown that natural
selection has power to increase that infertility just as it has
power to increase other favourable variations. Such an in-
crease of infertility will be beneficial, whenever new species arise
in the same area with the parent form ; and we thus see
how, out of the fluctuating and very unequal amounts of infer-
tility correlated with physical variations, there may have
arisen that larger and more constant amount which appears
usually to characterise well-marked species.

The great body of facts of which a condensed account has
been given in the present chapter, although from an experi-
mental point of view very insufficient, all point to the general
conclusion we have now reached, and afford us a not unsatis-
factory solution of the great problem of hybridism in relation
to the origin of species by means of natural selection. Further
experimental research is needed in order to complete the
elucidation of the subject ; but until these additional facts are
forthcoming no new theory seems required for the explanation
of the phenomena.

CHAPTER VIII

THE ORIGIN AND USES OF COLOUR IN ANIMALS

The Darwinian theory threw new light on organic colour The problem to
be solved The constancy of animal colour indicates utility Colour
and environment Arctic animals white Exceptions prove the rule
Desert, forest, nocturnal, and oceanic animals General theories of
animal colour Variable protective colouring Mr. Foulton's experi-
ments Special or local colour adaptations Imitation of particular
objects How they have been produced Special protective colouring
of butterflies Protective resemblance among marine animals Pro-
tection by terrifying enemies Alluring coloration The coloration
of birds' eggs Colour as a means of recognition Summary of the
preceding exposition Influence of locality or of climate on colour
Concluding remarks.

AMONG the numerous applications of the Darwinian theory
in the interpretation of the complex phenomena presented by
the organic world, none have been more successful, or are more
interesting, than those which deal with the colours of animals
and plants. To the older school of naturalists colour was a
trivial character, eminently unstable and untrustworthy in the
determination of species; and it appeared to have, in most cases,
no use or meaning to the objects which displayed it. The
bright and often gorgeous coloration of insect, bird, or flower,
was either looked upon as having been created for the enjoy-
ment of mankind, or as due to unknown and perhaps undis-
coverable laws of nature.

But the researches of Mr. Darwin totally changed our point
of view in this matter. He showed, clearly, that some of the
colours of animals are useful, some hurtful to them ; and he
believed that many of the most brilliant colours were developed
by sexual choice ; while his great general principle, that all

188 DARWINISM CHAP.

the fixed characters of organic beings have been developed
under the action of the law of utility, led to the inevitn bit-
conclusion that so remarkable and conspicuous a character as
colour, which so often constitutes the most obvious distinction
of species from species, or group from group, must also have
arisen from survival of the fittest, and must, therefore, in most
cases have some relation to the Avellbeing of its possessors.
Continuous observation and research, carried on by multitudes
of observers during the last thirty years, have shown this to
be the case ; but the problem is found to be far more complex
than was at first supposed. The modes in which colour is of
use to different classes of organisms is very varied, and have
probably not yet been all discovered ; while the infinite variety
and marvellous beauty of some of its developments are such
as to render it hopeless to arrive at a complete and satisfactory
explanation of every individual case. So much, however, has
been achieved, so many curious facts have been explained, and
so much light has been thrown on some of the most obscure
phenomena of nature, that the subject deserves a prominent
place in any account of the Darwinian theory.

The Problem to be Solved.

Before dealing with the various modifications of colour in
the animal world it is necessary to say a few words on colour
in general, on its prevalence in nature, and how it is that the
colours of animals and plants require any special explanation.
What we term colour is a subjective phenomenon, clue to tlu>
constitution of our mind and nervous system; while, objectively,
it consists of light- vibrations of different wave-lengths emitted
by, or reflected from, various objects. Every visible object
must be coloured, because to be visible it must send ra\^ of
light to our eye. The kind of light it sends is modified by the
molecular constitution or the surface texture of the object.
Pigments absorb certain rays and reflect the remainder, and
this reflected portion has to our eyes a definite colour, according
to the portion of the rays constituting white light which are
absorbed. Interference colours are produced cither by thin
films or by very fine stria? on the surfaces of bodies, which
cause, rays of certain wave lengt Its to neutralise each other,
leaving the remainder to produce the eU'ects ot colour. Such

vill ORIGIN AND' USES OF COLOUR IN ANIMALS 189

are the colours of soap-bubbles, or of steel or glass on which
extremely fine lines have been ruled ; and these colours often
produce the effect of metallic lustre, and are the cause of most
of the metallic hues of birds and insects.

As colour thus depends on molecular or chemical constitution
or on the minute surface texture of bodies, and, as the matter
of which organic beings are composed consists of chemical com-
pounds of great complexity and extreme instability, and is also
subject to innumerable changes during growth and development,
we might naturally expect the phenomena of colour to be more
varied here than in less complex and more stable compounds.
Yet even in the inorganic world we find abundant and varied
colours ; in the earth and in the water ; in metals, gems, and
minerals ; in the sky and in the ocean ; in sunset clouds and in
the many-tinted rainbow. Here we can have no question of
11. W to the coloured object, and almost as little perhaps in the
vivid red of blood, in the brilliant colours of red snow and
other low altfje and fund, or even in the universal mantle of

O O '

green which clothes so large a portion of the earth s surface.
The presence of some colour, or even of many brilliant colours,
in animals and plants would require no other explanation than
does that of the sky or the ocean, of the ruby or the emerald
that is, it would require a purely physical explanation
only. It is the wonderful individuality of the colours of animals
and plants that attracts our attention the fact that the colours
are localised in definite patterns, sometimes in accordance with
structural characters, sometimes altogether independent of
them ; while often differing in the most striking and fantastic
manner in allied species. We are thus compelled to look
upon colour not merely as a physical but also as a biological
characteristic, which has been differentiated and specialised
by natural selection, and must, therefore, find its explanation
in the principle of adaptation or utility.

The Constancy of Animal Colour indicates Utility.

That the colours and markings of animals have been
acquired under the fundamental law of utility is indicated by
a general fact which has received very little attention. As a
rule, colour and marking are constant in each species of wild
animal, while, in almost every domesticated animal, there arises

190 DARWINISM CHAP.

great variability. We see this in our horses and cattle, our
dogs and cats, our pigeons and poultry. Now, the essential
difference between the conditions of life of domesticated and
wild animals is, that the former are protected by man, while
the latter have to protect themselves. The extreme variations
in colour that immediately arise under domestication indicate
a tendency to vary in this way, and the occasional occurrence
of white or piebald or other exceptionally coloured individuals
of many species in a state of nature, shoAvs that this tendency
exists there also ; and, as these exceptionally coloured in-
dividuals rarely or never increase, there must be some con-
stant power at work to keep it in check. This power can
only be natural selection or the survival of the fittest, which
again implies that some colours are useful, some injurious, in
each particular case. With this principle as our guide, let
us see how far we can account both for the general and
special colours of the animal world.

Colour and Environment.

The fact that first strikes us in our examination of the
colours of animals as a whole, is the close relation that exists
between these colours and the general environment. Thus,
white prevails among arctic animals ; yellow or brown in desert
species ; while green is only a common colour in tropical ever-
green forests. If we consider these cases somewhat carefully
we shall find, that they afford us excellent materials for forming
a judgment on the various theories that have been suggested
to account for the colours of the animal world.

In the arctic regions there are a number of animals which are
wholly white all the year round, or which only turn white in
winter. Among the former are the polar bear and the American
polar hare, the snowy owl and the Greenland falcon ; among
the latter the arctic fox, the arctic hare, the ermine, and tin-
ptarmigan. Those which are permanently white remain among
the snow nearly all the year round, while those which change
their colour inhabit regions which are free from snow in
summer. The obvious explanation of this style of coloration
i-^, that it is protective, serving to conceal the herbivorous species
from their enemies, and enabling carnivorous animals to approach
their prey unperceived. Two other explanations have, how-

vni ORIGIN AND USES OF COLOUR IN ANIMALS 191

ever, been suggested. One is, that the prevalent white of the
arctic regions has a direct effect in producing the white colour
in animals, either by some photographic or chemical action on
the skin or by a reflex action through vision. The other is,
that the white colour is chiefly beneficial as a means of checking
radiation and so preserving animal heat during the severity of
an arctic winter. The first is part of the general theory that
colour is the effect of coloured light on the objects a pure
hypothesis which has, I believe, no facts whatever to support
it. The second suggestion is also an hypothesis merely,
since it has not been proved by experiment that a white
colour, per se, independently of the fur or feathers which is so
coloured, has any effect whatever in checking the radiation of
low-grade heat like that of the animal body. But both alike
are sufficiently disproved by the interesting exceptions to the
rule of Avhite coloration in the arctic regions, which exceptions
are, nevertheless, quite in harmony with the theory of pro-
tection.

Whenever we find arctic animals which, from whatever
cause, do not require protection by the white colour, then
neither the cold nor the snow-glare has any effect upon their
coloration. The sable retains its rich brown fur throughout
the Siberian winter ; but it frequents trees at that season and
not only feeds partially on fruits or seeds, but is able to
catch birds among the branches of the fir-trees, with the bark
of which its colour assimilates. Then we have that thoroughly
arctic animal, the musk-sheep, which is brown and conspicuous ;
but this animal is gregarious, and its safety depends on its
association in small herds. It is, therefore, of more im-
portance for it to be able to recognise its kind at a distance
than to be concealed from its enemies, against which it can
well protect itself so long as it keeps together in a compact
body. But the most striking example is that of the common
raven, which is a true arctic bird, and is found even in
mid- winter as far north as any known bird or mammal.
Yet it always retains its black coat, and the reason, from our
point of view, is obvious. The raven is a powerful bird
and fears no enemy, while, being a carrion-feeder, it has no
need for concealment in oixler to approach its prey. The
colour of the raven and of the musk- sheep are, therefore,

192 DARWINISM CHAI-.

both inconsistent with any other theory than that the white
colour of arctic animals has been acquired for concealment,
and to that theory both afford a strong support. Here we
have a striking example of the exception proving the rule.

In the desert regions of the earth we find an even more
general accordance of colour with surroundings. The lion,
the camel, and all the desert antelopes have more or less the
colour of the sand or rock among which they live. The
Egyptian cat and the Pampas cat are sandy or earth coloured.
The Australian kangaroos are of similar tints, and the
original colour of the wild horse is supposed to have been
sandy or clay coloured. Birds are equally well protected
by assimilative hues ; the larks, quails, goatsuckers, and
grouse which abound in the North African and Asiatic deserts
are all tinted or mottled so as closely to resemble the average
colour of the soil in the districts they inhabit. Canon
Tristram, who knows these regions and their natural history
so well, says, in an often quoted passage : " In the desert,
where neither trees, brushwood, nor even undulations of
the surface afford the slightest protection to its foes, a
modification of colour which shall be assimilated to that of
the surrounding country is absolutely necessary. Hence,
without exception, the upper plumage of every bird, whether
lark, chat, sylvain, or sand-grouse, and also the fur of all the
smaller mammals, and the skin of all the snakes and lizards,
is of one uniform isabelline or sand colour."

Passing on to the tropical regions, it is among their
evergreen forests alone that we find whole groups of birds
whose ground colour is green. Parrots are very generally
green, and in the East we have an extensive group of green
fruit-eating pigeons; while the barbets, bee-eaters, tnracos,
leaf -thrushes (Phyllornis), white-eyes (Zosterops), and many
other groups, have so much green in their plumage as to tend
greatly to their concealment among the dense foliage. There
can be no doubt that these colours have been acquired as a
protection, when we see that in all the temperate regions,
where the leaves are deciduous, the ground colour of the
great majority of birds, especially tin the upper surface, is a
rusty bro\vn of various shades, well corresponding with t-ho.
bark, withered leaves, ferns, and bare thickets among \vhich

viii ORIGIN AND USES OF COLOUR IN ANIMALS 193

they live in autumn and winter, and especially in early spring
when so many of them build their nests.

Nocturnal animals supply another illustration of the same
rule, in the dusky colours of mice, rats, bats, and moles, and in
the soft mottled plumage of owls and goatsuckers which,
while almost equally inconspicuous in the twilight, are such as
to favour their concealment in the daytime.

An additional illustration of general assimilation of colour
to the surroundings of animals, is furnished by the inhabitants
of the deep oceans. Professor Moseley of the Challenger
Expedition, in his British Association lecture on this subject,
says : " Most characteristic of pelagic animals is the almost
crystalline transparency of their bodies. So perfect is this trans-
parency that very many of them are rendered almost entirely
invisible when floating in the water, while some, even when
caught and held up in a glass globe, are hardly to be seen.
The skin, nerves, muscles, and other organs are absolutely
hyaline and transparent, but the liver and digestive tract
often remain opaque and of a yellow or brown colour, and
exactly resemble when seen in the water small pieces of
floating seaweed." Such marine organisms, however, as
are of larger size, and either occasionally or habitually float
on the surface, are beautifully tinged with blue above, thus
harmonising Avith the colour of the sea as seen by hovering
birds ; while they are white below, and are thus invisible
against the wave-foam and clouds as seen by enemies beneath
the surface. Such are the tints of the beautiful nudibranchiate
mollusc, Glaucus atlanticus, and many others.

General Theories of Animal Colour.

We are now in a position to test the general theories, or,
to speak more correctly, the popular notions, as to the origin
of animal coloration, before proceeding to apply the principle
of utility to the explanation of some among the many
extraordinary manifestations of colour in the animal world.
The most generally received theory undoubtedly is, that
brilliancy and variety of colour are due to the direct action
of light and heat ; a theory no doubt derived from the
abundance of bright - coloured birds, insects, and flowers
which are brought from tropical regions. There are, however,

O

194 DARWINISM CHAT.

two strong arguments against this theory. AVe have already
seen how generally bright coloration is wanting in desert
animals, yet here heat and light are both at a maximum,
and if these alone were the agents in the production of
colour, desert animals should be the most brilliant. Again,
all naturalists who have lived in tropical regions know that
the proportion of bright to dull coloured species is little if
any greater there than in the temperate zone, while there are
many tropical groups in which bright colours are almost en-
tirely unknown. No part of the world presents so many
brilliant birds as South America, yet there are extensive
families, containing many hundreds of species, which are as
plainly coloured as our average temperate birds. Such are the
families of the bush-shrikes and ant-thrushes (Formicariidae),
the tyrant-shrikes (Tyrannidse), the American creepers (Den-
drocolaptidffi), together with a large proportion of the wood-
warblers (Mniotiltidaj), the finches, the wrens, and some other
groups. In the eastern hemisphere, also, we have the babbling-
thrushes (Timaliidse), the cuckoo-shrikes (Campephagidne), the
honey-suckers (Meliphagidse), and several other smaller groups
which are certainly not coloured above the average standard
of temperate birds.

Again, there are many families of birds which spread over
the whole world, temperate and tropical, and among these the
tropical species rarely present any exceptional brilliancy of
colour. Such are the thrushes, goatsuckers, hawks, plovers,
and ducks ; and in the last-named group it is the temperate
and arctic zones that afford the most brilliant coloration.

The same general facts are found to prevail among insects.
Although tropical insects present some of the most gorgeous
coloration in the whole realm of nature, yet there are
thousands and tens of thousands of species which are as dull
coloured as any in our cloudy land. The extensive family of
the carnivorous ground-beetles (Carabidse) attains its greatest
brilliancy in the temperate zone; while by far the larger
proportion of the g7'eat families of the longicorns and the
weevils, are of obscure colours even in the tropics. In butter-
flies, there is undoubtedly a larger proportion of brilliant
colour in the tropics ; but if we compare families which are
almost equally developed over the globe as the rierida- or

viii ORIGIN AND USES OF COLOUR IN ANIMALS 195

whites and yellows, and the Satyridse or ringlets we shall find
no great disproportion in colour between those of temperate
and tropical regions.

The various facts which have now briefly been noticed are
sufficient to indicate that the light and heat of the sun are
not the direct causes of the colours of animals, although they
may favour the production of colour when, as in tropical
regions, the persistent high temperature favours the develop-
ment of the maximum of life. We will now consider the
next suggestion, that light reflected from surrounding coloured
objects tends to produce corresponding colours in the animal
world.

This theory is founded on a number of very curious facts
which prove, that such a change does sometimes occur and is
directly dependent on the colours of surrounding objects ; but
these facts are comparatively rare and exceptional in their
nature, and the same theory will certainly not apply to the in-
finitely varied colours of the higher animals, many of which
are exposed to a constantly varying amount of light and
colour during their active existence. A brief sketch of these
dependent changes of colour may, however, be advantageously
given here.

Variable Protective Colouring.

There are two distinct kinds of change of colour in animals
due to the colouring of the environment. In one case the
change is caused by reflex action set up by the animal seeing
the colour to be imitated, and the change produced can be
altered or repeated as the animal changes its position. In the
other case the change occurs but once, and is probably not
due to any conscious or sense action, but to some direct in-
fluence on the surface tissues while the creature is undergoing
a moult or change to the pupa form.

The most striking example of the first class is that of the
chameleon, which changes to white, brown, yellowish, or
green, according to the colour of the object on which it rests.
This change is brought about by means of two layers of
pigment cells, deeply seated in the skin, and of bluish and
yellowish colours. By suitable muscles these cells can be
forced upwards so as to modify the colour of the skin, which,

196 DARWINISM CHAP.

when they are not brought into action, is a dirty white.
These animals are excessively sluggish and defenceless, and the
power of changing their colour to that of their immediate sur-
roundings is no doubt of great service to them. Many of the
flatfish are also capable of changing their colour according to
the colour of the bottom they rest on ; and frogs have a
similar power to a limited extent. Some Crustacea also
change colour, and the power is much developed in the
Chameleon shrimp (Mysis Chama?leon) which is gray when on
sand, but brown or green when among brown or green seaweed.
It has been proved by experiment that when this animal is
blinded the change does not occur. In all these cases,
therefore, we have some form of reflex or sense action by
which the change is produced, probably by means of pigment
cells beneath the skin as in the chameleon.

The second class consists of certain larvae, and pupa?, which
undergo changes of colour when exposed to differently
coloured surroundings. This subject has been carefully
investigated by Mr. E. B. Poulton, who has communicated
the results of his experiments to the Royal Society. 1 It had
been noticed that some species of larvae which fed on several
different plants had colours more or less corresponding to the
particular plant the individual fed on. Numerous cases are
given in Professor Meldola's article on " Variable Protective
Colouring" (Proc. Zool. Soc., 1873, p. 153), and while the
general green coloration was attributed to the presence of
chlorophyll beneath the skin, the particular change in corn-
spondence to each food-plant was attributed to a special
function which had been developed by natural selection.
Later on, in a note to his translation of Weissmann's Tlu'uni
of Descent, Professor Meldola seemed disposed to think that
the variations of colour of some of the species might lie
phytophagic that is, due to the direct action of the differently
coloured leaves on which the insect fed. Mr. Poulton's
experiments have thrown much light on this question, since he
has conclusively proved that, in the case of the sphinx cater-
pillar of Smerinthus ocellatus, the change of colour is not due
to the food but to the coloured light reflected from the leave*.

1 Proceedings of the Itni/nl >'</./-///, No. 243, 1886 ; Transactions of the /.'(//<//
Society, vol. dxxviii. L. \<\>. 311-441.

vni ORIGIN AND USES OF COLOUR IN ANIMALS 197

This was shown by feeding two sets of larvae on the same
plant but exposed to differently coloured surroundings,
obtained by sewing the leaves together, so that in one case
only the dark upper surface, in the other the whitish under
surface was exposed to view. The result in each case was a
corresponding change of colour in the larvae, confirming the
experiments on different individuals of the same batch of
larvae which had been supplied with different food-plants or
exposed to a different coloured light.

An even more interesting series of experiments was made
on the colours of pupae, which in many cases were known to
be affected by the material on which they underwent their
transformations. The late Mr. T. W. Wood proved, in 1867,
that the pupae of the common cabbage butterflies (Pieris
brassiere and P. rapae) were either light, or dark, or green, ac-
cording to the coloured boxes they were kept in, or the colours
of the fences, walls, etc., against which they were suspended.
Mrs. Barber in South Africa found that the pupae of Papilio
Nireus underwent a similar change, being deep green when
attached to orange leaves of the same tint, pale yellowish-green
when on a branch of the bottle-brush tree whose half-dried
leaves were of this colour, and yellowish when attached to
the wooden frame of a box. A few other observers noted
similar phenomena, but nothing more was done till Mr.
Poulton's elaborate series of experiments with the larvae of
several of our common butterflies were the means of clearing;

O

up several important points. He showed that the action
of the coloured light did not affect the pupa itself but the
larva, and that only for a limited period of time. After
a caterpillar has done feeding it wanders about seeking a
suitable place to undergo its transformation. When this is
found it rests quietly for a day or two, spinning the web from
which it is to suspend itself ; and it is during this period of
quiescence, and perhaps also the first hour or two after its
suspension, that the action of the surrounding coloured
surfaces determines, to a considerable extent, the colour of
the pupa. By the application of various surrounding colours
during this period, Mr. Poulton was able to modify the colour
of the pupa of the common tortoise-shell butterfly from nearly
black to pale, or to a brilliant golden ; and that of Pieris rapae

198 DARWINISM CHAP.

from dusky through pinkish to pale green. It is interesting

to note, that the colours produced were in all cases such only
as assimilated with the surroundings usually occupied by the
species, and also, that colours which did not occur in such sur-
roundings, as dark red or blue, only produced the same effects
as dusky or black.

Careful experiments were made to ascertain whether the
effect was produced through the sight of the caterpillar. The
ocelli were covered with black varnish, but neither this, nor
cutting off the spines of the tortoise-shell larva to ascertain
whether they might be sense-organs, produced any effect on
the resulting colour. Mr. Poulton concludes, therefore, that
the colour-action probably occurs over the whole surface of
the body, setting up physiological processes which result in
the corresponding colour-change of the pupa. Such changes
are, however, by no means universal, or even common, in
protectively coloured pupae, since in Papilio machaon and
some others which have been experimented on, both in this
country and abroad, no change can be produced on the pupa
by any amount of exposure to differently coloured surround-
ings. It is a curious point that, with the small tortoise-shell
larva, exposure to light from gilded surfaces produced pupa)
with a brilliant golden lustre ; and the explanation is supposed
to be that mica abounded in the original habitat of the species,
and that the pupse thus obtained protection Avhen suspended
against micaceous rock. Looking, however, at the wide range
of the species and the comparatively limited area in which
micaceous rocks occur, this seems a rather improbable ex-
planation, and the occurrence of this metallic appearance is
still a difficulty. It does not, however, commonly occur in
this country in a natural state.

The two classes of variable colouring here discussed are
evidently exceptional, and can have little if any relation to
the colours of those more active creatures which arc continu-
ally changing their position with regard to surrounding objects,
and whose colours and markings are nearly constant through-
out the life of the individual, and (with the exception of
sexual differences) in all the individuals of the species. We
will now briefly pass in review the various characteristics and
uses of (lie colours which more generally prevail in nature ;

vin ORIGIN AND USES OF COLOUR IN ANIMALS 199

and having already discussed those protective colours which
serve to harmonise animals with their general environment,
we have to consider only those cases in which the colour
resemblance is more local or special in its character.

Special or Local Colour Adaptations.

This form of colour adaptation is generally manifested by
markings rather than by colour alone, and is extremely pre-
valent both among insects and vertebrates, so that we shall
be able to notice only a few illustrative cases. Among our
native birds we have the snipe and woodcock, whose markings
and tints strikingly accord with the dead marsh vegetation
among which they live the ptarmigan in its summer dress is
mottled and tinted exactly like the lichens Avhich cover the
stones of the higher mountains ; while young unfledged plovers
are spotted so as exactly to resemble the beach pebbles among
which they crouch for protection, as beautifully exhibited in
one of the cases of British birds in the Natural History
Museum at South Kensington.

In mammalia, we notice the frequency of rounded spots on
forest or tree haunting animals of large size, as the forest
deer and the forest cats ; while those that frequent reedy or
grassy places are striped vertically, as the marsh antelopes
and the tiger. I had long been of opinion that the brilliant
yellow and black stripes of the tiger were adaptive, but have
only recently obtained proof that it is so. An experienced
tiger-hunter, Major AValford, states in a letter, that the haunts
of the tiger are invariably full of the long grass, dry and pale
yellow for at least nine months of the year, which covers the
ground wherever there is water in the rainy season, and he
adds : " I once, while following up a wounded tiger, failed for
at least a minute to see him under a tree in grass at a distance
of about twenty yards jungle open but the natives saw
him, and I eventually made him out well enough to shoot
him, but even then I could not see at what part of him I was
aiming. There can be no doubt whatever that the colour of
both the tiger and the panther renders them almost invisible,
especially in a strong blaze of light, when among grass, and
one does not seem to notice stripes or spots till they are
dead." It is the black shadows of the vegetation that

200 DARWINISM CHAP.

assimilate with the black stripes of the tiger ; and, in like
manner, the spotty shadows of leaves in the forest so
harmonise with the spots of ocelots, jaguars, tjger-cats, and
spotted deer as to afford them a very perfect concealment.

In some cases the concealment is effected by colours and
markings which are so striking and peculiar that no one who
had not seen the creature in its native haunts would imagine
them to be protective. An example of this is afforded by the
banded fruit pigeon of Timor, whose pure white head and
neck, black wings and back, yellow belly, and deeply-curved
black band across the breast, render it a very handsome and
conspicuous bird. Yet this is what Mr. H. 0. Forbes says of
it : " On the trees the white-headed fruit pigeon (Ptilopus
cinctus) sate motionless during the heat of the day in numbers,
on well-exposed branches ; but it was with the utmost difficulty
that I or my sharp-eyed native servant could ever detect them,
even in trees where we knew they were sitting." l The trees
referred to are species of Eucalyptus which abound in Timor.
They have whitish or yellowish bark and very open foliage,
and it is the intense sunlight casting black curved shadows of
one branch upon another, with the white and yellow bark and
deep blue sky seen through openings of the foliage, that pro
duces the peculiar combination of colours and shadows to
which the colours and markings of this bird have become so
closely assimilated.

Even such brilliant and gorgeously coloured birds as the
sun-birds of Africa are, according to an excellent observer,
often protectively coloured. Mrs. M. E. Barber remarks
that " A casual observer would scarcely imagine that the
highly varnished and magnificently coloured plumage of the
various species of Noctarinea could be of service to them, yet
this is undoubtedly the case. The most unguarded moments
of the lives of these birds are those that are spent amongst
the flowers, and it is then that they are less wary than at an}'
other time. The different species of aloes, which blossom in
succession, form the principal sources of their winter supplies
of food ; and a legion of other gay flowering plants in spring
and summer, the aloe blossoms especially, are all brilliantly
coloured, and they harmonise admirably with the gay plumage
1 A Naturalist's Wanderings in the Eastern Archipelago, p. 460.

via ORIGIN AND USES OF COLOUR IN ANIMALS 201

of the different species of sun-birds. Even the keen eye of a
hawk will fail to detect them, so closely do they resemble the
flowers the)' frequent. The sun-birds are fully aware of this
fact, for no sooner have they relinquished the flowers than they
become exceedingly wary and rapid in flight, darting arrow-
like through the air and seldom remaining in exposed situations.
The black sun-bird (Xcctarinea ainetln stina) is never absent
from that magnificent forest-tree, the ' Kaffir Boom ' (Erythrina
cafl'ra) ; all day long the cheerful notes of these birds may be
heard amongst its spreading branches, yet the general aspect
of the tree, which consists of a huge mass of scarlet and purple-
blaek blossoms without a .single green leaf, blends and har-
monises with the colours of the blaek sun bird to such an extent
that a dozen of them may be. feeding amongst its blossoms
without being conspicuous, or even visible." 1

Some other cases will still further illustrate how the colours
of even very conspicuous animals may be adapted to their
peculiar haunts.

The late Mr. Swinhoe says of the Kerivoula picta, which
he observed in Formosa: "The body of this bat was of an
orange colour, but the wings were painted with orange-yellow
and black. It was caught suspended, head downwards, on a
cluster of the fruit of the longan tree (Nephelium longanum).
Now this tree is an evergreen, and all the year round some
portion of its foliage is undergoing decay, the particular leaves
being, in such a stage, partially orange and black. This bat
can, therefore, at all seasons suspend from its branches and
elude its enemies by its resemblance to the leaves of the
tree." 2

Even more curious is the case of the sloths defenceless
animals which feed upon leaves, and hang from the branches
of trees with their back downwards. Most of the species have
a curious buff-coloured spot on the back, rounded or oval in
shape and often with a darker border, which seems placed
there on purpose to make them conspicuous ; and this was a
great puzzle to naturalists, because the long coarse gray or
greenish hair was evidently like tree-moss and therefore
protective. But an old writer, Baron von Slack, in his Voyage

1 Trans. Phil. Soc. (? of S. Africa), 1878, part iv. p. 27.
2 Proc. Zool. Soc., 1862 p. 357.

202 DARWINISM CHAP.

to ^iiri/t'tm (1810), had already explained the matter. He
says : "The colour and even the shape of the hair are much
like withered moss, and serve to hide the animal in the trees,
but particularly when it has that orange-coloured spot between
the shoulders and lies close to the tree ; it looks then exactly
like a piece of branch where the rest has been broken off, by
which the hunters are often deceived." Even such a huge
animal as the giraffe is said to be perfectly concealed by its
colour and form when standing among the dead and broken
trees that so often occur on the outskirts of the thickets where
it feeds. The large blotch-like spots on the skin and the
strange shape of the head and horns, like broken branches, so
tend to its concealment that even the keen-eyed natives have
been known to mistake trees for giraffes or giraffes for trees.

Innumerable examples of this kind of protective colouring
occur among insects ; beetles mottled like the bark of trees or
resembling the sand or rock or moss on which they live, with
green caterpillars of the exact general tints of the foliage they
feed on ; but there are also many cases of detailed imitation of
particular objects by insects that must be briefly described. 1

Protective Imitation of Particular Objects.

The insects which present this kind of imitation most per-
fectly are the Phasmidre, or stick and leaf insects. The well-

1 With reference to this general resemblance of insects to their environment
the following remarks by Mr. Poultou are very instructive. He says :
" Holding the larva of Sphinx ligustri in one hand and a twig of its food-
plant iu the other, the wonder we feel is, not at the resemblance but at the
difference ; we are surprised at the difficulty experienced in detecting so con-
spicuous an object. And yet the protection is very real, for the larva 1 , will be
passed over by those who are not accustomed to their appearance, although the
searcher may be told of the presence of a large caterpillar. An experienced
entomologist may also fail to find the larvae till after a considerable search.
This is general protective resemblance, and it depends upon a general harmony
between the appearance of the organism and its whole environment. It is
impossible to understand the force of this protection for any larva, without
seeing it on its food-pl.-mt and in an entirely normal condition. The artistic.
effect of green foliage is more complex than we often imagine ; numberless
modifications are wrought by varied lights and shadows upon colours which are
in themselves far from uniform. In the larva of Papilio machaon the pro-
tection is very real when the larva is on the food-plant, and can hardly
be appreciated at all when the two are apart." Numerous other examples are
given in the chapter on "Mimicry and other Protective Resemblances among
Animals." in my CuatributwHS to the Theory </ Natural Xelcdiun.

viii ORIGIN AND USES OF COLOUR IX ANIMALS 203

known leaf-insects of Ceylon and of Java, species of Phyllium,
are so wonderfully coloured and veined, with leafy expansions
on the legs and thorax, that not one person in ten can see
them when resting on the food-plant close beneath their eyes.
Others resemble pieces of stick with all the minutire of knots
and branches, formed by the insects' legs, which are stuck out
rigidly and unsymmetrically. I have often been unable to
distinguish between one of these insects and a real piece of
stick, till I satisfied myself by touching it and found it to !><
alive. One species, which was brought me in Borneo, was
covered with delicate semitransparent green foliations, exactly
resembling the hepatica? which cover pieces of rotten stick in
the damp forests. Others resemble dead leaves in all their
varieties of colour and form ; and to show how perfect is the
protection obtained and how important it is to the possessors
of it, the following incident, observed by Mr. Belt in Nicaragua,
is most instructive. Describing the armies of foraging ants in
the forest which devour every insect they can catch, he says :
" I was much surprised with the behaviour of a green leaf-
like locust. This insect stood immovably among a host of ants,
many of which ran over its legs without ever discovering there
was food within their reach. So fixed was its instinctive
knowledge that its safety depended on its immovability, that
it allowed me to pick it up and replace it among the ants without
making a single effort to escape. This species closely resembles
a green leaf." l

Caterpillars also exhibit a considerable amount of detailed
resemblance to the plants on which they live. Grass -feeders
are striped longitudinally, while those on ordinary leaves are
always striped obliquely. Some very beautiful protective
resemblances are shown among the caterpillars figured in
Smith and Abbott's Lepidopterous Insects of Gevrij'm, a work
published in the early part of the century, before any theories
of protection were started. The plates in this work are
most beautifully executed from drawings made by Mr. Abbott,
representing the insects, in every case, on the plants which
they frequented, and no reference is made in the descriptions
to the remarkable protective details which appear upon the
plates. We have, first, the larva of Sphinx fuciformis feeding
1 The Naturalist in Nicaragua, p. 19.

204 DARWINISM CHAP.

on a plant Avith linear grass-like leaves and small blue flowers ;
and we find the insect of the same green as the leaves, striped
longitudinally in accordance with the linear leaves, and with
the head blue corresponding both in size and colour with the
flowers. Another species (Sphinx tersa) is represented feeding
on a plant with small red flowers situated in the axils of the
leaves ; and the larva has a row of seven red spots, unequal
in size, and corresponding very closely with the colour and
size of the flowers. Two other figures of sphinx larva? are
very curious. That of Sphinx pampinatrix feeds on a wild
vine (Vitis indivisa), having green tendrils, and in this species
the curved horn on the tail is green, and closely imitates in
its curve the tip of the tendril. But in another species
(Sphinx cranta), which feeds on the fox -grape (Vitis vulpina),
the horn is very long and red, corresponding with the long red-
tipped tendrils of the plant. Both these larvre are green with
oblique stripes, to harmonise with the veined leaves of the
vines ; but a, figure is also given of the last-named species after
it has done feeding, when it is of a decided brown colour and
has entirely lost its horn. This is because it then descends to
the ground to bury itself, and the green colour and red
horn would be conspicuous and dangerous ; it therefore loses
both at the last moult. Such a change of colour occurs in
many species of caterpillars. Sometimes the change is seasonal ;
and, in those which hibernate with us, the colour of some
species, which is brownish in autumn in adaptation to the
fading foliage, becomes green in spring to harmonise with the
newly -opened leaves at that season. 1

Some of the most curious examples of minute imitation
are afforded by the caterpillars of the geometer moths, which
are always brown or reddish, and resemble in form little
twigs of the plant on which they feed. They have the habit,
when at rest, of standing out obliquely from the branch, to
which they hold on by their hind pair of prolegs or claspers,
and remain motionless for hours. Speaking of these pro-
tective resemblances Mr. Jenner Weir says: "After being
thirty years an entomologist I was deceived myself, and took
out my pruning scissors to cut from a plum tree a spur which
I thought I had overlooked. This turned out to be the larva

1 R. Meldola, iu Proc. Zool. Soc., 1873, p. 155.

vin ORIGIN AND USES OF COLOUR IN ANIMALS 205

of a geometer two inches long. I showed it to several members
of my family, and defined a space of four inches in which it
\\as to be seen, but none of them could perceive that it was a
caterpillar." 1

One more example of a protected caterpillar must be
given. Mr. A. Everett, writing from Sarawak, Borneo, says:
" I had a caterpillar brought me, which, being mixed by my
boy with some other things, I took to be a bit of moss with
two exquisite pinky-white seed capsules ; but I soon saw that
it moved, and examining it more closely found out its real
character : it is covered with hair, with two little pink spots
on the upper surface, the general hue being more green. Its
motions are very slow, and when eating the head is with-
drawn beneath a fleshy mobile hood, so that the action of
feeding does not produce any movement externally. It wa>
found in the limestone hills at Busan, the situation of all
others where mosses are most plentiful and delicate, and
where they partially clothe most of the protruding masses
of rock."

How these Imitations have been Produced.

To many persons it will seem impossible that such beauti-
ful and detailed resemblances as those now described and
these are only samples of thousands that occur in all parts of
the world can have been brought about by the preservation
of accidental useful variations. But this will not seem so
surprising if we keep in mind the facts set forth in our
earlier chapters the rapid multiplication, the severe struggle
for existence, and the constant variability of these and
all other organisms. And, further, we must remember
that these delicate adjustments are the result of a process
which has been going on for millions of years, and that we
now see the small percentage of successes among the myriads
of failures. From the very first appearance of insects and
their various kinds of enemies the need of protection arose,
and was usually most easily met by modifications of colour.
Hence, we may be sure that the earliest leaf- eating insects
acquired a green colour as one of the necessities of their
existence ; and, as the species became modified and specialised,

1 Mature, vol. iii. p. 166.

206 DARWINISM CHAP.

those feeding on particular species of plants would rapidly
acquire the peculiar tints and markings best adapted to
conceal them upon those plants. Then, every little variation
that, once in a hundred years perhaps, led to the preservation
of some larva which was thereby rather better concealed than
its fellows, would form the starting-point of a further
development, leading ultimately to that perfection of imitation
in details which now astonishes us. The researches of Dr.
Weissmann illustrate this progressive adaptation. The very
young larvae of several species are green or yellowish without
any markings ; they then, in subsequent moults, obtain certain
markings, some of which are often lost again before the larva
is fully grown. The early stages of those species which,
like elephant hawk -moths (Chserocampa), have the anterior
segments elongated and retractile, with large eye -like spots
to imitate the head of a vertebrate, are at first like those of
non-retractile species, the anterior segments being as large as
the rest. After the first moult they become smaller, com-
paratively ; but it is only after the second moult that the
ocelli begin to appear, and these are not fully defined till after
the third moult. This progressive development of the in-
dividual the ontogeny gives us a clue to the ancestral
development of the whole race the phylogeny ; and we are
enabled to picture to ourselves the very slow and gradual
steps by which the existing perfect adaptation has been
brought aboiit. In many larvre great variability still exists,
and in some there are two or more distinctly-coloured forms
usually a dark and a light or a brown and a green form.
The larva of the humming-bird hawk -moth (Macroglossa
stellatarum) varies in this manner, and Dr. Weissmann raised
five varieties from a batch of eggs from one moth. It feeds
on species of bedstraw (Galium verum and G. mollugo), and
as the green forms are less abundant than the brown, it has
probably undergone some recent change of food -plant or
of habits which renders brown the more protective colour.

Special Protective Colouring of Butterflies.

We will now consider a few cases of special protective
colouring in the perfect butterfly or moth. Mr. Man sol
Weale states that in South Africa there is a great prevalence

vin ORIGIN AND USES OF COLOUR IN ANIMALS 207

of white and silvery foliage or bark, sometimes of dazzling
brilliancy, and that many insects and their larvae have brilliant
silvery tints which are protective, among them being three
species of butterflies whose undersides are silvery, and which
are thus effectually protected when at rest. 1 A common
African butterfly (Aterica meleagris) always settles on the
ground with closed wings, which so closely resemble the soil
of the district that it can with difficulty be seen, and the
colour varies with the soil in different localities. Thus
specimens from Senegambia were dull brown, the soil being
reddish sand and iron- clay; those from Calabar and Came-
roons were light brown with numerous small white spots, the
soil of those countries being light brown clay with small
quartz pebbles; while in other localities where the colours <!'
the soil were more varied the colours of the butterfly varied
also. Here we have variation in a single species which has
become specialised in certain areas to harmonise with the
colour of the soil. 2

Many butterflies, in all parts of the world, resemble dead
leaves on their under side, but those in which this form
of protection is carried to the greatest perfection are the
species of the Eastern genus Kallima. In India K. inachis,
and in the larger Malay islands K. paralekta, are very com-
mon. They are rather large and showy butterflies, orange
and bluish on the upper side, with a very rapid flight, and
frequenting dry forests. Their habit is to settle always where
there is some dead or decaying foliage, and the shape and
colour of the wings (on the under surface), together with the
attitude of the insect, is such as to produce an absolutely
perfect imitation of a dead leaf. This is effected by the
butterfly always settling on a twig, with the short tail of the
hind wings just touching it and forming the leaf -stalk.
From this a dark curved line runs across to the elongated tip
of the upper wings, imitating the midrib, on both sides of
which are oblique lines, formed partly by the nervures and
partly by markings, which give the effect of the usual veining
of a leaf. The head and antennae fit exactly between the
closed upper wings so as not to interfere with the outline,

1 Trans. Ent. Soc. Lond., 1878, p. 185.
2 Ibid. (Proceedings, p. xlii.)

208 DARWINISM CHAP.

which has just that amount of irregular curvature that is seen
in dry and withered leaves. The colour is very remarkable
for its extreme amount of variability, from deep reddish-brown
to olive or pale yellow, hardly two specimens being exactly
alike, but all coming within the range of colour of leaves in
various stages of decay. Still more curious is the fact that
the paler wings, which imitate leaves most decayed, are
usually covered with small black dots, often gathered into
circular groups, and so exactly resembling the minute fungi
on decaying leaves that it is hard at first to believe that the
insects themselves are not attacked by some such fungus.
The concealment produced by this wonderful imitation is
most complete, and in Sumatra I have often seen one enter a
bush and then disappear like magic. Once I was so fortunate
as to see the exact spot on which the insect settled ; but even
then I lost sight of it for some time, and only after a per-
sistent search discovered that it was close before my eyes. 1
Here we have a kind of imitation, which is very common in a
less developed form, carried to extreme perfection, with the
result that the species is very abundant over a considerable
area of country.

Protective Resemblance among Marine Animals.

Among marine animals this form of protection is very
common. Professor Moseley tells us that all the inhabitants
of the Gulf-weed are most remarkably coloured, for purposes
of protection and concealment, exactly like the weed itself.
" The shrimps and crabs which swarm in the weed are of
exactly the same shade of yellow as the weed, and have white
markings upon their bodies to represent the patches of Mem-
branipora. The small fish, Antennarius, is in the same way
weed-colour with white spots. Even a Planarian worm, which
lives in the weed, is similarly yellow-coloured, and also a
mollusc, Scyllrea pelagica." The same writer tells us that " a
number of little crabs found clinging to the floats of the blue-
shelled mollusc, lanthina, were all coloured of a corresponding
blue for concealment." 5

1 Wallace's Malay Archipelago, vol. i. p. 204 (fifth edition, p. 130), with
figure.

2 Moseley's Notes by a Naturalist on tin- C/ml/i ni/rr.

ORIGIN" AND USES OF COLOUR IN ANIMALS 209

Professor E. 8. Morse of Salem, Mass., found that most
of the New England marine mollusca were protectively
coloured ; instancing among others a little red chiton on rocks
clothed with rod calcareous alga?, and Crepidula plana, liv-
ing within the apertures of the shells of larger species of
(iasteropuds and of a pure white colour corresponding to its
habitat, while allied species living on seaweed or on the
outside of dark shells were dark brown. 1 A still more
interesting case has been recorded by Mr. (leorge Brady. He
says : " Amongst the Xullipore which matted together the
laniinaria roots in the Firth of Clyde were living numerous
small starfishes (Ophiocoma bellis) which, except \\lim their
writhing movements betrayed them, wen; quite undistinguish-
ablo from the calcareous 1 tranches of the alga; their rigid
angularly twisted rays had all the appearance of the coralline,
and exactly assimilated to its dark purple colour, so that
though I held in my hand a root in which were half a dozen
of the starfishes, I was really unable to detect them until
revealed by their movements." :

These feu examples are sufficient to show that the principle
of protective coloration extends to the ocean as well as over
the earth ; and if we consider how completely ignorant we
are of the habits and surroundings of most marine animals, it
may well happen that many of the colours of tropical fishes,
which seem to us so strange and so conspicuous, are really
protective, owing to the number of equally strange and
brilliant forms of corals, sea-anemones, sponges, and sea-
weeds among which they live.

Protection IHJ Terrifying Enemies.

A considerable number of quite defenceless insects obtain
protection from some of their enemies by having acquired a
resemblance to dangerous animals, or by some threatening or
unusual appearance. This is obtained either by a modifica-
tion of shape, of habits, of colour, or of all combined. The
simplest form of this protection is the aggressive attitude of
the caterpillars of the Sphingidse, the forepart of the body

1 Proceedings of the Boston Soc. of Nat. Hist., vol. xiv. 1871.
- Nature, 1870, p. 376.

P

210 DARWINISM CHAP.

being erected so as to produce a rude resemblance to the figure
of a sphinx, hence the name of the family. The protection is
carried further by those species which retract the first three
segments and have large ocelli on each side of the fourth
segment, thus giving to the caterpillar, when the forepart of
its body is elevated, the appearance of a snake in a threaten-
ing attitude.

The blood-red forked tentacle, thrown out of the neck of
the larvae of the genus Papilio when alarmed, is, no doubt, a
protection against the attacks of ichneumons, and may, per-
haps, also frighten small birds ; and the habit of turning up
the tail possessed by the harmless rove-beetles (Staphylinidse),
giving the idea that they can sting, has, probably, a similar
use. Even an unusual angular form, like a crooked twig or
inorganic substance, may be protective ; as Mr. Poulton thinks
is the case with the curious caterpillar of Notodonta ziczac,
which, by means of a few slight protuberances on its body,
is able to assume an angular and very unorganic-looking
appearance. But perhaps the most perfect example of this
kind of protection is exhibited by the large caterpillar of
the Eoyal Persimmon moth (Bombyx regia), a native of
the southern states of North America, and known there as
the " Hickory -horned devil." It is a large green cater-
pillar, often six inches long, ornamented with an immense
crown of orange-red tubercles, which, if disturbed, it erects
and shakes from side to side in a very alarming manner.
In its native country the negroes believe it to be as deadly
as a rattlesnake, whereas it is perfectly innocuous. The
green colour of the body suggests that its ancestors were
once protectively coloured ; but, growing too large to be
effectually concealed, it acquired the habit of shaking its head
about in order to frighten away its enemies, and ultimately
developed the crown of tentacles as an addition to its terrify-
ing powers. This species is beautifully figured in Abbott and
Smith's Lepidopterous Insects of Georgia.

Alluring Coloration.

Besides those numerous insects which obtain protection
through their resemblance to the natural objects among which
they live, there are some whose disguise is not used for

vni ORIGIN AND USES OF COLOUR IN ANIMALS 211

concealment, but as a direct means of securing their prey by
attracting them within the enemy's reach. Only a few cases
of this kind of coloration have yet been observed, chiefly
among spiders and mantida? ; but, no doubt, if attention
were given to the subject in tropical countries, many more
would be discovered. Mr. H. 0. Forbes has described a
most interesting example of this kind of simulation in
lava. While pursuing a large butterfly through the jungle,
he was stopped by a dense bush, on a leaf of which he
observed one of the skipper butterflies sitting on a bird's
dropping. "I had often," he says, "observed small Blues
at rest on similar spots on the ground, and have wondered
what such a refined and beautiful family as the Lycsenidre
could find to enjoy, in food apparently so incongruous
for a butterfly. I approached with gentle steps, but
ready net, to see if possible how the present species was
engaged. It permitted me to get quite close, and even to
seize it between my fingers ; to my surprise, however, part of
the body remained behind, adhering as I thought to the
excreta. I looked closely, and finally touched with my finger
the excreta to find if it were glutinous. To my delighted
astonishment I found that my eyes had been most perfectly
deceived, and that what seemed to be the excreta Avas a
most artfully coloured spider, lying on its back with its feet
crossed over and closely ad pressed to the body." Mr. Forbes
then goes on to describe the exact appearance of such excreta,
and how the various parts of the spider are coloured to
produce the imitation, even to the liquid portion which
usually runs a little down the leaf. This is exactly imitated
by a portion of the thin web which the spider first spins
to secure himself firmly to the leaf ; thus producing, as Mr.
Forbes remarks, a living bait for butterflies and other insects
so artfully contrived as to deceive a pair of human eyes, even
Avhen intently examining it. 1

A native species of spider (Thomisus citreus) exhibits a
somewhat similar alluring protection by its close resemblance
to buds of the wayfaring tree, Viburnum lantana. It is pure
creamy-white, the abdomen exactly resembling in shape and
colour the unopened buds of the flowers among which it takes
1 A Naturalist's Wanderings in the Eastern Archipelago, p. 63.

212 DARWINISM CHAP.

its station ; and it has been seen to capture flies which came
to the flowers.

But the most curious and beautiful case of alluring protec-
tion is that of a wingless Mantis in India, which is so formed
and coloured as to resemble a pink orchis or some other
fantastic flower. The whole insect is of a bright pink colour,
the large and oval abdomen looking like the labellura of
an orchid. On each side, the two posterior legs have im-
mensely dilated and flattened thighs which represent the
petals of a flower, while the neck and forelegs imitate the
upper sepal and column of an orchid. The insect rests
motionless, in this symmetrical attitude, among bright green
foliage, being of course very conspicuous, but so exactly
resembling a flower that butterflies and other insects settle
upon it and are instantly captured. It is a living trap,
baited in the most alluring manner to catch the unwary
flower-haunting insects. 1

The Coloration of Birds' Eggs.

The colours of birds' eggs have long been a difficulty on
the theory of adaptive coloration, because, in so many cases
it has not been easy to see what can be the use of the par-
ticular colours, which are often so bright and conspicuous that
they seem intended to attract attention rather than to be con-
cealed. A more careful consideration of the subject in all its
bearings shows, however, that here too, in a great number of
cases, we have examples of protective coloration. When,
therefore, we cannot see the meaning of the colour, we may
suppose that it has been protective in some ancestral form,
and, not being hurtful, has persisted under changed condi-
tions which rendered the protection needless.

We may divide all eggs, for our present purpose, into two

1 A beautiful drawing of this rare insect, Hymenopits bicornis (in the
nymph or active pupa state), was kindly sent me by Mr. Wood-Mason, Curator
of the Indian Museum at Calcutta. A species, very similar to it, inhabits Java,
where it is said to resemble a pink orchid. Other Mantidtv, of the genus
Gon r ylus, have the anterior part of the thorax dilated and coloured either
white, pink, or purple ; and they so closely resemble flowers that, according
to Mr. Wood- Mason, one of them, having a bright violet-blue protlioraric,
shield, was found in Pegu by a botanist, and was for a moment mistaken by
him for a flower. See Proc. Ent. Sue. Lond., 1878, p. liii.

via ORIGIN AND USES OF COLOUR IN ANIMALS 213

great divisions ; those which are white or nearly so, and those
which are distinctly coloured or spotted. Egg-shells being com-
posed mainly of carbonate of lime, we may assume that the
primitive colour of birds' eggs was white, a colour that pre-
vails now among the other egg-bearing vertebrates lizards,
crocodiles, turtles, and snakes ; and we might, therefore, expect
that this colour would continue where its presence had no
disadvantages. Now, as a matter of fact, we find that in all
the groups of birds which lay their eggs in concealed places,
whether in holes of trees or in the ground, or in domed or
covered nests, the eggs are cither pure white or of very pah;
uniform coloration. Such is the case with kingfishers, bee-
ratcrs, penguins, and puffins, which nest in holes in the
ground; with the great parrot family, the woodpeckers, the
rollers, hoopoes, trogons, owls, and some others, which build in
holes in trees or other concealed places; while martins, wrens,
willow-warblers, and Australian finches, build domed or covered
nests, and usually have white eggs.

There are, however, many other birds which lay their
white eggs in open nests ; and these afford some very in-
teresting examples of the varied modes by which concealment
may be obtained. All the duck tribe, the grebes, and the
pheasants belong to this class; but these birds all have the
habit of covering their eggs with dead leaves or other material
whenever they leave the nest, so as effectually to conceal
them. Other birds, as the short-eared owl, the goatsucker,
the partridge, and some of the Australian ground pigeons,
lay their white or pale eggs on the hare soil ; but in these
cases the birds themselves are protectively coloured, so that,
when sitting, they are almost invisible ; and they have the
habit of sitting close and almost continuously, thus effectually
concealing their eggs.

Pigeons and doves offer a very curious case of the protec-
tion of exposed eggs. They usually build very slight and
loose nests of sticks and twigs, so open that light can be
seen through them from below, while they are generally well
concealed by foliage above. Their eggs are white and
shining ; yet it is a difficult matter to discover, from beneath,
whether there are eggs in the nest or not, while they are well
hidden by the thick foliage above. The Australian podargi

214 DARWINISM CHAP.

huge goatsuckers build very similar nests, and their white
eggs are protected in the same manner. Some large and
powerful birds, as the swans, herons, pelicans, cormorants, and
storks, lay white eggs in open nests ; but they keep careful
watch over them, and are able to drive away intruders. On
the whole, then, we see that, while white eggs are conspicuous,
and therefore especially liable to attack by egg-eating animals,
they are concealed from observation in many and various ways.
We may, therefore, assume that, in cases where there seems
to be no such concealment, we are too ignorant of the whole
of the conditions to form a correct judgment.

We now come to the large class of coloured or richly
spotted eggs, and here we have a more difficult task, though
many of them decidedly exhibit protective tints or markings.
There are two birds which nest on sandy shores the lesser
tern and the ringed plover, and both lay sand-coloured eggs,
the former spotted so as to harmonise with coarse shingle, the
latter minutely speckled like fine sand, which are the kinds
of ground the two birds choose respectively for their nests.
" The common sandpipers' eggs assimilate so closely with
the tints around them as to make their discovery a matter
of no small difficulty, as every oologist can testify who has
searched for them. The pewits' eggs, dark in ground
colour and boldly marked, are in strict harmony with the
sober tints of moor and fallow, and on this circumstance
alone their concealment and safety depend. The divers'
eggs furnish another example of protective colour ; they
are generally laid close to the water's edge, amongst drift
and shingle, Avhere their dark tints and black spots conceal
them by harmonising closely with surrounding objects. The
snipes and the great army of sandpipers furnish innumer-
able instances of protectively coloured eggs. In all the
instances given the sitting - bird invariably leaves the eggs
uncovered when it quits them, and consequently their safety
depends solely on the colours which adorn them." 1 The
wonderful range of colour and marking in the eggs of the
guillemot may be imputed to the inaccessible rocks on which

1 C. Dixon, iu Seebolim's History of British Birds, vol. ii. Introduction, p.
xxvi. Many of the other examples here cited are taken from the same valu-
able work,

vni ORIGIN AND USES OF COLOUR IN ANIMALS 215

it breeds, giving it complete protection from enemies. Thus
the pale or bluish ground colour of the eggs of its allies, the
auks and puffins, has become intensified and blotched and
spotted in the most marvellous variety of patterns, owing to
there being no selective agency to prevent individual variation
having full sway.

The common black coot (Fulica atra) has eggs which are
coloured in a specially protective manner. Dr. William
Marshall writes, that it only breeds in certain localities where
a large water reed (Phragmites arundinacea) abounds. The
eggs of the coot are stained and spotted with black on a
yellowish-gray ground, and the dead leaves of the reed are of
the same colour, and arc stained black by small parasitic fungi
of the Uredo family ; and these leaves form the bed on which
the eggs are laid. The eggs and the leaves agree so closely
in colour and markings that it is a difficult thing to dis-
tinguish the eggs at any distance. It is to be noted that
the coot never covers up its eggs, as its ally the moor-hen
usually doos.

The beautiful blue or greenish eggs of the hedge-sparrow,
the song-thrush, the blackbird, and the lesser redpole seem at
first sight especially calculated to attract attention, but it is
very doubtful whether they are really so conspicuous when
seen at a little distance among their usual surroundings. For
the nests of these birds are either in evergreens, as holly or
ivy, or surrounded by the delicate green tints of our early
spring vegetation, and may thus harmonise very well with the
colours around them. The great majority of the eggs of our
smaller birds are so spotted or streaked with brown or black
on variously tinted grounds that, when lying in the shadow of
the nest and surrounded by the many colours and tints of
bark and moss, of purple buds and tender green or yellow
foliage, with all the complex glittering lights and mottled
shades produced among these by the spring sunshine and by
sparkling raindrops, they must have a quite different aspect
from that which they possess when we observe them torn
from their natural surroundings. We have here, probably,
a similar case of general protective harmony to that of the
green caterpillars with beautiful white or purple bands and
spots, which, though gaudily conspicuous when seen alone,

216 DARWINISM CHAP.

become practically invisible among the complex lights and
shadows of the foliage they feed upon.

In the case of the cuckoo, which lays its eggs in the nests
of a variety of other birds, the eggs themselves are subject
to considerable variations of colour, the most common type,
however, resembling those of the pipits, wagtails, or warblers,
in whose nests they are most frequently laid. It also often
lays in the nest of the hedge-sparrow, whose bright blue eggs
are usually not at all nearly matched, although they are
sometimes said to be so on the Continent. It is the opinion
of many ornithologists that each female cuckoo lays the same
coloured eggs, and that it usually chooses a nest the owners
of which lay somewhat similar eggs, though this is by no
means universally the case. Although birds which have
cuckoos' eggs imposed upon them do not seem to neglect them
on account of any difference of colour, yet they probably do
so occasionally ; and if, as seems probable, each bird's eggs are
to some extent protected by their harmony of colour with their
surroundings, the presence of a larger and very differently
coloured egg in the nest might be dangerous, and lend to the
destruction of the Avhole set. Those cuckoos, therefore, Avhich
most frequently placed their eggs among the kinds Avhich they
resembled, would in the long run leave most progeny, and
thus the very frequent accord in colour might have been
brought about.

Some writers have suggested that the varied colours of
birds' eggs are primarily due to the effect of surrounding
coloured objects on the female bird during the period pre-
ceding incubation ; and have expended much ingenuity in
suggesting the objects that may have caused the eggs of one
bird to be blue, another brown, and another pink. 1 But no
evidence has been presented to prove that any effects what-
ever are produced by this cause, while there seems no difficulty
in accounting for the facts by individual variability and the
action of natural selection. The changes that occur in the
conditions of existence of birds must sometimes render the
concealment less perfect than it may once have been ; and
when any danger arises from this cause, it may be met either

1 See A. H. S. Lucas, in Proceedings of Royal Society of Victoria, 1887,
1>. 56.

VHI ORKilN AM) VSKS OF COLOUR IN ANIMALS 217

l>y some change in the colour of the eggs, or in the structure
or position of the nest, or l>y the increased care which the
parents bestow upon the eggs. In this way the various
divergences which now so often puzzle us may have arisen.

I '<>!,, II I' i/s If MntHS (if Hl'ClHJII if/oil.

If we consider the habits and life-histories of those animals
which are more or less gregarious, comprising a large pro-
portion of the herbivora, some carnivora, and a considerable
number of all orders of birds, we shall see that a means of
ready recognition of its own kind, at a distance or during
rapid motion, in the dusk of twilight or in partial cover,
must be of the greatest advantage and often lead to the pre-
servation of life. Animals of this kind will not usually
receive a stranger into their midst. While they keep together
they are generally safe from attack, but a solitary straggler
becomes an easj prey to the enemy; it is, therefore, of the
highest importance that, in such a case, the wanderer should
have every facility for discovering its companions with cer-
tainty at any distance within the range of vision.

Some means of easy recognition must be of vital im-
portance to the young and inexperienced of each flock, and it
also enables the sexes to recognise their kind and thus avofd
the evils of infertile crosses ; and I am inclined to believe that
its necessity has had a more widespread influence in deter-
mining the diversities of animal coloration than any other
cause whatever. To it may probably be imputed the singular
fact that, whereas bilateral symmetry of coloration is very
frequently lost among domesticated animals, it almost uni-
versally prevails in a state of nature; for if the two sides of
an animal were unlike, and the diversity of coloration among
domestic animals occurred in a wild state, easy recognition
would be impossible among numerous closely allied forms. 1

1 Professor Win. H. Brewer of Yale College has shown that the white
marks or the spots of domesticated animals are rarely symmetrical, Imt have
a tendency to appear more frequently on the left side. This is the case with
horses, cattle, clogs, and swine. Among wild animals the skunk varies con-
siderably in the amount of white on the body, and this too was found to be
usually greatest on the left side. A close examination of numerous striped or
spotted species, as tigers, leopards, jaguars, zebras, etc., showed that the
bilateral symmetry was not exact, although the general effect of the two sides

218 DARWINISM CHAP.

The wonderful diversity of colour and of marking that pre-
vails, especially in birds and insects, may be due to the fact
that one of the first needs of a new species would be, to keep
separate from its nearest allies, and this could be most readily
done by some easily seen external mark of difference. A few
illustrations will serve to show how this principle acts in nature.

My attention was first called to the subject by a remark
of Mr. Darwin's that, though, " the hare on her form is a
familiar instance of concealment through colour, yet the
principle partly fails in a closely allied species, the rabbit ; for
Avhen running to its burrow it is made conspicuous to the
sportsman, and no doubt to all beasts of prey, by its upturned
white tail." 1 But a little consideration of the habits of the
animal will show that the white upturned tail is of the greatest
value, and is really, as it has been termed by a writer in The
Field, a " signal flag of danger." For the rabbit is usually a
crepuscular animal, feeding soon after sunset or on moonlight
nights. When disturbed or alarmed it makes for its burrow,
and the white upturned tails of those in front serve as guides
and signals to those more remote from home, to the young and
the feeble ; and thus each following the one or two before it, all
are able with the least possible delay to regain a place of
comparative safety. The apparent danger, therefore, becomes
a most important means of security.

The same general principle enables us to understand the
singular, and often conspicuous, markings on so many gregarious
herbivora which are yet, on the whole, protectively coloured.
Thus, the American prong- buck has a white patch behind
and a black muzzle. The Tartarian antelope, the Ovis poli
of High Asia, the Java wild ox, several species of deer, and a
large number of antelopes have a similar conspicuous white
patch behind, which, in contrast to the dusky body, must enable
them to be seen and followed from a distance by their fellows.
AVhere there are many species of nearly the same general size
and form inhabiting the same region as with the antelopes

was the same. This is precisely what we should expect if the symmetry is not
the result of a general law of the organisation, but has been, in part at least, pro-
duced and preserved for the useful purpose of recognition by the animal's
fellows of the same species, and especially by the sexes and the young. See
Proc. nf the Am. Ass. for Advancement of Science, vol. xxx. p. 246,
1 Descent of Man, p. 542.

VIII

ORIGIN AND USES OF COLOUR IN ANIMALS

219

of Africa we find many distinctive markings of a similar
kind. The gazelles have variously striped and banded faces,
besides white patches behind and on the flanks, as shown
in the woodcut. The spring-bok has a white patch on the
face and one on the sides, with a curiously distinctive white
stripe above the tail, which is nearly concealed when the
animal is at rest by a fold of skin but comes into full view
when it is in motion, being thus quite analogous to the

FIG. 18. Gazella scemmerringi.

upturned white tail of the rabbit. In the pallah the
white rump-mark is bordered with black, and the peculiar
shape of the horns distinguishes it when seen from the
front. The sable-antelope, the gems-bok, the oryx, the hart-
beest, the bonte-bok, and the addax have each peculiar white
markings ; and they are besides characterised by horns so
remarkably different in each species and so conspicuous, that
it seems probable that the peculiarities in length, twist, and
curvature have been differentiated for the purpose of recogni-
tion, rather than for any speciality of defence in species whose
general habits are so similar.

220 DARWINISM CHAP.

It is interesting to note that these markings for recognition
are very slightly developed in the antelopes of the woods and
marshes. Thus, the grys-bok is nearly uniform in colour, except
the long black-tipped ears ; and it frequents the wooded moun-
tains. The duyker-bok and the rhoode-bok are wary bush-
haunters, and have no marks but the small white patch
behind. The wood-haunting bosch-bok goes in pairs, and has
hardly any distinctive marks on its dusky chestnut coat, but
the male alone is horned. The large and handsome koodoo
frequents brushwood, and its vertical white stripes are no
doubt protective, while its magnificent spiral horns afford easy
recognition. The eland, which is an inhabitant of the open
country, is uniformly coloured, being sufficiently recognisable
by its large size and distinctive form ; but the Derbyan eland
is a forest animal, and has a protectively striped coat. In like
manner, the fine Speke's antelope, which lives entirely in the
swamps and among reeds, has pale vertical stripes on the
sides (protective), Avith white markings on face and breast for
recognition. An inspection of the figures of antelopes and
other animals in Wood's Natural History, or in other illustrated
works, will give a better idea of the peculiarities of recognition
markings than any amount of description.

Other examples of such coloration are to be seen in the
dusky tints of the musk-sheep and the reindeer, to whom
recognition at a distance on the snowy plains is of more
importance than concealment from their few enemies. The
conspicuous stripes and bands of the zebra and the quagga arc
probably due to the same cause, as may be the singular crests
and face-marks of several of the monkeys and lemurs. 1

1 It may be thought that such extremely conspicuous markings as those of
the zebra would be a great danger in a country abounding with lions, leopards,
and other beasts of prey ; but it is not so. Zebras usually go in bauds, ami
are so swift and wary that they are in little danger during the day. It is in
the evening, or on moonlight nights, when they go to drink, that they are chiefly
exposed to attack ; and Mr. Francis Galton, who has studied these animals in their
native haunts, assures me, that in twilight they are not at all conspicuous,
the stripes of white and black so merging together into a gray tint that it is
very difficult to see them at a little distance. We have here an admirable
illustration of how a glaringly conspicuous style of marking for recognition may
be so arranged as to become also protective at the time when protection is
most needed ; and we may also learn how impossible it is for us to decide
on the inutility of any kind of coloration without a careful study of the
habits of the species in its native country.

vni ORIOIN AND USES OF COLOUR IN ANIMALS 221

.--
.-

en *i

ff ^

o

f-c O

=2
d

222 DARWINISM CHAP.

Among birds, these recognition marks are especially
numerous and suggestive. Species which inhabit open
districts are usually protectively coloured ; but they generally
possess some distinctive markings for the purpose of being
easily recognised by their kind, both when at rest and during
flight. Such are, the white bands or patches on the breast
or belly of many birds, but more especially the head and
neck markings in the form of white or black caps, collars,
eye- marks or frontal patches, examples of which are seen in
the three species of African plovers figured on page 221.

Recognition marks during flight are very important for all
birds which congregate in flocks or which migrate together ;
and it is essential that, Avhile being as conspicuous as possible,
the marks shall not interfere with the general protective tints
of the species when at rest. Hence they usually consist of
well -contrasted markings on the wings and tail, which are
concealed during repose but become fully visible when the
bird takes flight. Such markings are well seen in our four
British species of shrikes, each having quite different white
marks on the expanded wings and on the tail feathers ; and
the same is the case with our three species of Saxicola the
stone-chat, whin-chat, and wheat-ear which are thus easily
recognisable on the Aving, especially when seen from above, as
they would be by stragglers looking out for their companions.
The figures opposite, of the wings of two African species
of stone- curlew which are sometimes found in the same
districts, well illustrates these specific recognition marks.
Though not very greatly different to our eyes, they are no
doubt amply so to the sharp vision of the birds themselves.

Besides the white patches on the primaries here shown, the
secondary feathers are, in some cases, so coloured as to afford
very distinctive markings during flight, as seen in the central
secondary quills of two African coursers (Fig. 21).

Most characteristic of all, however, are the varied markings
of the outer tail-feathers, whose purpose is so well shown by
their being almost always covered during repose by the two
middle feathers, which are themselves quite unmarked and
protectively tinted like the rest of the upper surface of the
body. The figures of the expanded tails of two species of East
Asiatic snipe, whose geographical ranges overlap each other,

viii ORIGIN AND USES OF COLOUR IN ANIMALS 223

FIG. 20. CEdicnemus venniculatus (above). CE. senegalensis (below).

224

DARWINISM

CHAP.

will serve to illustrate this difference ; which is frequently much
greater and modified in an endless variety of ways (Fig. 2'2).
Numbers of species of pigeons, hawks, finches, warblers,
ducks, and innumerable other birds possess this class of mark-
and they correspond so exactly in general character with

ings

Cursorius chalcopterus. C. gallicus.

FIG. 21. Secondary quills.

those of the mammalia, already described, that we cannot
doubt they serve a similar purpose. 1

Those birds which are inhabitants of tropical forests, and
which need recognition marks that shall be at all times
visible among the dense foliage, and not solely or chiefly
during flight, have usually small but brilliant patches of colour

1 The principle of colouring for recognition was; I believe, lirst stated in
my article on " The Colours of Animals and Plants " in Macmillan's M<nj<i:.iii<',
and more fully in my volume on Tropical Nature. Subsequently Mrs. l!:irli<T
gave a few examples under the head of " Indicative or Banner Colours," but
she applied it to the distinctive colours of the malrs of birds, which I explain
on another principle, though this may assist.

vin ORIGIN AND USES OF COLOUR IN ANIMALS 225

FIG. 22. Suolopax megala (upper). S. stenura (lower),

Q

226 DARWINISM CHAP.

on the head or neck, often not interfering with the generally
protective character of their plumage. Such are the bright
patches of blue, red, or yellow, by which the usually green
Eastern barbets are distinguished ; and similar bright patches
of colour characterise the separate species of small green
fruit-doves. To this necessity for specialisation in colour, by
which each bird may easily recognise its kind, is probably due
that marvellous variety in the peculiar beauties of some groups
of birds. The Duke of Argyll, speaking of the humming
birds, made the objection that "A crest of topaz is no
better in the struggle for existence than a crest of sapphire.
A frill ending in spangles of the emerald is no better in the
battle of life than a frill ending in spangles of the ruby. A
tail is not affected for the purposes of flight, whether its
marginal or its central feathers are decorated with white ;"
and he goes on to urge that mere beauty and variety for
their own sake are the only causes of these differences. But,
on the principles here suggested, the divergence itself is useful,
and must have been produced pari passu with the structural
differences on which the differentiation of species depends ;
and thus we have explained the curious fact that prominent
differences of colour often distinguish species otherwise very
closely allied to each other.

Among insects, the principle of distinctive coloration for
recognition has probably been at work in the production of
the wonderful diversity of colour and marking we find every-
where, more especially among the butterflies and moths ; and
here its chief function may have been to secure the pairing
together of individuals of the same species. In some of the
moths this has been secured by a peculiar odour, which
attracts the males to the females from a distance ; but there is
no evidence that this is universal or even general, and among
butterflies, especially, the characteristic colour and marking,
aided by size and form, afford the most probable means of
recognition. That this is so is shown by the fact that " the
common white butterfly often flies down to a bit of paper on
the ground, no doubt mistaking it for one of its own species ;"
while, according to Mr. Collingwood, in the Malay Archipelago,
"a dead butterfly pinned upon a conspicuous twig Avill often
arrest an insect of the same species in its headlong flight, and

vin ORIGIN AND USES OF COLOUR IN ANIMALS 227

bring it down within easy reach of the net, especially if it be
of the opposite sex." 1 In a great number of insects, no doubt,
form, motions, stridulating sounds, or peculiar odours, serve to
distinguish allied species from each other, and this must be
especially the case with nocturnal insects, or with those whose
colours are nearly uniform and are determined by the need of
protection ; but by far the larger number of day-flying and
active insects exhibit, varieties of colour and marking, forming
the most obvious distinction between allied species, and which
have, therefore, in all probability been acquired in the process
of differentiation for the purpose of checking the intercrossing
of closely allied forms. 2

Whether this principle extends to any of the less highly
organised animals is doubtful, though it may perhaps have
affected the higher mollnsca. But in marine animals it seems
probable that the colours, however beautiful, varied, and
brilliant they may often be, are in most cases protective,
assimilating them to the various bright-coloured seaweeds, or
to some other animals which it is advantageous for them to
imitate. 3

Summary of the Preceding Exposition.

Before proceeding to discuss some of the more recondite
phenomena of animal coloration, it will be well to consider
for a moment the extent of the ground we have already
covered. Protective coloration, in some of its varied forms,
has not improbably modified the appearance of one-half of
the animals living on the globe. The white of arctic animals,
the yellowish tints of the desert forms, the dusky hues of
crepuscular and nocturnal species, the transparent or bluish
tints of oceanic creatures, represent a vast host in themselves;
but we have an equally numerous body whose tints are
adapted to tropical foliage, to the bark of trees, or to the soil

1 Quoted by Darwin in Descent of Man, p. 317.

2 In the American Naturalist of March 1888, Mr. J. E. Todd lias an
article on " Directive Coloration in Animals," in which he recognises many of
the cases here referred to, and suggests a few others, though I think he
includes many -forms of coloration as "paleness of belly and inner side of
legs" which do not belong to this class.

3 For numerous examples of this protective colouring of marine animals
see Moseley's Voyage of the C]i<iU?n<j<:r, and Dr. E. S. Morse in Proc. ofBost.
Soc. of Nat. Hist., vol. xiv. 1871.

228 DARWINISM CHAP.

or dead leaves on or among which they habitually live. Then
we have the innumerable special adaptations to the tints and
forms of leaves, or twigs, or flowers ; to bark or moss ; to rock
or pebble ; by which such vast numbers of the insect tribes
obtain protection ; and we have seen that these various forms
of coloration are equally prevalent in the waters of the seas
and oceans, and are thus coextensive with the domain of life
upon the earth. The comparatively small numbers which
possess "terrifying" or "alluring" coloration may be classed
under the general head of the protectively coloured.

But under the next head colour for recognition we have
a totally distinct category, to some extent antagonistic or
complementary to the last, since its essential principle is
visibility rather than concealment. Yet it has been shown, I
think, that this mode of coloration is almost equally im-
portant, since it not only aids in the preservation of existing
species and in the perpetuation of pure races, but was, per-
haps, in its earlier stages, a not unimportant factor in their
development. To it we owe most of the variety and much
of the beauty in the colours of animals ; it has caused at
once bilateral symmetry and general permanence of type ;
and its range of action has been perhaps equally extensive
with that of coloration for concealment.

Influence of Locality or of Climate on Colour.

Certain relations between locality and coloration have long
been noticed. Mr. Gould observed that birds from inland or
continental localities were more brightly coloured than those
living near the sea-coast or on islands, and he supposed that
the more brilliant atmosphere of the inland stations was the
explanation of the phenomenon. 1 Many American naturalists
have observed similar facts, and they assert that the intensity
of the colours of birds and mammals increases from north to
south, and also with the increase of humidity. This change
is imputed by Mr. J. A. Allen to the direct action of the en-
vironment. He says: "In respect to the correlation of intensity
of colour in animals with the degree of humidity, it would
perhaps be more in accordance with cause and effect to express
the law of correlation as a decrease of intensity of colour with
1 See Origin of Species, p. 107.

vin ORIGIN AND USES OF COLOUR IN ANIMALS 229

a decrease of humidity, the paleness evidently resulting from
exposure and the blanching effect of intense sunlight, and a
dry, often intensely heated atmosphere. With the decrease of
the aqueous precipitation the forest growth and the protection
afforded by arborescent vegetation gradually also decreases, as
of course does also the protection afforded by clouds, the
excessively humid regions being also regions of extreme
cloudiness, while the dry regions are comparatively cloudless
districts." Almost identical changes occur in birds, and are
imputed by Mr. Allen to similar causes.

It will be seen that Mr. Gould and Mr. Allen impute
opposite effects to tjhe same cause, brilliancy or intensity of
colour being due to a brilliant atmosphere according to the
former, while paleness of colour is imputed by the latter to
a too brilliant sun. According to the principles which have
been established by the consideration of arctic, desert, and
forest animals respectively, we shall be led to conclude that
there has been no direct action in this case, but that the effects
observed are due to the greater or less need of protection.
The pale colour that is prevalent in arid districts is in harmony
with the general tints of the surface ; while the brighter tints
or more intense coloration, both southward and in humid
districts, are sufficiently explained by the greater shelter due
to a more luxuriant vegetation and a shorter winter. The
advocates of the theory that intensity of light directly affects
the colours of organisms, are led into perpetua