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LL.D., F.L.S., KICJ. 






First Edition published May 1889 
/? mtcd A ugust and October 1889 ; 181,0 


THE present work treats the problem of the Origin of Species 
on the same general lines as were adopted by Darwin ; but 
from the standpoint reached after nearly thirty years of 
discussion, with an abundance of new facts and the advocacy 
of many new or old theories. 

While not attempting to deal, even in outline, with the 
vast subject of evolution in general, an endeavour has been 
made to give such an account of the theory of Natural Selec- 
tion as may enable any intelligent reader to obtain a clear 
conception of Darwin's work, and to understand something 
of the power and range of his great principle. 

Darwin wrote for a generation which had not accepted 
evolution, and which poured contempt on those who upheld 
the derivation of species from species by any natural law of 
descent. He did his work so well that " descent with 
modification " is now universally accepted as the order of 
nature in the organic world ; and the rising generation of 
naturalists can 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 Darwin's theory apply, solely, 
to the particular means by which the change of species has 
been brought about, not to the fact of that change. The 
objectors seek 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 


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. 


1 have also made what appears to me an important change 
in the arrangement of the subject. Instead of treating first 
the comparatively difficult and unfamiliar details of variation, 
1 commence with the Struggle for Existence, which is really 
the fundamental phenomenon on which natural selection 
depends, while the particular facts which illustrate it are 
comparatively familiar and very interesting. It has the 
further advantage that, after discussing variation and the 
effects of artificial selection, we proceed at once to explain 
how natural selection acts. 

Among the subjects of novelty or interest discussed in this 
volume, and which have important bearings on the theory of 
natural selection, are : (1) A proof that all specific characters 
are (or once have been) either useful in themselves or cor- 
related with useful characters (Chap. VI) ; (2) a proof that 
natural selection can, in certain ca^es, increase the sterility of 
crosses (Chap. VII) ; (3) a fuller discussion of the colour 
relations of animals, with additional facts and arguments on 
the origin of sexual differences of colour (Chaps. VIII-X) ; 
(4) an attempted solution of the difficulty presented by the 
occurrence of both very simple and very complex modes of 
securing the cross-fertilisation of 'plants (Chap. XI) ; (5) some 
fresh facts and arguments on the wind-carriage of seeds, and 
its bearing on the wide dispersal of many arctic and alpine 
plants (Chap. XII) ; (6) some new illustrations of the non- 
heredity of acquired characters, and a proof that the effects of 
use and disuse, even if inherited, must be overpowered by 
natural selection (Chap. XIV) ; and (7) a new argument as to 
the nature and origin of the moral and intellectual faculties 
of man (Chap. XV). 

Although 1 maintain, and even enforce, my differences 
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. 


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, arid 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 




Definition of _ species Special creation The early transmutationists 
Scientific opinion before Darwin The problem before Darwin The 
change of opinion effected by Darwin jHie Darwinian jheory Pro- 
posed mode of treatment of the subject . . .Pages 1-13 



Its importance Tho struggle among plants Among animals Illustrative 
cases Succession of trees in forests of Denmark The struggle for 
existence on the PampasIncrease of organisms in a geometrical 
ratio Examples of rapid increase of animals Rapid increase and 
wulc 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 . . . 14-40 



Importance of variability Popular ideas regarding it Variability of the 
lower animals Tho variability of insects Variation among lizards 


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 



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 selectjoii_by man Conditions favour- 
able to variation- Concluding remarks . , . 83*101 



Effect^j^struggle for existence^ under ^ ur^c]ianged__5Qjidilions The effecj; 
Iu^ej:jahangejof^qnditions 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 Jbo 

life TJjej? 011 tinned existence of low 

forms pf_ life. Extinction of low_ types among the higher animals 
Circiimstances Javourabl e 3 ^. le _ or . igi u _ _o? n ?. w species Probable 
origin of the dippers Jho importance of isolation On the advance 
Qf_^rganisatiou_by natural selection Summary of the first five 
chapters . . . . . . . 102-125 



CM (V 

Difficulty as to ^mallness of variations As to the right variations occur- 
ring whejijt^fluj^ed 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 


giants 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- 
_in^|nten3ossing Gulick on the effects of isolation Cases in which 
isolation is ineffective ..... Pages 126-151 



Statement of the problem Extreme susceptibility of the reproductive 
functions Reciprocal crosses Individual differences in respect to 
cross - fertilisation Dimorphism and trirnorphism among plants 
4-Cases of the fertility of hybrids and of the infertility of mongrels 
The effects of close interbreeding 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 ..... 152-186 



The Darwinian theory threw now light on organic colour The problem to 
be solved The constancy of animal colour indicates utility Colour 
and environment Arctic animals white Exceptions prove tho rule 
Desert, forest, nocturnal, and oceanic animals General theories of 
animal colour Variable protective colouring Mr. Poulton'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 ..... 187-231 




The skunk as an example of warning colorationWarning colours among 
insects Butterflies Caterpillars Mimicry How mimicry has been 
produced Helicon idoe 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 



Sex colours in the mollusca and crustacca 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 duo 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 
plumes 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 



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 


of additional facts bearing on insect fertilisation Fertilisation of 
flowers by birds Self-fertilisation of flowers Difficulties and con- 
tradictions Intercrossing not necessarily advantageous Supposed 
evil results of close interbreeding How the struggle for existence 
acts among flowers Flowers the product of insect agency Concluding 
remarks on colour in nature .... Pages 301-337 



The facts to be explained The conditions which have determined dis- 
tribution The permanence of oceans Oceanic and continental areas 
Madagascar and New Zealand The teachings of the thousand- 
fathom line The distribution of marsupials The distribution of 
tapirs Powers of dispersal as illustrated by insular organisms Birds 
and insects at sea Insects at great altitudes The dispersal of plants 
Dispersal of seeds by the wind Mineral matter carried by the wind 
Objections to the theory of wind-dispersal answered Explanation 
of north temperate plants in the southern hemisphere No proof of 
glaciation in the tropics Lower temperature not needed to explain 
the facts Concluding remarks .... 338-374 



What we may expect The number of known species of extinct animals 
Causes of the imperfection of tho geological record Geological 
evidences of evolution Shells Crocodiles The rhinoceros tribe 
The pedigree of the horse tribe Development of deer's horns Brain 
development Local 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 
remarks 375-409 




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 paiis by variation and selection Direct action 
of the environment The American school of evolutionistsOrigin 
of the feet of the- ungulates Supposed action of animal intelligence 
Semper on the direct influence of the environment Professor Ocddes'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 



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 
antiquity 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 


PORTRAIT OF AUTHOR . ... Frontispiece 

MAP SHOWING THE 1000-FATHOM LINE . . To face page 349 



2. ,, VARIATION OF LIZARDS . . . , . 48 








11. ,, CURVES OF VARIATION . . . .64 






17. PRIMULA VRRIS (Cowslip), From Darwin's Forms of Floivers . 157 

18. GAZELLA SOSMMERRINGI (to show recognition marks) . . 219 


Charadriadcc ....... 221 



LENSIS (from Seebolim's Charadriarfcc) , . . 223 


(from Seebolim's Charadrladcc) , . . 224 


Seebolim's Cfiaradrindru) ..... 225 



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

ceedings of the Entomological Society) .... 251 



Animal Life) . . . . . . .260 


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

29. LYTHKUM 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 




American Addresses') ...... 388 


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


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


Animal Life) . . . . . . .427 




Definition of species Special creation The early Transmntationists 
Scientific opinion before Darwin Tlio problem before Darwin 
The change of opinion effected by Darwin The Darwinian theory 
Proposed mode of treatment of the subject. 

THE title of Mr. Darwin's great work is On the Origin of 
Species by means of Natural Selection and the Preservation of 
Favoured Ilaces in the Struggle for Life. In order to ap- 
preciate fully the aim and object of this work, and the 
change which it has effected not only in natural history but 
in many other sciences, it is necessary to form a clear con- 
ception of the meaning of the term " species," to know what 
was the general belief regarding them at the time when Mr. 
Darwin's book first appeared, arid to understand what he 
meant, and what was generally meant, by discovering their 
" origin." It is for want of this preliminary knowledge that 
the majority of educated persons who are not naturalists are 
so ready to accept the innumerable objections, criticisms, and 
difficulties of its opponents as proofs that the Darwinian 
theory is unsound, while it also renders them unable to ap- 
preciate, or even to comprehend, the vast change which that 
theory has effected in the whole mass of thought and opinion 
on the great questiofi of evolution. 

The term " species " was thus defined by the celebrated 
botanist De Candolle : * ^ A species is a collection of all the 

jndmduals._which. resemblo.. eacb_ other mojre than they 

resemble anything else, which _ jean. Jbjr. J^ut^l^jcundation 



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 

And the zoologist Swainson gives a somewhat similar defini- 
tion ' u A species, in the usual_acccptation of the term, is an 
animal which, _m A JitaJte j)^ kyjcertajn 

peculiarities^ of form, size.,, colour, qr^ other circumstances^ from 
jmother__ajnimal. It pn^gates^aft^ 
pcrf ec^_r^seml^^^^j.)areiit ; its peciiljaritieSi therefore i 

To illustrate thesy definitions we will take two common 
English birds, the rook (Corvus frugilegus) and the crow 
(Corvus corone). These arc 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 versa. 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 
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 Animals, p. 350, 

and also obsc. 
siderably in their tot. 
that they might be all produce 
most eminent of these writers was u, fc 
Lamarck, who published an elaborate work, the j. , 
Zoologique, in which he endeavoured to prove that all c, 
mals whatever are descended from other species of animals. 
He attributed the change of species chiefly to the effect of 
changes in the conditions of life such as climate, food, etc. 
and especially to the desires arid efforts of the animals them- 
selves to improve their condition, leading to a modification of 
form or size in certain parts, owing to the well-known physio- 
logical law that all organs are strengthened by constant use, 
while they are weakened or even completely lost by disuse. 
The arguments of Lamarck did not, however, satisfy naturalists, 
and though a few adopted the view that closely allied species 
had descended from each other, the general belief of the 
educated public was, that each species was a " special creation " 
quite independent of all others ; while the great body of 
naturalists equally 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 work 
dealing with the question was the celebrated Vestiges of 
Creation, 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 

.xxore slightly 

^ecies, nor was any 

j ou constant differences should 

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 of 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 


" special creations " in each country ; and these conclusions 
were arrived at after a careful study of Lamarck's work, a full 
abstract of which is given in the earlier editions of the 
Principles of Geology. 1 

Professor Agassiz, one of the greatest naturalists of the last 
generation, went even further, and maintained not only that 
each species was specially created, but that it was created in 
the proportions and in the localities in which we now find it 
to exist. The following extract from his very instructive 
book on Lake Superior explains this view: "There are in 
animals peculiar adaptations which are characteristic of their 
species, and which cannot be supposed to have arisen from 
subordinate influences. Those which live in shoals cannot be 
supposed to have been created in single pairs. Those which 
are made to bo the food of others cannot have been created 
in the same proportions as those which live upon them. 
Those which are everywhere found in innumerable specimens 
must have been introduced in numbers capable of maintaining 
their normal proportions to those which live isolated and are 
comparatively and constantly fewer. For we know that this 
harmony in the numerical proportions between animals is 
one of the great laws of nature. The circumstance that 
species occur within definite limits where no obstacles prevent 
their wider distribution leads to the further inference that 
these limits were assigned to them from the beginning, and 
so we should come to the final conclusion that the order 
which prevails throughout nature is intentional, that it is 
regulated by the limits marked out on the first day of 
creation, and that it has been maintained unchanged through 
ages with no other modifications than those which the higher 
intellectual powers of man enable him to impose on some 
few animals more closely connected with him." 2 

These opinions of some of the most eminent and influential 
writers of the pro -Darwinian age seem to us, now, either 
altogether obsolete or positively absurd ; but they never- 
theless exhibit the mental condition of even the most 
advanced section of scientific men on the problem of the 

1 These expressions occur in Chapter IX. of the earlier editions (to the 
ninth) of the Principles of Geology, 

2 L. Agassiz, Lake Superior, p. 377. 


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 Geoffrey 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 desgrnded 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 was 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 was understood by the " origin " 
or the "transmutation" of species before Darwin's work 
appeared, was the comparatively simple question whether 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 laws 
and conditions still in action and capable of being thoroughly 
investigated. If any naturalist had been asked at that day 
whether, supposing it to be clearly shown that all the different 
species of each genus had been derived from some one 
ancestral species, and that a full and complete explanation 
were to be given of how each minute difference in form, 
colour, or structure might have originated, and how the 


several peculiarities of habit and of geographical distribution 
might have been brought about whether, if this were done, 
the "origin of species" would be discovered, the great 
mystery solved, he would undoubtedly have replied in the 
ailirmative. He would probably have added that he never 
expected any such marvellous discovery to be made in 
his lifetime. But so much as this assuredly Mr. Darwin has 
done, riot only in the opinion of his disciples and admirers, 
but by the admissions of those who doubt the completeness 
of his explanations. For almost all their objections and 
difficulties apply to those larger differences which separate 
genera, families, and orders from each other, not to those which 
separate one species from the species to which it is most nearly 
allied, and from the remaining species of the same genus. They 
adduce such difficulties as the first development of the eye, or 
of the milk-producing glands of the mammalia ; the wonderful 
instincts of bees and of ants ; the complex arrangements for 
the fertilisation of orchids, and numerous other points of 
structure or habit, as not being satisfactorily explained. But 
it is evident that these peculiarities had their origin at a very 
remote period of the earth's history, and no theory, however 
complete, can do more than afford a probable conjecture as to 
how they were produced. Our ignorance of the state of the 
earth's surface and of the conditions of life at those remote 
periods is very great ; thousands of animals and plants must 
have existed of which we have no record ; while wo are 
usually without any information as to the habits and general 
life-history even of those of which we possess some fragmentary 
remains ; so that the truest and most complete theory would 
not enable us to solve all the difficult problems which the 
whole course of the development of life upon our globe 
presents to us. 

What we may expect a true theory to do is to enable us 
to comprehend and follow out in some detail those changes in 
the form, structure, and relations of animals and plants which 
are effected in short periods of time, geologically speaking, 
and which are now going on around us. We may expect it 
to explain satisfactorily most of the lesser and superficial 
differences which distinguish one species from another. We 
may expect it to throw light on the mutual relations of the 


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 over-acting laws in nature, 
new species are necessarily produced, while the old species 
become extinct ; and jfc 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 amount ; 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 
whole 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 


were believed to have been always as distinct and separate as 
they are now, and to have originated by some totally unknown 
process so far removed from ordinary reproduction that it was 
usually spoken of as " special creation." There was, then, no 
question of the origin of families, orders, and classes, because 
the very first step of all, the " origin of species," was believed 
to be an insoluble problem. But now this is all changed. The 
whole scientific and literary world, even the whole educated 
public, accepts, as a matter of common knowledge, the origin 
of species from other allied species by the ordinary process of 
natural birth. The idea of special creation or any altogether 
exceptional mode of production is 'absolutely extinct ! Yet 
more : this is held also to apply to many higher groups as 
well as to the species of a genus, and not even Mr. Darwin's 
severest critics venture to suggest that the primeval bird, 
reptile, or fish must have been " specially created." And this 
vast, this totally unprecedented change in public opinion has 
been the result of the work of one man, and was brought 
about in the short space of twenty years ! This is the answer 
to those who continue to maintain that the "origin of species" is 
not yet discovered ; that there are still doubts and difficulties ; 
that there are divergencies of structure so great that we 
cannot understand how they had their beginning. Wo may 
admit all this, just as we may admit that there are enormous 
difficulties in the way of a complete comprehension of the 
origin and nature of all the parts of the solar system and of 
the stellar universe. But we claim for Darwin that he is the 
Newton of natural history, and that, just so surely as that the 
discovery and demonstration by Newton of the law of gravita- 
tion established order in place of chaos and laid a sure founda- 
tion for all future study of the starry heavens, so surely has 
Darwin, by his discovery of the law of natural selection 
and his demonstration of the great principle of the preserva- 
tion of useful variations in the struggle for life, not only thrown 
a flood of light on the process of development of the whole 
organic world, but also established a firm foundation for all 
future study of nature. 

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 Origin of 


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 erroneous. 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 oth^r 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 which, as he himself says, " will long remain 

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 


find it to harmonise with the development hypothesis, that 
Darwin devoted the whole of his life to collecting facts and 
making experiments, the record of a portion of which he has 
given us in a series of twelve masterly volumes. 

Proposed Mode of Treatment of the Subject. 

It is evidently of the most vital importance to any theory 
that its foundations should be absolutely secure. It is 
therefore necessary to show, by a wide and comprehensive 
array of facts, that animals and plants do perpetually vary in 
the manner and to the amount requisite ; and that this takes 
place in wild animals as well as in those which are domesti- 
cated. It is necessary also to prove that all organisms do 
tend to increase at the great rate alleged, and that this 
increase actually occurs, under favourable conditions. We 
have to prove, further, that variations of all kinds can be 
increased and accumulated by selection ; and that the struggle 
for existence to the extent here indicated actually occurs in 
nature, and leads to the continued preservation of favourable 

These matters will be discussed in the four succeeding 
chapters, though in a somewhat different order the struggle 
for existence and the power of rapid multiplication, which is 
its cause, occupying the first place, as comprising those facts 
which are the most fundamental and those which can be 
perfectly explained without any reference to the less generally 
understood facts of variation. These chapters will be followed 
by a discussion of certain difficulties, and of the vexed question 
of hybridity. Then will come a rather full account of tho 
more important of tho complex relations of organisms to each 
other and to the earth itself, which are either fully explained 
or greatly elucidated by the theory. The concluding chapter 
will treat of the origin of man and his relations to the lower 



Its importance The struggle aujong 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, arid hardly ever think of, the means by which 
this 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 


a number of the flowers. It is not so commonly known that 
if a garden is left to become altogether wild, the weeds that 
first take possession of it, often covering the whole surface of 
the ground with two or three different kinds, will themselves 
be supplanted by others, so that in a few years many of 
the original flowers and of the earliest weeds may alike have 
disappeared. This is one of the very simplest cases of the 
struggle for existence, resulting in the successive displacement 
of one set of species by another ; but the exact causes of this 
displacement are by no means of such a simple nature. All 
the plants concerned may be perfectly hardy, all may grow 
freely from seed, yet when left alone for a number of years, 
each set is in turn driven out by a succeeding set, till at the 
end of a considerable period a century or a few centuries 
perhaps hardly one of the plants which first monopolised 
the ground would be found there. 

Another phenomenon of an analogous kind is presented by 
the different behaviour of introduced wild plants or animals 
into countries apparently quite as well suited to them as 
those which they naturally inhabit. Agassiz, in his work on 
Lake Superior, states that the roadside weeds of the north- 
eastern United States, to the number of 130 species, are all 
European, the native weeds having disappeared westwards ; 
and in New Zealand there are no less than 250 species of 
naturalised European plants, more than 100 species of which 
have sDread widely over the country, often displacing the 
native 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 become common. 
Even attempts to naturalise suitable plants usually fail ; for 
A. de Candolle states that several botanists of Paris, Geneva, 
and especially of Montpellicr, have sown the seeds of many 
hundreds of species of hardy exotic plants in what appeared 
to be the most favourable situations, 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 Gtographie Botaniqiie, p. 798, 


wild in New Zealand. But Sir Joseph Hooker informs us 
that the late Mr. Bid^ell 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 

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 t^is 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 

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 
when 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 
when 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 Orif/in of Species, p. 53. 


enemy in the forces of inorganic nature. Each species can 
sustain a certain amount of heat and cold, each requires a 
certain amount of moisture at the right season, each wants 
a proper amount of light or of direct sunshine, each needs 
certain elements in the soil ; the failure of a due proportion 
in these inorganic conditions causes weakness, and thus leads 
to speedy death. The struggle for existence in plants is, 
therefore, threefold in character and infinite in complexity, 
and the result is seen in their curiously irregular distribution 
over the face of the earth. Not only has each country its 
distinct plants, but every valley, every hillside, almost every 
hedgerow, has a different set of plants from its adjacent valley, 
hillside, or hedgerow if not always different in the actual 
species yet very different in comparative abundance, some 
which are rare in the one being common in the other. Hence 
it happens that slight changes of conditions often produce 
great changes in the flora of a country. Thus in 1740 and 
the two following years the larva of a moth (Phalama graminis) 
committed such destruction in many of the meadows of 
Sweden that the grass was greatly diminished in quantity, 
and many plants which were before choked by the grass 
sprang up, and the ground became variegated with a multi- 
tude of different species of flowers. The introduction of goats 
into the island of St. Helena led to the entire destruction of 
the native forests, consisting of about a hundred distinct species 
of trees and shrubs, the young plants being devoured by 
the goats as fast as they grew up. The camel is a still greater 
enemy to woody vegetation than the goat, and Mr. Marsh 
believes that forests would soon cover considerable tracts of 
the Arabian and African deserts if the goat and the camel 
were removed from them. 1 Even in many parts of our own 
country the existence of trees is dependent on the absence of 
cattle. Mr. Darwin observed, on some extensive heaths near 
Farnham, in Surrey, a few clumps of old Scotch firs, but no 
young 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 young fir-trees growing too close 
together for all to live ; and these were not sown or planted, 
nothing having been done to the ground beyond enclosing it 

1 The J&trth as Modified ly Human Action, p. 51. 


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 wddld 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 


was greater than is often seen in passing from one quite 
different soil to another. Besides a great change in the pro- 
portional numbers of the native heath-plants, twelve species 
which could not be found on the heath flourished in the 
plantations. The effect on the insect life must have been still 
greater, for six insectivorous birds which were very common 
in the plantations were not to be seen on the heath, which 
was, however, frequented by two or three different species of 
insectivorous birds. It would have required continued study 
for several years to determine all the differences in the 
organic life of the two areas, but the facts stated by Mr. 
Darwin are sufficient to show how great a change may be 
effected by the introduction of a single kind of tree and the 
keeping out of cattle. 

The next case I will give in Mr. Darwin's own words : 
"In several parts of the world insects determine the existence 
of cattle. Perhaps Paraguay offers the most curious instance 
of this ; for here neither cattle nor horses nor dogs have ever 
run wild, though they swarm southward and northward in a 
feral state ; and Azara and Rcnggcr have shown that this is 
caused by the greater numbers, in Paraguay, of a certain fly 
which lays its eggs in the navels of these animals when first 
born. The increase of these flies, numerous as they are, 
must be habitually checked by some means, probably by other 
parasitic insects. Hence, if certain insectivorous birds were 
to decrease in Paraguay, the parasitic insects would probably 
increase ; and this would lessen the number of the navel- 
frequenting flics then cattle and horses would become feral, 
and this would greatly alter (as indeed I have observed in 
parts of South America) the vegetation : this again would 
largely affect the insects, and this, as we have just seen in 
Staffordshire, the insectivorous birds, and so onward in ever- 
increasing circles of complexity. Not that under nature the 
relations will ever be as simple as this. Battle within battle 
must be continually recurring with varying success; and yet in 
the long run the forces are so nicely balanced, that the face 
of nature remains for a long time uniform, though assuredly 
the merest trifle would give the victory to one organic being 
over another." 1 

1 The Origin of Species, p. 56. 


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 rfntrue, 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 pratensc), 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, who 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, which 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, which 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- 


Blangsted, strikingly illustrates our subject. 1 The chief com- 
batants are the beech and the birch, the former being every- 
where successful in its invasions. Forests composed wholly 
of birch are now only found in sterile, sandy tracts ; every- 
where else the trees are mixed, and wherever the soil is 
favourable the beech rapidly drives out the birch. The latter 
loses its branches at the touch of the beech, and devotes all 
its strength to the upper part where it towers above the beech. 
It may live long in this way, but it succumbs ultimately in 
the fight of old age if of nothing else, for the life of the 
birch in Denmark is shorter than that of the beech. The 
writer believes that light (or rather shade) is the cause of the 
superiority of the latter, for it has a greater development of 
its branches than the birch, which is more open and thus 
allows the rays of the sun to pass through to the soil below, 
while the tufted, bushy top of the beech preserves a deep 
shade at its base. Hardly any young plants can grow under 
the beech except its own shoots ; and while the beech can 
flourish under the shade of the birch, the latter dies im- 
mediately under the beech. The birch has only been saved 
from total extermination by the facts that it had possession of 
the Danish forests long before the beech ever reached the 
country, and that certain districts are unfavourable to the 
growth of the latter. But wherever the soil has been enriched 
by the decomposition of the leaves of the birch the battle 
begins. The birch still flourishes on the borders of lakes and 
other marshy places, where its enemy cannot exist. In the 
same way, in the forests of Zeeland, the fir forests are dis- 
appearing before the beech. Left to themselves, the firs are 
soon displaced by the beech. The struggle between the latter 
and the oak is longer and more stubborn, for the branches and 
foliage of the oak are thicker, and offer much resistance to the 
passage of light. The oak, also, has greater longevity ; but, 
sooner or later, it too succumbs, because it cannot 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 raised, and the 
climate grew milder ; then > the fir came and formed large 
forests. This tree ruled for centuries, and then ceded the 

1 See Nature, vol. xxxi. p, 63. 


first place to the holm-oak, which is now giving way to the 
beech. Aspen, birch, fir, oak, and beech appear to bo 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 was the most abundant. The 
beech is a tree essentially of the temperate zone, having its 
northern limit considerably southward of the oak, fir, lurch, 
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 which 
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 was 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 
whole west coast down to Terra de! Fuego; and it is inconsistent 
with what we know of the rapid variation and adaptation of 
species to new conditions. What seems a more satisfactory 
explanation has been given by Mr. Edwin Clark, a civil 
engineer, who resided nearly two years in the country and 


paid much attention to its natural history. He says : " The 
peculiar characteristics of these vast level plains which descend 
from the Andes to the great river basin in unbroken monotony, 
are the absence of rivers or water-storage, and the periodical 
occurrence of droughts, or ' siccos/ in the summer months. 
These conditions determine the singular character both of its 
flora aiid fauna. 

" The soil is naturally fertile and favourable for the growth 
of trees, and they grow luxuriantly wherever they are pro- 
tected. The eucalyptus is covering large tracts wherever it 
is enclosed, arid willows, poplars, and the fig surround every 
estancia when fenced in. 

"The open plains are covered with droves of horses 
and cattle, and overrun by numberless wild rodents, the 
original tenants of the pampas. During the long periods 
of drought, which are so great a scourge to the country, these 
animals are starved by thousands, destroying, in their efforts to 
live, every vestige of vegetation. In one of these f siccos/ at 
the time of my visit, no less than 50,000 head of oxen and 
sheep and horses perished from starvation and thirst, after tearing 
deep out of the soil every trace of vegetation, including the 
wiry roots of the pampas-grass. Under such circumstances 
the existence of an unprotected tree is impossible. The only 
plants that hold their own, in addition to the indestructible 
thistles, grasses, and clover, are a little herbaceous oxalis, pro- 
ducing viviparous buds of extraordinary vitality, a few poisonous 
species, such as the hemlock, and a few tough, thorny dwarf- 
acacias and wiry rushes, which even a starving rat refuses. 

" Although the cattle are a modern introduction, the 
numberless indigenous rodents must always have effectually 
prevented the introduction of any other species of plants ; 
large tracts are still honeycombed by the ubiquitous biscacho, 
a gigantic rabbit ; and numerous other rodents still exist, in- 
cluding rats and mice, pampas-hares, and the great nutria and 
carpincho (capybara) on the river banks." 1 

Mr. Clark further remarks on the desperate struggle for 

existence which characterises the bordering fertile zones, 

where rivers and marshy plains permit a more luxuriant and 

varied vegetable and animal life. After describing how the 

1 A Visit to South America, 1878 ; also Nature, vol. xxxi. pp. 263-339. 


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 with 
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 were such as were either 
poisonous, or thorny, or resinous, or indestructibly tough. 
Ilence we had only a gr^at 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, bigrionias, 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 
oeriotheras, 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 arid law, arid 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-tip 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. 


Increase of Organisms in a Geometrical Ratio. 

The facts which now been adduced, sufficiently prove 
that there is a continual competition, and struggle, and war 
going on in nature, and that each species of animal and 
plant affects many others in complex and often unexpected 
ways. We will now proceed to show 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 the fact 
of the rapid increase, in a geometrical ratio, of all the species 
of animals and plants. In the lower orders this increase is 
especially rapid, a single flesh-fly (Musca carnaria) producing 
20,000 larvae, and these growing so quickly that they reach 
their full size in five days ; hence the great Swedish naturalist, 
Linmeus, asserted that a dead horse would be devoured by three 
of these flies as quickly as by a lion. Each of these Iarva3 remains 
in the pupa state about five or six days, so that each parent fly 
may be increased ten thousand-fold in a fortnight. Supposing 
they went on increasing at this rate during only three months 
of summer, there would result one hundred millions of millions 
of millions for each fly at the commencement of summer, a 
number greater probably than exists at any one time in the 
whole world. And this is only one species, while there are 
thousands of other species increasing also at an enormous rate ; 
so that, if they were unchecked, the whole atmosphere would 
be dense with flies, and all animal food and much of animal 
life would be destroyed by them. To prevent this tremendous 
increase there must be incessant war against these insects, by 
insectivorous birds and reptiles as well as by other insects, in 
the larva as well as in the perfect state, by the action of the 
elements in the form of rain, hail, or drought, and by other 
unknown causes ; yet wo see nothing of this ever-present war, 
though by its means alone, perhaps, we are saved from famine 
and pestilence. 

Let us now consider a less extreme and more familiar 
case. We possess a considerable number of birds which, 
like the redbreast, sparrow, the four common titmice, the 
thrush, and the blackbird, stay with us all the year round 
These lay on an average six eggs, but, as several of them have 


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 thaft 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? It is evident 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 
which 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 


the parents is killed ; while they offer a defenceless prey to 
jackdaws, jays, and magpies, and not a few are ejected from 
their nests by their foster-brothers the cuckoos. As soon as 
they are fledged and begin to leave the nest great numbers 
are destroyed by buzzards, sparrow-hawks, and shrikes. Of 
those which migrate in autumn a considerable proportion are 
probably lost at sea or otherwise destroyed before they reach a 
place of safety ; while those which remain with us are greatly 
thinned by cold and starvation during severe winters. Exactly 
the same thing goes on with every species of wild animal and 
plant from the lowest to the highest. All breed at such a rate, 
that in a few years the progeny of any one species would, if 
allowed to increase unchecked, alone monopolise the land ; 
but all alike are kept within bounds by various destructive 
agencies, so that, though the numbers of each may fluctuate, 
they can never permanently increase except at the expense of 
some others, which must proportionately decrease. 

Cases showing the Great Powers of Increase of Animals. 

As the facts now stated are the very foundation of the 
theory we are considering, and the enormous increase and 
perpetual destruction continually going on require to be kept 
ever present in the mind, some direct evidence of actual cases 
of increase must 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 8000 
head were not uncommon. Cattle were afterwards taken 
from this island to Mexico and to other parts of America, and 
in 1587, sixty- five years after the conquest of Mexico, the 
Spaniards exported 64,350 hides from that country and 
35,444 from 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 Buenos Ayres there were, at 
the end of the last century, about twelve million cows and 
three million horses, besides great numbers in all other parts 


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 thorn they all fell upon 
him and did not cease biting arid kicking till they left him 
dead. Hogs were 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 m great numbers over a largo 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 weeds over a large portion of the tropics. 
White clover (Trifolium repensj? spreads over all the temperate 
regions of the world, 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. 


tcnax), a large plant with iris-like leaves 5 or 6 feet high. 
Mr. W. L. Travers has paid much attention to the effects of 
introduced plants in New Zealand, and notes the following 
species as being especially remarkable. The common knot- 
grass (Polygonum aviculare) grows most luxuriantly, single 
plants covering a space 4 or 5 feet in diameter, and send- 
ing their roots 3 or 4 feet deep. A large sub-aquatic 
dock (Itumcx obtusifolius) abounds in every river-bed, even 
far up among the mountains. The common sow-thistle 
(Sonchus oleraceus) grows all over the country up to an 
elevation of 6000 feet. The water-cress (Nasturtium officinale) 
gmws with amazing vigour in many of the rivers, forming 
stems 12 feet long and | inch in diameter, and completely 
choking them up. It cost 300 a year to keep the Avon 
at Christehurch free from it. The sorrel (Rumex acetosella) 
covers hundreds of acres with a sheet of red. It forms a 
dense mat, exterminating other plants, and preventing cultiva- 
tion. It can, however, be itself exterminated by sowing the 
ground with red clover, which will also vanquish the 
Polygonum aviculare. The most noxious weed in New 
Zealand appears, however, to be the Hypocha?ris radicata, a 
coarse yellow -flowered composite not uncommon in our 
meadows and waste places. This has been introduced with 
grass seeds from England, and is very destructive. It is 
stated that excellent pasture was in three years destroyed by 
this weed, which absolutely displaced every other plant on the 
ground. It grows in every kind of soil, and is said even to 
drive out the white clover, which is usually so powerful in 
taking possession of the soil. 

In Australia another composite plant, called there the Cape- 
weed (Cryptostemma calendulaceum),did much damage, and was 
noticed by Baron Von Hugcl in 1833 as "an uriexterminable 
weed " ; but, after forty years' occupation, it was found to give 
way to the dense herbage formed by lucerne and choice 

In Ceylon we are told by Mr. Thwaites, in his Enumera- 
tion of Ceylon Plants, that a plant introduced into the 
island less than fifty years ago 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 


from the West Indies, which appears to have found in Ceylon a 
soil arid climate exactly suited to it. It now covers thousands 
of acres with 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 tin's 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 noi essential to llapid 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 (Kctopistcs 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 exists in myriads at St. Kilda 
and other haunts of the species, 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 composite, 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 


American naturalist, Alexander Wilson, will be read with 

" Not far from Shelby ville, in the State of Kentucky, 
about five years ago, there was one of these breeding-places, 
which stretched through the woods in nearly a north and 
south direction, was several miles in breadth, and was said to 
be upwards of 40 miles in extent. In this tract almost 
every tree was furnished with nests wherever the branches 
could accommodate them. The pigeons made their first 
appearance there about the 10th of April, and left it 
altogether with their young before the 25th of May. As 
soon as the young were fully grown and before they left the 
nests, numerous parties of the inhabitants from all parts of 
the adjacent country came with waggons, axes, beds, cooking 
utensils, many of them accompanied by the greater part of 
their families, and encamped for several days at this immense 
nursery. Several of them informed me that the noise was 
so great as to terrify their horses, and that it was difficult for 
one person to hear another without bawling in his ear. The 
ground was strewed with broken limbs of trees, eggs, and 
young squab pigeons, which hud been precipitated from above, 
and on which herds of hogs were fattening. Hawks, buzzards, 
and eagles were sailing about in great numbers, and seizing 
the squabs from the nests at pleasure ; while, from 20 feet 
upwards to the top of the trees, the view through the woods 
presented a perpetual tumult of crowding and fluttering 
multitudes of pigeons, their wings roaring like thunder, 
mingled with the frequent crash of falling timber ; for now 
the axemen were at work cutting down those trees that seemed 
most crowded with nests, and contrived to fell them in such 
a manner, that in their descent they might bring down several 
others ; by which means the falling of one largo 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 
squab 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 tliat two eggs are laid and usually two 
young produced, but it may be that in most cases only one of these comes to 


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. 1 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 Shelby ville, 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 


instead of a diminution of this prodigious procession, it seemed 
rather to increase, both in numbers and rapidity ; and anxious 
to reach Frankfort before night, I rose and went on. About 
four o'clock in the afternoon I crossed Kentucky River, at the 
town of Frankfort, at which time the living torrent above my 
head seemed as numerous and as extensive as ever. Long 
after this I observed them in large bodies that continued to 
pass for six or eight minutes, and these again were followed 
by other detached bodies, all moving in the same south-east 
direction, till after six o'clock in the evening. The great 
breadth of front which this mighty multitude preserved would 
seem to intimate a corresponding breadth of their breeding- 
place, which, by several gentlemen who had lately passed 
through part of it, was stated to mo at several miles." 

From these various observations, Wilson calculated that 
the number of birds contained in the mass of pigeons which 
he saw on this occasion was at least two thousand millions, 
while this was only one of many similar aggregations known 
to exist in various parts of the United States. The 
picture here given of these defenceless birds, and their still 
more defenceless young, exposed to the attacks of numerous 
rapacious enemies, brings vividly before us one of the phases 
of the unceasing struggle for existence ever going on ; but 
when we consider the slow rate of increase of these birds, 
and the enormous population they are nevertheless able to 
maintain, we must be convinced that in the case of the 
majority of birds which multiply far more rapidly, and yet 
are never able to attain such numbers, the struggle against 
their numerous enemies and against the adverse forces of 
nature must be even more severe or more continuous. 

Struggle for Life between closely allied Animals and Plants 
often the most severe. 

The struggle we have hitherto been considering has been 
mainly that between 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 struggle often going on at the same time between 
closely related species, which almost always terminates in tho 
destruction of one of them. As an example of what is 



meant, Darwin states that the recent increase of the missel- 
thrush in parts of Scotland has caused the decrease of the 
song-thrush. 1 The l>lack 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 
Volga, 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 alle<je 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 llussia 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, arid 
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 arid so yield more seed, and 
will consequently in a few years supplant the other varieties. 

1 Origin of Species, p. 59. Professor A. Newton, however, informs me that 
these species do not interfere with one another in the way here stated. 


As an effect of this principle, we seldom find closely allied 
species of animals or plants living together, but often in 
distinct though adjacent districts where the conditions of life 
are somewhat different. Thus wo may find cowslips (Primula 
vcris) growing in a meadow, and primroses (P. vulgaris) in an 
adjoining wood, each in abundance, but not often intermingled. 
And for the same reason the old turf of a pasture or heath 
consists of a great variety of plants matted together, so much 
so that in a patch little more than a yard square Mr. Darwin 
found twenty distinct species, belonging to eighteen distinct 
genera and to eight natural orders, thus showing their extreme 
diversity of organisation. For the same reason a number of 
distinct grasses and clovers are sown in order to make a good 
lawn instead of any one species ; and the quantity of hay 
produced has been found to be greater from a variety of very 
distinct grasses than from any one species of grass. 

It may be thought that forests are an exception to this 
rule, since in the north-temperate and arctic regions we find 
extensive forests of pines or of oaks. But these are, after all, 
exceptional, and characterise those regions only where the 
climate is little favourable to forest vegetation. In the 
tropical and all the warm temperate parts of the earth, where 
there is a sufficient 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 so great a variety 
of forms, and they are so thoroughly intermingled, that the 
traveller often finds it difficult to discover a second specimen 
of any particular species which he 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 we 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- 


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 riot 
so readily observed. As illustrations we may refer to the 
wolf, ranging over Europe and Northern Asia, while the jackal 
inhabits Southern Asia and Northern Africa; the tree- 
porcupines, of which there are two closely allied species, one 
inhabiting the easterj*, 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 JBrandti) 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 


benevolent ruler of the universe. Thus, a brilliant writer 
says : " Pain, grief, disease, and death, are these the inventions 
of a loving God? That no animal shall rise to excellence 
except by being fatal to the life of others, is this the law of 
a kind Creator? It is useless to say that pain has its 
benevolence, that massacre has its mercy. Why is it so 
ordained that bad should be the raw material of good ? Pain 
is not the less pain because it is useful ; murder is not less 
murder because it is conducive to development. Here is 
blood upon the hand still, and all the perfumes of Arabia will 
not sweeten it." 1 

Even so thoughtful a writer as Professor Huxley adopts 
similar views. In a recent article on " The Struggle for 
Existence " he speaks of the myriads of generations of herbiv- 
orous animals which "have been tormented and devoured by 
carnivores " ; of the carnivores and herbivores alike " subject to 
all the miseries incidental to old age, disease, and over-multi- 
plication"; and of the "more or less enduring suffering," 
which is the meed of both vanquished and victor. And he 
concludes that, since thousands of times a minute, were our 
ears sharp enough, we should hear sighs and groans of pain 
like those heard by Dante at the gate of hell, the world 
cannot be governed by what we call benevolence.- 

Now there is, I think, good reason to believe that all this 
is greatly exaggerated ; that the supposed " torments " and 
" miseries " of animals have little real existence, but are the 
reflection of the imagined sensations of cultivated men and 
women in similar circumstances ; and that the amount of actual 
suffering caused by the struggle for existence among animals 
is altogether insignificant. Let us, therefore, endeavour to 
ascertain what are the real facts on which these tremendous 
accusations are founded. 

In the first place, we must remember that animals are 
entirely spared the pain we suffer in the anticipation of death 
a pain far greater, in most cases, than the reality. This leads, 
probably, to an almost perpetual enjoyment of their lives ; 
since their constant watchfulness against danger, and even 
their actual flight from an enemy, will be the enjoyable 

1 Winwood Reade's Martyrdom of Man, p. 520. 

2 Nineteenth Century, February 1888, pp. 162, 163. 


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 hon 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 rue 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 which there was no sense of 
pain or feeling of terror, though I was quite conscious of all 
that was happening. It was like what patients partially 
under the influence of chloroform describe, who see all the 
operation, but feel riot 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 which causes 
a general shock to the system, Mr. Whymper describes an 
accident to himself during one of his preliminary explorations 
of the Matterhorn, when he fell several hundred feet, bounding 
from rock to rock, till fortunately embedded in a snow-drift 
near the edge of a tremendous precipice. He declares that 
while falling and feeling blow after blow, he neither lost 
consciousness nor suffered pain, merely thinking, calmly, that 
a few more blows would finish him. We have therefore a 
right to conclude, that when death follows soon after any 
great shock it is as easy and painless a death as possible ; and 
this is certainly what happens when an animal is seized by a 
beast of prey. For the enemy is one which hunts for food, 
not for pleasure or excitement ; and it is doubtful whether any 
carnivorous animal in a state of nature begins to seek after 


prey till driven to do so by hunger. When an animal is 
caught, therefore, it is very soon devoured, arid thus the first 
shock is followed by an almost painless death. Neither do 
those which die of cold or hunger suffer much. Cold is 
generally severest at night and has a tendency to produce 
sleep and painless extinction. Hunger, on the other hand, is 
hardly felt during periods of excitement, and when food is 
scarce the excitement of seeking for it is at its greatest. It 
is probable, also, that when hunger presses, most animals will 
devour anything to stay their hunger, and will die of gradual 
exhaustion and weakness not necessarily painful, if they do 
not fall an earlier prey to some enemy or to cold. 1 

Now let us consider what are the enjoyments of the lives 
of most animals. As a rule they come into existence at a 
time of year when food is most plentiful and the climate most 
suitable, that is in the spring of the temperate zone and at 
the commencement of the dry season in the tropics. They 
grow vigorously, being supplied with abundance of food ; and 
when they reach maturity their lives are a continual round of 
healthy excitement and exercise, alternating with complete 
repose. The daily search for the daily food employs all their 
faculties and exercises every organ of their bodies, while this 
exercise leads to the satisfaction of all their physical needs. 
In our own case> we can give no more perfect definition of 
happiness, than this exercise and this satisfaction ; and we 
must therefore conclude that animals, as a rule, enjoy all the 
happiness of which they are capable. And this normal state 
of happiness is not alloyed, as with us, by long periods 
whole lives often of poverty or ill-health, and of the un- 
satisfied longing for pleasures which others enjoy but to which 
we cannot attain. Illness, and what answers to poverty in 
animals continued hunger are quickly followed by unantici- 
pated and almost painless extinction. Where we err is, in 
giving to animals feelings and emotions which they do not 
possess. To us the very sight of blood and of torn or mangled 
limbs is painful, while the idea of the suffering implied by it 

1 The Kestrel, which usually feeds on mice, birds, and frogs, sometimes 
stays its hunger with earthworms, as do some of the American buzzards. 
The Honey-buzzard sometimes eats not 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. 


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 

lt 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 

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 



Importance of variability Popular ideas regarding it Variability of the 
lower animals The variability of insects Variation among lizards 
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 
Concluding remarks. 

THE foundation of the Darwinian theory is the variability of 
species, and it is quite useless to attempt even to understand 
that theory, much less to appreciate the completeness of the 
proof of it, unless we first obtain a clear conception of the 
nature and extent of this variability. The most frequent and 
the most misleading of the objections to the efficacy of natural 
selection arise from ignorance of this subject, an ignorance 
shared by many naturalists, for it is only since Mr. Darwin 
has taught us their importance that varieties have been 
systematically collected and recorded ; and even now very 
few collectors or students bestow upon them the attention 
they deserve. By the older naturalists, indeed, varieties 
especially if numerous, small, and of frequent occurrence 
wore looked upon as an unmitigated nuisance, because they 
rendered it almost impossible to give precise definitions of 
species, then considered the chief end of systematic natural 
history. Hence it was the custom to describe what was 
supposed to be the " typical form " of species, and most 
collectors were satisfied if they possessed this typical form 
in their cabinets. 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 


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 arid 

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 wh^n 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 Oriyin of Spcdes ; while a 
considerable body of facts has been made known since the 
publication of the last edition of that work. 

J'arwbUily of the Lower Animals. 

Among the lowest and most ancient marine organisms are 
the Foramimfera, 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 studied 
with great care by many eminent naturalists, and the late Dr. 
W. B. Carpenter in his great work the Introduction 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 


specimens : " The range of variation is so great among the 
Foraminifera as to include not merely those differential char- 
acters which have been usually accounted specific, but also 
those upon which the greater part of the genera of this group 
have been founded, and even in some instances those of its 
orders. n l 

Coming now to a higher group the Sea- Anemones Mr. P. 
H. Gossc and other writers on these creatures often refer to 
variations in size, in the thickness and length of the tentacles, 
the form of the disc and of the mouth, and the character of 
surface of the column, while the colour varies enormously in 
a great number of the species. Similar variations occur in all 
the various groups of marine invertebrata, and in the great 
sub-kingdom of the mollusca they are especially numerous. 
Thus, Dr. S. P. Woodward states that many present a most 
perplexing amount of variation, resulting (as he supposes) 
from supply of food, variety of depth and of saltness of the 
water ; but we know that many variations are quite inde- 
pendent of such causes, and we will now consider a few cases 
among the land-mollusca in which they have been more care- 
fully studied. 

In the small forest region of Oahu, one of the Sandwich 
Islands, there have been found about 175 species of land-shells 
represented by 700 or 800 varieties ; and we are told by the 
Rev. J. T. Gulick, 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 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 2*resents 
a minute gradation of forms between the more divergent types 
found in the more widely separated localities^ 

In most land-shells there is a considerable amount of varia- 
tion in colour, markings, size, 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 
198 varieties of the common wood -snail (Helix nemoralis), 
while of the equally common garden -snail (Helix hortensis) 
ninety varieties have been described. Fresh-water shells are also 
1 Foraminifera, preface, p. x. 


subject to great variation, so that there is much uncertainty as 
to the number of species ; and variations are especially frequent 
in the Planorbida3, 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 Keport 
on the Kecent 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. Yernon 
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 Calathus niollis as having "the hind wings at 
one time ample, at another rudimentary, arid at a third nearly 
obsolete ;" and of the same irregularity as to the wings being 
characteristic of many Orthoptera and of the Homopterous 
Fulgorida3. Mr. Westwood in his Modern Classification of 
Insects states that " the species of Gerris, Hydrornetra, 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 
1 United States Geological Survey of the Territories, 1874. 


Orolina exhibits four distinct varieties, all occurring together, 
and these differ not only in colour but in form, one variety 
being described as having the fore wings much elongated in the 
male, while another is much larger and has "the hind wings in 
the male different in shape." Of Heliconius Numata Mr. Bates 
says : " This species is so variable that it is difficult to find two 
examples exactly alike," while " it varies in structure as well 
as in colours. The wings are sometimes broader, some- 
times narrower ; and their edges are simple in some examples 
and festooned in others." Of another species of the same 
genus, H. melpomenc, ten distinct varieties are described 
all more or less connected by intermediate forms, and four 
of these varieties were obtained at one locality, Serpa on 
the north hank of the Amazon. Ceratina Ninonia is another 
of these very unstable species exhibiting many local varieties 
which are, however, incomplete and connected by intermediate 
forms ; while the several species of the genus Lycorea all 
vary to such an extent as almost to link them together, so 
that Mr. Bates thinks they might all fairly be considered as 
varieties of one species only. 

Turning to the Eastern Hemisphere we have in Papilio 
Scverus a species which exhibits a large amount of simple 
variation, in the presence or absence of a pale patch on the 
upper wings, in the brown submarginal marks on the lower 
wings, in the form and extent of the yellow band, and in 
the size of the specimens. The most extreme forms, as well 
as the intermediate ones, are often found in one locality and 
in company with each other. A small butterfly (Terias hecabe) 
ranges over the whole of the Indian and Malayan regions to 
Australia, and everywhere exhibits great variations, many of 
which have been described as distinct species ; but a gentle- 
man in Australia bred two of these distinct forms (T. hecabe 
and T. yEsiope), with several intermediates, from one batch of 
caterpillars found feeding together on the same plant. 1 It is 
therefore very probable that a considerable number of supposed 
distinct species are only individual varieties. 

Cases 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 the Entomological Society of London^ 1875, p. vii. 


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 larvae 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 (Deilophela galii). 

Variation among Lizards. 

Passing on from the lower animals to the vertcbrata, 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 



7 3 5 7 p 77 



n _/,. 


^l -* 




Mean length. Sin. 



Nech _ 






Af era/7 length. 1. 18in. 

Fore Leg. 




^ , 




Mean length. 1.05in. 

Hind Leg. . 














Mean length. 1.90in, 

Toeof Hind Foot 








Mean length. 0. 70in. 


The lengths in the table are given in millimetres, which are here reduced 
to inches for the means. 

Fio. 1. -Variations of Lacerta muralis. 



Lacerta ocellata 

^^^^^H Tail 

Lacerta uiridis ' J! y . . 

14. Hind Legs 

' ' '...Tail 

M Neck 

** Body 

Lacerta agilis 

M- ///'/</ Aegs 

(4 Body 

Lacerta muralis 

JJind Legs 

i JJeck 

Lacerta uelox ^^ '. 

M Tail 


Lacerta deserti ... . . 

M Hind Legs 

Length of Head jmrnmrnammmj taken as the 
standard In each of the above -named species 

Fio. 2. Variation of Lizards. 


i . 

each specimen are then laid down in the same manner at 
convenient distances apart for comparison ; and we see that 
their variations bear no definite relation to those of the body, 
and not much to those of each other. With the exception of 
No. 5, in which all the parts agree in being large, there is a 
marked independence of each part, shown by the lines often 
curving in opposite directions ; which proves that in those 
specimens one part is large while the other is small. The 
actual amount of the variation is very great, ranging from 
one-sixth of the mean length in the neck to considerably more 
than a fourth in the hind leg, and this among only fourteen 
examples which happen to be in a particular museum. 

To prove that this is not an isolated case, Professor Milne 
Edwards also gives a table showing the amount of variation in 
the museum specimens of six common species of lizards, also 
taking the head as the standard, so that the comparative 
variation of each part to the head is given. In the accompany- 
ing diagram (Fig. 2) the variations are exhibited by means of 
lines of varying length. It will be understood that, however 
much the specimens varied in size, if they had kept the same 
proportions, the variation line would have been in every case 
reduced to a point, as in the neck of L. velox which exhibits 
no variation. The different proportions of the variation lines 
for each species may show a distinct mode of variation, or may 
be merely due to the small and differing number of specimens ; 
for it is certain that whatever amount of variation occurs 
among a few specimens will be greatly increased when a much 
larger number of specimens are examined. That the amount of 
variation is large, may be seen by comparing it with the actual 
length of the head (given below the diagram) which was used 
as a standard in determining the variation, but which itself 
seems not to have varied. 1 

Variation among Birds. 

Coming now to the class of Birds, wo 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 circuni- 
1 Ann. des Sci. Nat., torn. xvi. p. 50. 


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 Fauna) of Eastern North America," 
published in the Bulletin of the Museum of Comparative, 
Zoology at Harvard College, Cambridge, Massachusetts, in 
1871. In this work exaj^t 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 1 5 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 14 to 21 per cent of the mean length, and the 
tail from 13 '8 to 23*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- 


mencing with the outer one. As an example of the irregular 
variation constantly met with, the following occurred among 
twenty -five specimens of Dendroeca coronata. Numbers 
bracketed imply that the corresponding feathers were of 
equal length. 1 



Second in 

Thml in 

Foni th in 

Fifth in 

Sixth in 














! 4 S 




i ) 






! ' 






Here we have five very distinct proportionate lengths of 
the wing feathers, any one of which is often thought sufficient 
to characterise a distinct species of bird ; and though this is 
rather an extreme case, Mr. Allen assures us that " the com- 
parison, extended in the table to only a few species, has been 
carried to scores of others with similar results." 
* Along with this variation in size and proportions there occurs 
a large amount of variation in colour and markings. " The 
difference in intensity of colour between the extremes of a 
series of fifty or one hundred specimens of any species, collected 
at a single locality, and 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 Itirds of Florida, p. 206, Table F. 


song sparrow (Melospiza melodia), the fox-coloured sparrow 
(Passerella iliaca), the swamp sparrow (Melospiza pains tris), the 
black and white creeper (Mniotilta varia), the water-wagtail 
(Seiurus novreboracencis), in Turdus fusccscons 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 qp 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 

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. 














.^ - 






-, ' 







^ - 

.0 . 




< c 

o Q. 


<a> "S 



S Q> 


1 s 


1 | 

S . c 


' 1 


o *^ 


*o* "o" 

^ N c ^S 

v v\ 

^ 3 C5i 



' | " 



; | | 



! E i 




; 1 





; ! 1 


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 

It maybe said, however, that these arc 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 tho 
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 Sec Table I, p. 211, of Allen's Winter Birds of Florida. 



FIG. 4. Dohchonyx pryzivorus. 20 Males. 




/ 5 70 15 20 25 30 35 40 

10 15 20 25 30 35 40 

Fio. 5. Agelteus pboeniccus, 40 Males, 


sidercd to give tho sizo of the bird), from the shortest 
to tho longest, and the same number of vertical lines are 
drawn, numbered from one to twenty. In this case (and 
wherever practicable) tho body-length is measured from the 
lower line of the diagram, so that the actual length of the bird 
is exhibited as well as the actual variations of length. These 
can be well estimated by means of the horizontal line drawn 
at the mean between the two extremes, and it will be seen 
that one-fifth of the total number of specimens taken on either 
side exhibits a very large amount of variation, which would of 
course be very much greater if a hundred or more specimens 
were compared. The lengths of the wing, tail, and other parts 
are then laid down, and the diagram thus exhibits at a glance 
the comparative variation of these parts in every specimen as 
well as the actual amount of variation in the twenty specimens ; 
and we are thus enabled to arrive at some important con- 

We note, first, that the variations of none of the parts follow 
the variations of the body, but are sometimes almost in an 
opposite direction. Thus the longest wing corresponds to a 
rather small body, the longest tail to a medium body, while 
the longest leg and toes belong to only a moderately large body. 
Again, even related parts do not constantly vary together but 
present many instances of independent variation, as shown by 
the want of parallelism in their respective variation-lines. In 
No. 5 (see Fig. 4) tho wing is very long, the tail moderately 
so ; while in No. 6 tho wing is much shorter while the tail is 
considerably longer. The tarsus presents comparatively little 
variation ; and although the three toes may be said to vary in 
general together, there are many divergencies ; thus, in passing 
from No. 9 to No. 10, the outer toe becomes longer, while the 
hind toe becomes considerably shorter ; while in Nos. 3 and 4 
the middle too varies 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 (Agelaeus phoeniceus), 
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, 



16 10 15 20 25 30 




8. $5 1 







6 10 16 20 25 30 


though showing some amount of correlated variation, yet in 
no less than nine cases vary in opposite directions as compared 
with the preceding species. 

The next diagram (Fig. G), showing the variations of thirty- 
one males of the Cardinal bird (Cardinalis virginianus), exhibits 
these features much more strongly. The amount of variation 
in proportion to the size of the bird is very much greater ; 
while the variations of the wing arid tail not only have no 
correspondence with that of the body but very little with each 
other. In no less than twelve or thirteen instances they vary 
in opposite directions, while even where they correspond in 
direction the amount of the variation is often very dispropor- 

As the proportions of the tarsi and toes of birds have great 
influence on their mode of life and habits and are often used 
as specific or even generic characters, I have prepared a 
diagram (Fig. 7) to show the variation in these parts only, among 
twenty 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 vertical 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 is really very large ; 
while the diverging curves and angles show that each part 
varies, to a great 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 Schlegel's Catalogue of the Birds in the 
Leyden Museum, in which 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 











Middle Toe. 

Outer Toe.... 
Hind Toe.., 

i * 

Tarsus . 

Middle Toe. 
Outer Toe^. 
Hind Toe. 



Tarsus . 

Middle Toe. 
Outer Toe.... 
Hind Toe 



| | 


Middle Toe., 

Outer Toe.... 

Hind Toe 

/ 2 3 4 
From Table G, in Allen's Birds of Florida. 

FIG. 7. Variation of Tarsus and Toes. 








^ *1 ! 



Phonygama atra 111 



Oriolus galbula I 




Pica caudaia 




Semeioptera wallacei 



Pyrrhocorax alpinus 

1 I 



FIG. 8. Variation of Birds in Leyden Museum. 


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 which occur are " infinitesimal " or even 
"small. 1 ' 

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 riot 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 Agelaeus phceniceus, this approach to an 
equable spreading of the variations is still more apparent ; 
while in Fig. 12, where fifty-eight specimens of Cardinalis 


virginianus are registered, we see a remarkable spreading out 
of the spots, showing in some of the characters a tendency to 
segregation into two or more groups of individuals, each vary- 
ing considerably from the mean. 

In order fully to appreciate the teaching of these diagrams, 







A. f 





\e Toe. 






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. 
We know, from the general law which governs variations 
from a mean value, that with increasing numbers the range 





Length] of Bill. 

. :.:*:.::>.: . 

Total Len 


^th of Bird. 



of Tail. 


Length of Wing. 

Amount of 





WING. 1 

FIG. 10. 

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

Curves of Variation 

Fio. 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 


of the dots, when even a few specimens are compared, shows 
'that the curve, with high numbers, would be a flat one like the 
lower curve in the illustration here given. This being the case it 
would follow that a very large proportion of the total number of 
individuals constituting a species would diverge considerably 
from its average condition as regards each part or organ ; arid 
as we know from the previous diagrams of variation (Figs. 1 
to 7) that each part varies to a considerable extent, inde- 
pendently, the materials constantly ready for natural selection 

CARDINALIS VIRGINIANUS. 58 specimens. Florida. 



of Bird. 



VAVte A*. 


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

FIG. 12 

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

Variation in the Mammalia. 

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



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 
mariner, 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 

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 bo 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 

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



10 15 20 25 80 32 

16 10 15 20 2S 30 32 

Fia. 13. Sciurus carolinonsia. 32 specimens. Florida. 


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 spermatheca3 spherical pouches without 

diver ticulse 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 


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

(1) The number of the spermathecae 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 

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


" In some species of Shrews (Sorex) and in some field-mice 
(Arvicola), the Rev. L. Jeuyun (Ann. Nat. Hist., vol. vii. pp. 267, 
272) found the proportional length of the intestinal canal to 
vary considerably. He found the same variability in the 
number of the caudal vertebras. In three specimens of an 
Arvicola he found the gall-bladder having a very different 
degree of development, and there is reason to believe it is 
sometimes absent. Professor Owen has shown that this is 
the case with the gall-bladder of the giraffe." 

Dr. Crisp (Proc. Zool. Soc., 1862, p. 137) found the gall- 
bladder present in some specimens of Cervus superciliaris while 
absent in others ; and he found it to be absent in three 
giraffes which he dissected. A double gall-bladder was 
found in a sheep, and in a small mammal preserved in the 
Hunterian Museum there are three distinct gall-bladders. 

The length of the alimentary canal varies greatly. In three 
adult giraffes described by Professor Owen it was from 124 to 
136 feet long; one dissected in France had this canal 211 
feet long ; while Dr. Crisp measured one of the extraordinary 
length of 254 feet, and similar variations are recorded in 
other animals. 1 

The number of ribs varies in many animals. Mr. St. George 
Mivart 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 Colobus there are 
generally seven, but sometimes eight pairs of true ribs. In 
the Cebidas there are generally seven or eight pairs, but in 
Ateles sometimes nine" (Proc. Zool, Soc., 1865, p. 568). In 
the same paper it is stated that the number of dorsal vertebrae 
in man is normally twelve, very rarely thirteen. In the 
Chimpanzee there are normally thirteen dorsal vertebras, but 
occasionally there are fourteen or only twelve. 

Variations in the Skull. 

Among the nine adult male Orang-utans, collected by 
myself in Borneo, the skulls differed 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 
1 Proc. Zool Soc., 1864, p. 64. 




8 9 10 




1 2 3 4 6 6 7 8 

FIG. 14.- Variation of Skull of Wolf. 10 specimens. 



size. I noted particularly tha't these variations bore no 
necessary relation to each other, so that a large temporal 
muscle and zygomatic aperture might exist either with a 
large or a small cranium ; and thus was explained the curious 
difference between the single-crested and the double-crested 
skulls, which had been supposed to characterise distinct species. 
As an instance of the amount of variation in the skulls of 
fully adult male orangs, I found the width between the orbits 
externally to be only 4 inches in one specimen and fully 
5 inches in another. 

Exact measurements of large series of comparable skulls of 
the mammalia are not easily found, but from those available 
I have prepared three diagrams (Figs. 14, 15, and 16), in order 
to exhibit the facts of variation in this very important organ. 
The first shows the variation in ten specimens of the common 
wolf (Canis lupus) from one district in North America, and 
we sec that it is not only largo in amount, but that each 
part exhibits a considerable independent variability. 1 

In Diagram 15 we have the variations of eight skulls of 
the Indian Honey-bear (Ursus labiatus), as tabulated by the 
late Dr. J. M Gray of the British Museum. For such a 
small number of specimens the amount of variation is very 
large from one-eighth to one-fifth of the mean size, while 
there are an extraordinary number of instances of inde- 
pendent variability. In Diagram 16 we have the length arid 
width of twelve skulls of adult males of the Indian wild boar 
(Sus cristatus), also given by Dr. Gray, exhibiting in both sets 
of measurements a variation of more than one-sixth, combined 
with a very considerable amount of independent variability. 2 

The few facts now given, as to variations of the internal 
parts of animals, might bo multiplied indefinitely by a search 
through the voluminous writings of comparative anatomists. 
But the evidence already adduced, taken in conjunction with 
the much fuller evidence of variation in all external organs, 
leads us to the conclusion that wherever variations are looked 
for among a considerable number of individuals of the more 

1 J. A. Allen, on Geographical Variation among North American Mammals, 
Bull. U. S. OeoL and Geog. Survey, vol. ii. p. 314 (1876). 
3 Proc. Zool. Soc. Land., 1864, p. 700, aud 1868, p. 28. 





Mean 11li in. 


Mean 7% in. 


Mean 6 1 A in. 


Mean win. 



1 234667 
(From Table by Dr. J,E. Gray. P.Z.S. 1864, p. 700.) 

Fm. 15. Variation of 8 skulls (Ursus labiatus). 





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 scries 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 tke 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 Gal ton 
in his researches on the theory of variability, the upper lino 
(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 of London, 1887, p. 24. 


The Kea (Nestor notabilis) is a curious parrot inhabiting the 
mountain ranges of the Middle Island of New Zealand. It 
belongs to the family of Bmsh-tongued parrots, and naturally 
feeds on the honey of flowers and the insects which frequent 
them, together with such fruits or berries as arc found in the 
region. Till quite recently this comprised its whole diet, but 
since the country it inhabits has become occupied by Europeans 
it has developed a taste for a carnivorous diet, with alarming 
results. It began by picking the sheepskins hung out to dry 
or the meat in process of being cured. About 1868 it was 
f\rst observed to attack living sheep, which had frequently 
been found with raw and bleeding wounds on their backs. 
Since then it is stated that the bird actually burrows into the 
living sheep, eating its way down to the kidneys, which form 
its special delicacy. As a natural consequence, the bird is 
being destroyed as rapidly as possible, and one of the rare 
and curious members of the New Zealand fauna will no 
doubt shortly cease to exist. The case affords a remark- 
able instance of how the climbing feet and powerful hooked 
beak developed for one set of purposes can be applied to 
another altogether different 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 golden eagle, was taught by its foster -parent to 
eat flesh, which it continued to do regularly and apparently 
with great relish. 1 

Change of habits appears to be often a result of imitation, 
of which Mr. Tegetmeier gives some good examples. He states 
that if pigeons are reared exclusively with small grain, as 
wheat or barley, they will starve before eating beans. But 
when they are thus 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 maize, 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- 
meier, who grows only these latter colours, found that after 
1 Nature, vol. xix. p. 554. 


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 0. Abbott 
well remarks : "In studying the habits of our American birds 
and I suppose it is true of birds everywhere it must at all 
times bo 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 reallv are, in every portion of the terri- 
tory it inhabits." 1 * 

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 hangriests (Icteridie), with the 
exception that the cavity is at the top. Clearly these New 
Zealand chaffinches were at a loss for a design when 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. 


many large genera. As examples we may take the roses, the 
brambles, and the willows as well illustrating this fact. In Mr. 
Baker's Revision of the British Jioses (published by the Linnean 
Society in 1863), he includes under the single species, Kosa 
cariina the common dog-rose no less than twenty -eight 
named varieties distinguished by more or less constant characters 
and often confined to special localities, and to these are 
referred about seventy of the species of British and continental 
botanists. Of the genus Rubus or bramble, five British species 
are given in Bentham's Handbook of the British Flora, while 
in the fifth edition of Babington's Manual of British Botany, 
published about the same time, no less than forty-five species 
are described. Of willows (Salix) the same two works 
enumerate fifteen and thirty -one species respectively. The 
hawkweeds (Ilieracium) are equally puzzling, for while Mr. 
Bentham admits only seven British species, Professor Babing- 
ton describes no less than thirty -two, besides several named 

A French botanist, Mons. A. Jordan, has collected numerous 
forms of a common little plant, the spring whitlow- grass 
(Draba verna) ; he has cultivated these for several successive 
years, and declares that they preserve their peculiarities un- 
changed ; he also says that they each come true from seed, 
and thus possess all the characteristics of true species. He 
has described no less than fifty-two such species or permanent 
varieties, all found in the south of France ; and he urges 
botanists to follow his example in collecting, describing, and 
cultivating all such varieties as may occur in their respective 
districts. Now, as the plant is very common almost all over 
Europe 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 must certainly be held 
to prove that in many large genera and in some single species 
there is a very large amount of variation, which renders it 
quite impossible for experts to agree upon the limits of species. 
We will now adduce a few striking cases of individual 

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


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 blunt ; (5) in the base being sharp, 
blunt, 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 blun^; (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 

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 

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. We 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 bo 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 one locality flowers varying from inch to 1| inch in 
diameter ; the bracts varying from 1 J inch to 4 inches across ; 
and the petaloid sepals either broad or narrow, and varying 
in number from five to ten. Though generally pure white 
on their upper surface, some specimens arc a full pink, while 
others have a decided bluish tinge. 

Mr. Darwin states that he carefully examined a large number 
of plants of Geranium phseum and G. pyrenaicum (not perhaps 
truly British but frequently found wild), which had escaped 
from cultivation, and had spread by seed in an open planta- 
tion ; and he declares that "the seedlings varied in almost 
every single character, both in their flowers and foliage, to a 
degree which I have never seen exceeded ; yet they could not 
have been exposed to any great change of their conditions." 1 

The following examples of variation in important parts of 
plants were collected by Mr. Darwin and have been copied 
from his unpublished MSS. : 

" Do Candollo (Mem. Soc. Phys. de Geneve, torn. ii. part ii. 
p. 217) states that Papavcr bracteatum and P. orientalc present 
indifferently two sepals and four petals, or three sepals and 
six petals, which is sufficiently rare with other species of the 

" In the Primulacere and in the great class to which this 
family belongs the unil ocular ovarium is free, but M. Dubury 
(Hem. Soc. Phys. de Geneve, torn. ii. p. 406) has often found 
individuals in Cyclamen hcdenriolium, in which the base of 
the ovary was connected for a third part of its length with 
the inferior part of the calyx." 

" M. Aug. St. Hilaire (Sur la Gynobase, Mem. des Mus. 
d'Hist. Nat., torn. x. p. 134), speaking of some bushes of the 
Gomphia oleoefolia, which he at first thought formed a quite 
distinct species, says : * Voila done dans un meme individu 
des loges et un style qui se rattachent tantot a un axe vertical, 
et tantot a un gynobase ; done celui-ci n'est qu'un axe veri- 
table ; mais cet axe est dcprim6 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 Kutacese to produce from 
a single multilocular ovary, one-styled and symmetrical, 
several unilocular ovaries, each with its own style. 1 And he 
1 Animals and Plants under Domestication, vol. ii. p. 258. 


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

" Do Candolle has divided the Cruciferae into five sub-orders 
in accordance with the position of the radicle and cotyledons, 
yet Mons. T. Gay (Ann. des Scien. Nat., ser. i. torn. vii. p. 389) 
found in sixteen seeds of Petrocallis Pyrenaica the form of the 
embryo so uncertain tnat he could not tell whether it ought 
to be placed in the sub-orders ' Pleurorhizue ' or 'NotorhizeV; 
so again (p. 400) in Cochlearia saxatilis M. Gay examined 
twenty-nine embryos, and of these sixteen were vigorously 
' pleurorhize"es/ nine had characters intermediate between 
pleuro- and no tor- hiz6es, arid four were pure notorhize'es." 

"M. Raspail asserts (Ann. des Scien. NaL, 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 


Species which vary lime. 

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 


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 specimens 
of a rare species than of a species that was very common. 
Now as these rare species do really vary much less than the 
common species, 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 species which become 
the parents of new forms, and thus the non-variability of any 
number of rare or local species offers no difficulty whatever in 
the way of the theory of evolution. 

Concluding Remarks. 

We have now shown in some detail, at the risk of being 
tedious, that individual variability is a general character of all 
common and widespread species of animals or plants ; arid, 
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. Yet more important is the fact that 
each part or organ varies to a considerable extent inde- 
pendently of other parts. Again, we have shown, by abundant 
evidence, that the variation that occurs is very large in 
amount usually reaching 10 or 20, and sometimes even 25 
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 


the last edition of The Origin 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 
is 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 " (Origin, p. 66). And 
again, after speaking of changed conditions "affording a better 
chance of the occurrence of favourable variations," he adds : 
u Unless such occur natural selection can do nothing" (Origin, 
p. 64). These expressions are hardly consistent with the 
fact of the constant and large ampunt 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. 



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. 

HAVING so fully discussed variation under nature it will be 
unnecessary to devote so much space to domesticated animals 
and cultivated plants, especially as Mr. Darwin has published 
two 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 how 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 increased 
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 facts 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 


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 diner 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 
quantity of it as possible, and when the crop is ripe we 
carefully choose a few of the very largest seeds, or wo may 
by means of a sieve sort out a quantity of the largest 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 


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. 

Proofs of the Generality of Variation. 

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 two 
of the numerous possible modes of variation occur 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 in 
that particular direction, 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 


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 tfil 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 selection 
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, while the 
leaves and flowers usually differ so little that they are hardly 
distinguishable except to a very close observer. 

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


Variations of Apples and of Melons. 

The most remarkable varieties are afforded by the apple 
and the melon, and some account of these will be given as 
illustrating the effects of slight variations accumulated by 
selection. All our apples are known to have descended from 
the common crab of our hedges (Pyrus mains), and from this 
at least a thousand distinct varieties have been produced. 
These differ greatly in the size and form of the fruit, in its 
colour, and in the texture of the skin. They further differ in 
the time of ripening, in their flavour, and in their keeping 
properties ; but apple trees also differ 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 differ 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, have to be fertilised with the 
pollen from other varieties in order to produce fruit. The 
pips or seeds differ also in shape, size, 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 six years 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, 


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 tlio same genus as our 
currant and gooseberry, which we have cultivated for their 
fruits, there are some ornamental species, as the Kibes 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, arid we have 
zonal geraniums, variegated ivies, gold and silver marked 
hollies, and many others. 

Variations of Domestic Animals. 

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 wo only breed it 
in sufficient quantity, watch carefully for the required varia- 
tions, and carry on selection with patience arid 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. 


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 different 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 differ nearly so much in size, form, arid proportions. 
We have here a remarkable proof that variation is not con- 
fined to superficial characters to the colour, hair, or external 
appendages, when we 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 Newfoundland 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 Pigeons. 

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 and Plants under Domestication, vol. i. pp. 40-42. 


colour, the tail has a dark band across the end, the wings 
have two black bands, arid 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 fan tails 
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 arid 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 

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 arid logs and stand almost upright, so as to 
present a very distinct appearance. Their skeleton has 
become modified, the ribs being broader and the vertebras 
more numerous than in other pigeons. 


RACK II. Carriers. These arc 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 wide. There arc several sub -races, one 
being called Dragons. 

RACE ITT. Emits. These arc 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 the 
skin of the neck is often red. There arc several sub-races, 
and these differ very much, forming a series of links between 
the wild rock-pigeon and the carrier. 

RACK IV. Barbs. 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 have also 
a naked carunculated skin round the eyes, and the skin over 
the nostrils swollen. 

RACK V. Fanfails. Short-bodied arid 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 back its slender neck, so that in highly -bred 
varieties the head touches the tail. The feet are small, and 
they walk stiffly. 

RACE VI. Turbits and Owls. 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 Turbits 
also have a crest on the head, and both have the beak 
exceedingly short. 

RACE VII. Tumblers. 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 will go on tumbling till they die. 


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 ily 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 bo one over which they have no control, 
an involuntary movement which 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 
while 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 scutelte. 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. 


RACK VIII. Indian Frill-lack. In these birds the beak is 
very short, and the feathers of the whole body arc reversed 
or turn backwards. 

RACE IX. Jacobin. These curious birds have a hood of 
feathers almost enclosing the head and meeting in front of 
the neck. The wings and tail arc unusually long. 

RACE X. Trumpeter. 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 Laughers, Frill -lacks, Nans, Spots, and 
Swallows. 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 wing-feathers black or 
red. The Spots are white, with the tail and a spot on the 
forehead red. The Swallows are 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 bo distinguished. It is interesting to note that almost 
every part of the bird, whose vacations 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 vertebrae 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, arid 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 


of the scutellse also vary. The eggs also vary somewhat in 
si/e 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 flight, and the voice, all exhibit modifications of the 
most remarkable kind. 1 


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 
bo adduced showing that such constitutional variation does 

Among animals the cases arc 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. 


that killed the flowers and buds of all other kinds of pears. 
Wheat, which is grown over so large a portion of the world, has 
become adapted to special climates. Wheat imported from 
India and sown in good wheat soil in England produced the 
most meagre ears ; while wheat taken from France to the 
West Indian Islands produced either wholly barren spikes or 
spikes furnished with two or three miserable seeds, while 
West Indian seed by its side yielded an enormous harvest. The 
orange was very tender when first introduced into Italy, and 
continued so as long as it was propagated by grafts, but 
when trees were raised from seed many of these were found 
to be hardier, and the orange is now perfectly acclimatised in 
Italy. Sweet-peas (Lathyrus odoratus) imported from England 
to the Calcutta Itotanic Gardens produced few blossoms and 
no seed ; those from France flowered a little better, but still 
produced no seed, but plants raised from seed brought from 
Darjeeling in the Himalayas, but originally derived from 
England, flower and seed profusely in Calcutta. 1 

An observation by Mr. Darwin himself is perhaps even 
more instructive. He says: "On 24th May 1864 there 
was a severe 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 wide 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 jDomestication, vol. ii. pp. 307-311. 


been long domesticated in the East, the gold and silver carps ; 
and these present great variation, riot 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 by 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 arc 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 effect 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, arid 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 bo 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 


and Mr. Burgess. Mr. Youatt, one of the greatest authorities 
on breeding 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 
desire to deviate from the original breed, and the difl'ercnce 
must have arisen from some slight difference of taste or judg- 
ment in selecting, each year, the parents for the next year's 
stock, 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 scats of civilisation in 
Western Asia or Egypt, and have therefore been the subjects 
of human care and selection for some thousands of years, the 
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 arid cow are nowhere found in a 
wild state, and they have 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 1400 B.C. It was introduced into Europe 
about 600 B.C. Several distinct breeds were known to the 
Romans about tho commencement of tho 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 Candollc 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 



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 Capo of Good Hope, not- 
withstanding that they both possess an exceedingly rich and 
varied flora. These countries having boon, 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 largo 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 any 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 largo 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. 


Although I believe, for reasons which 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 qualities 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 with. 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 

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 Remarks. 

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 


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 ho 
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 

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. 


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 vn nations 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. 




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 will enable us nbw to deal with the 
very core of our subject the formation,-!)! .species_ by_ means 
j)QiatuEaJ selection. We have seen how tremendous is the 
struggle for existence always Agoing on in nature owing to tho 
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 arid for the most part independ- 
ently ; and we have seen that this variability is both large in 
its amount in proportion to tho 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 


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

Effect of Struggle for Existence under Unchanged Conditions. 

Let us first consider what will be the effect oi 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 
species is exposed to numerous and varied dangers throughout 
its entire existence, and that it is only by means of the exact 
adaptation of its organisation^ including its instincts and habits 
to its surroundings that it is enabled to live till it produces 
offspring which may take its place when it ceases to exist. 
We have seen also that, of the whole annual increase only a 
very small fraction survives; and though the survival in indi- 
vidual cases may sometimes be due rather to accident than 
to any real superiority, yet we cannot doubt that, in the long 
run, those survive which are best 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 every 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 which 
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 


throughout all geological time and in every part of the world. 
Land and water have been continually shifting their positions ; 
some regions arc 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 coiitirifciits 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 arid 
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, only, 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. ' 


Divergence of Character. 

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 where 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 ^idarjt^them to their new habits would soon ^lle 
brought about^ because we know "that v<ariations in all the 
external organs and all their separate parts axe 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 shcepj 
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 night, 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 zone, 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 be 
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 haveJbe^B produced^ That 
this is the true explanation of this singular fact is proved by 
much corroborative 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. We thus see that there is no general 
tendency to the abortion, of wings in Madeira, TmF~that Ttis 
simplyaT^age^ of adaptation to new conditions. Those insects 

1 Origin of Species, p. 71. 


to whom wings were not absolutely essential escaped a serious 
danger by not using them,. and the wings therefore became 
reduced or were completely lostj 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.3 Many flying insects, not varying fast enough, would bo 
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 (Cicindelidoi), the chafers (Melolonthidae), the 
click-beetles (Elateridoo), 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 wej*(^ncajmble_of jljght, and most 
of them entirely without wings. They includecl 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 nrjuriu usjto^them. J 

It is no douUTdue 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 urticoe) inhabiting the Isle of Man, which is only 
about half the size of the same species in England or Irelapd ; 
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 
aflbrded by our red grouse. This bird, tho Lagopus scotieus ,of 


naturalists, is entirely confined to the British Isles. It is, 
however, very closely allied to the willow grouse (Lagopus 
albus), a bird which ranges all over Europe, Northern Asia, 
and North America, but which, unlike our species, changes to 
white in winter. No 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 arid in habits, the two species are generally con- 
sidered to be distinct.i 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 grouse was 
identical with that of the rest of Europe. But 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 means of concealment. The colour was, there- 
fore, gradually 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 
winter 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 arid 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 
1 Yarrell's British Birds, fourth edition, vol. iii. p. 77. 


more vegetarian diet, eating seeds as well as insects, and 
feeding on the ground as well as among trees. The delicate 
little blue 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/hree common pipits the tree-pipit 
(Anthus arborcus), 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. 
oenanthe) are more or less 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 
character 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 usually seize it 
on or near the ground, feeding on hares, rabbits, squirrels, 
grouse, pigeons, and poultry. Kites and buzzards,- on the 


other hand, seize their prey upon the ground, and the former 
feed largely on reptiles and offal as well as on birds and 
quadrupeds. Others have adopted fish as their chief food, 
and the osprey snatches its prey from the water 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 arc carrion-eating, insect-eating, and 
fruit-eating crows. Even kingfishers are, some aquatic, some 
terrestrial in their habits ; some live on fish, some on insects, 
some on reptiles. Lastly, among the primary divisions of birds 
we find a purely terrestrial group the Katita 1 , including the 
ostriches, cassowaries, etc. ; other great groups, including the 
ducks, cormorants, gulls, penguins, etc., are aquatic ; while the 
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 ilosh-cater as well as a grain-cater, 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. We 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, arid 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 


by rivals, or is in process of extinction by enemies, to save 
itself by adopting nevy habits or by occupying vacant places 
in nature. This is the immediate and obvious effect 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 bo supported therex 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 bo 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 these 
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 

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 diifer 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, 1 00 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 


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 plants are diversified for different 
habits of life, so will a greater number of individuals be 
capable of there supporting themselves." 1 

The most closely allied Species inhabit distinct Areas. 

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 but a naturalist are usually 
not found in the same but in widely separated countries.^ 
Thus, the nearest allies to our European golden plover are 
found in North America and East Asia ; the nearest ally 
of our European jay is found in Japan, although there are 
several other 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 (Parus azureus) ; the Parus ledouci of Algeria 
is very near our coal tit, and the Parus lugubris of South- 
Eastern Europe and Asia Minor is nearest to our marsh tit. 
So, our four species of wild pigeons the ring-dove, stock- 
dove, rock-pigeon, and turtle-dove are riot 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 
arid 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. Bernardi), but rarely are two 
1 Origin of Specics t p. 89. 


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 will now consider some 
other modes in whij^li natural selection will act, to adapt 
organisms to changed conditions. 

Adaptation to Conditions at Various 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 arid 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 


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

The larvae of insects have thus been wonderfully modified 
in order to escape the numerous enemies to whose attacks 
they are 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 brown instead of a 
green colour is protective. Others have acquired curious 

attitudes and large ocelli, which cause them to e the 

head 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 arc 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 eggs 
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 



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 atfd developed into higher forms ? 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 arc 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 diatomaceas 
and conferva), 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 


arc the lowest mammals the echidna nnd ornithorhynchus of 
Australia; the lowest birds the apteryx of New Zealand 
and the cassowaries of the New Guinea region; while the 
lowest fish the amphioxus or lancelet, is completely isolated, 
and has apparently survived only by its habit of burrow- 
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 arid more specialised groups must have been 

Circumstances favourable fo the Origin of New Species ly 
Natural 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, since, 
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, will 
be that some species will increase and others will diminish ; 
and in cases where 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, however, 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 without some considerable change in structure or 
habits, that change would, in some cases, be brought about by 
variation and natural selection, and thus new varieties or new 
species might bo formed. We have to consider, then, which 


arc the species that would bo 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 Cinclidoo 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 -larvoc, 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, 


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 arc known, all those of the old world being so closely 
allied to our British bird that some ornithologists consider 
them to bo merely local races of one species ; while in North 
America and the northern Andes there are two other 

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 ancestor 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 div T "ig and flying under water was acquired by a 
true land-bird. 

That such habits mi'ijht be acquired under stress of need 
is rendered highl} m^x iblo by the facts stated by the well- 
known American nau.v&list, Dr. Abbott. Ho 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 beneUth the surface, so that often, for 


several seconds, a large part of the body is submerged. Now 
these birds still have the plumage pervious to water, and so 
arc 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, 8. 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 
Muller has described a caddis-tly larva which lives among these 
leaves, and which has been modified in the pupa state in 
accordance with its surroundings. The pupaj 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 now abode. Bromeliye 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. 1 ritz 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 aqur^ P 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. * :x. p. 30. 


The Importance of Isolation. 

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. .But this question of the 
swamping effects of intercrossing will be considered in another 

Mr. Darwin was of opinion that, on the whole, the largeness 
of the area occupied by 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 will 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 


and recognition by members of the same variety or incipient 
species. It has also been observed that each differently 
coloured variety of wild animals, or of domesticated animals 
which have rim 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 ly Natural Selection. 

As natural selection acts solely by the preservation of use- 
ful variations, or those yhich 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 wo 
should expect that the larger groups in each class of animals 
and plants those which have persisted arid 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 mcsozoic rocks, have developed into the marvellous wealth 
of forms of the higher dicotyledons that now adorn the earth. 

But this remarkable advance iri tho 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 


forms, and also of absolute degradation or degeneration. Ser- 
pents, for example, have "been developed from some lizard-like 
type 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 
same remark will apply to the whale tribe among mammals ; 
to the blind amphibia and insects of the great caverns ; arid 
among plants to the numerous cases in which flowers, once 
specially 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 burnet, and even, as some botanists maintain, our 
rushes, sedges, and grasses. The causes which have led to 
this degeneration will be 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 retrogression, 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 boon 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 


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 
now 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, wo must always bear in mind that what 
goes on in the case of the individual or family group we may 


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 chance 
determines which shall live and which die. For, although in 
many individual cases death may he 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 works. 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 arc 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, the fittest will survive, 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 
variation as something exceptional and comparatively rare as 
an abnormal deviation from the uniformity and stability of the 


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 ^f 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 versd 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 


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. Darwin 
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 arid* 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. 



Difficulty as to smallness of variations As to tlio 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 Dellxeuf s law No " specific " character proved to be 
useless The swamping effects of intercrossing Isolation as prevent- 
ing intercrossing Gnlick 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 arid 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 Species, was never made use 
of by Darwin himself, who 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 


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. We 
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 
1)0 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 (Uiwunt 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 The Origin of Species, 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. Darwin 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 


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 oolight ; 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 which 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 

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, 
when 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 


difficult cases, and this Mr. Darwin has done. One or two of 
these may be briefly given here, but the whole series should 
be carefully read by any one who wishes to see how many 
curious facts and observations have been required in order to 
elucidate them ; whence we 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 tlio.y really deserved that name, the young may have 
been nourished by a fluid secreted by the interior surface of 
the mal-supial sack, as is believed to be the case with the 
iish (Hippocampus) whose eggs are hatched within a some- 
what similar sack. This being the case, those individuals 
which secreted a 'more nutritious fluid, and those whose 
young were able ho obtain and swallow a more constant supply 
by suction, wo\t ; be more likely to live and come to a healthy 
maturity, and rould therefore be preserved by natural selec- 

In another case which has been adduced as one of special 
difficulty, a, more complete explanation is given. Soles, 
turbots, and other flatfish arc, 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. Darwin shows by 
reference to the researches of Malm and others, the young of 
these iish 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. 176-193. 


this time soft and flexible, the constant repetition of this effort 
causes the eye gradually to move round the head till it conies 
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 K)st, 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 ^ot unintelligible ; 
granting of course the sensitiveness to light f some forms of 
nervous tissue. For he shows that there arc, i 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 
form the first rudiment of a lens. Many of the succeeding 
steps are lost, as would necessarily be the case, owing to the 
reat advantage of each modification which gave increased 
distinctness of vision, the creatures possessing it inevitably 
surviving, while those below them became extinct. .But wo 
can w r ell 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 tho spherical aberration is absolutely 
corrected ; while long- and short- sightedness, and the various 
diseases and imperfections to which the eye is liable, may bo 
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 tho matter may study carefully 



the whole of the sixth and seventh chapters of the last edition 
of The Or if/in of ti2 }C<: ' ies > m which these and many other cases 
are discussed in considerable detail. 

Useless or non-adaptive Char ctcfcrs. 

Many naturalists seem to be of opinion that a considerable 
number of the characters which distinguish species are of no 
service whatever to their possessors, and therefore cannot have 
been produced or increased by natural selection. Professors 
Bronn and IJroca have urged this objection on the continent. 
In America, Dr. Cope, the well-known paleontologist, has long 
since put forth the same objection, declaring that non-adaptive 
characters are as numerous as those which are adaptive ; but 
he differs completely from most who hold the same general 
opinion in considering that they occur chiefly " in the 
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 G. J. Romanes has urged this 
difficulty in his paper on "Physiological Selection" (Journ. 
Linn. Sor., vol. xix. pp. 338, 314). 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 tho 
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 


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 Niigejj 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 Orif/in of Species I perhaps attributed too much to the 
action of natural selection or the survival of the fittest. 1 
have altered the fifth edition of the Origin so as to confine my 
remarks to adaptive changes of structure, but I am conrinced, 
from the light gained duriny even the lad few years, that very 
many structures which now appear to us useless, trill hereafter be 
proved to be useful, and will therefore come within the raiuje of 
natural selection. Nevertheless I did not formerly consider 
sufficiently the existence of structures which, as far as we am 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 which 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 character's, 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, 


and colours of the flowers, 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 
was 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, 
even the streaks and spots with which they arc 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 tft 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 Kcrner 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 ilowcr-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 
been 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 


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 spinevS, hairs, or 
down with which various pirts 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, cither 
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. Largo 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 


poisonous fang, 1 * often 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 bands, spots, or patches of white, 
or of bright colour, which vary in every species, and are often 
concealed when the creature is at rest but 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 quickly recognised, even at a distance, by 
its fellows, especially the parents by their young and the two 
sexes by each other ; and this recognition must 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 tho character of flight, facility for running, 
or climbing, for inhabiting chiefly the ground or trees, and 
the kind of food that can bo 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 tho length or shapo of the ears 
and tail, in the proportions of the limbs, or in the length 
and quality of the hair on different parts of tho body. As 
regards tho ears and tail, one of the objections by Professor 


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. Schubl 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 Hies are injurious or even fatal to 
large mammals, wo 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. 


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 flanks. 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 bo 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 

1 Nature, vol. xxxviii. p. 328. 

2 A very remarkable illustration of function in an apparently useless 
ornament is given by Semper. He says, "It is known that the skin of 
reptiles encloses the body with scales. These scales are distinguished by 
very 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 


Instability of Non-adaptive Characters. 

One very weighty objection to the theory that specific 
characters can ever be wholly 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., wfcich 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 casting, 
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 Pi ofessor 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 <jeneric> not specific^ characters ; and the three 
genera adduced are somewhat isolated, implying considerable antiquity and 
the 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. 10. 


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" (Journ. Linn. tioc. t vol. xix. 
p. 3 84) 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. But, 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 p.irt would be iisc/ul, and would therefore be 
subject to the law of survival of the fittest. The genera Ateles and Colobus 
are two of the most put el y arboreal types of monkeys, and it is not difficult 
to conceive that the constant use of the elongated fingers for climbing from 
tree to tree, and catching on to branches while making great leaps, might 
require all the nervous energy and muscular growth to be directed to the 
fingers, the small thumb remaining useless. The case of the Potto is more 
difficult, 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 meie 
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 
evidently 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. The 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, 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 tho 
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. 


that they art 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, rfs 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. 
Tegctmeier 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 marie arid 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 would 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 bo 
useful, or both may be useful in unequal degrees. 

The difficulty as to how individual differences or sports can 
become fixed and perpetuated, jf altogether useless, is evaded 
by those who hold that such character^ 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 shoulc then be allowed 
to perpetuate themselves by heredity," until they are finally 


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 
correlate/! with some useful and important peculiarities. 

As bearing upon this question we may refer to what is 
termed Delbamfs law, which has been thus briefly stated by 
Mr. Murphy in his work on Habit and Intelligence, p. 

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

It is not impossible that some definite varieties, such as the 
melariic form of the jaguar and the bridled variety of the guille- 
mot are duo 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 
selection. 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 species 
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 Species, p. 72. 


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/ulmit, 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 Renew 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 which 
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. Romanes 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, \itility of specific characters. In a letter to Semper (30th Nov. 
1878) he writes : " As onr 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, and 
as not in any way due to natural selection" (Life of Darwin, vol. iii. p. 161), 


it as one of the difficulties which can alone be 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 ; while he admits that "if the 
assumption were granted there would be 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 some further discussion and 
elucidation of the subject are desirable. ' 

In one of the preliminary chapters of Mr. Seebohm's recent 
work on the Chamdriidcr, he 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 largo 
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 


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 nut 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 ot 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 exaggerated 
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 


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 will 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 may 
explain the origin of so many flightless and rather bulky birds 
in oceanic islands, as the dodo, the cassowary, and the extinct 
inoas. 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 wo find in the separate 
islands of the Galapagos, 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, wo 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 tho 
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 


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 th/ 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. 80. 


new species. This was the keynote of Mr. Yernon Wollas ton's 
essay on "Variation of Species," published in 1850, and it is 
adopted by the Uev. J. G-. Gulick in his paper on " Diversity 
of Evolution under one Set of External Conditions " (Jo urn. 
Linn. tfoc. ZooL, vol. xi. p. 490). 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 \iew 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 Achatinellido. 1 , found 
nowhere else in the world. The interesting point is the 
extreme restriction of the species and varieties. The 
average range of each species is only five or six miles, 
while some are restricted to but 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 live or six 
miles in breadth ; and this small territory furnishes about 
175 species, represented by 700 or 800 varieties. Mr. 
Gulick states, that the vegetation 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 


another on the same side an equal distance apart. In a very 
lengthy paper, presented to the Linncan Society last year, on 
"Divergent Evolution through Cumulative Segregation," Mr. 
Gulick endeavours to Avork 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 conditions 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. p. 215. 


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 will 
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 arc 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 bo 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 


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 essence 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. fine. Zod , vol. xx. pp. 189- 
274) he discusses the various forms of isolation above referred to, under i.o 
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 arid 
often recondite modes by which practical isolation may be brought about, 
may help to remove one of the popular difficulties in the way of the action 
of natural selection in the origination of species. 


if this were a vcra causa, we 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 ]ess 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 this, 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 difficulties and objections 
relating to the question of hybridity, which arc so important 
as to require a separate chapter for their adequate discussion. 



Statement of the problem 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. Ilnth'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 are 
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 a&' distinct from a variety ; and so 
long as it was believed that species were separate creations, or 


at all events had an origin quite distinct from that of varieties, 
this law could have no exceptions, because, if any two species 
had been found to be fertile when crossed and their hybrid 
offspring to be also fertile, this fact would have been held to 
prove them to be not species but nineties. On the other hand, 
if two varieties 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 

The elaborate and careful examination of the whole subject 
by Mr. Darwin, 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 physiological 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. 

Extreme Susceptibility of 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, 


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 riot ; 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 riot 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 at 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 Bechstein 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 


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. 
])arwin. The two distinct species of plants, Mirabilis jalapa 
and M. longillora, 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, Knlrcuter, 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. longiHora. 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. 

Individual 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. ]>p. 163-170. 


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 ; arid 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, JDianthus 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 corolla, while the 


stylo is long, the globular stigma appearing just in. the centre 
of the open ilower. 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). 

hong-styled form. Short-styled form. 

FIG. 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 versd. 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, while it would never 
reach the stigma of another plant of the short- styled form. 


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 present similar phenomena, one or 
two of the most curious of which must be referred to. The 
beautiful crimson ilax (Linum grandiflorum) has also two 
forms, the styles only differing in length ; and in this case Mr. 
Darwin found by numerous experiments, which 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 (whereas 
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 ho 
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 


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

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 tJie 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 arc fertile inter se; and then to consider why it is that 
such cases are so few in number. 

The common domestic goose (Anscr ferns) 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 Ilutton, 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 bo 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 oil The 
Different Forms of Flowers in Plants of the same Species, chaps, i.-iv. 

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


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

Dogs have been frequently crossed with wolves and with 
jackals, and their hybrid offspring have been found to be fertile 
inter 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 Britain, 
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 bo 
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 arid short ; the 
litters diminish in frequency, arid 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 same species 
are bred under similar adverse conditions. JBut 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 Or eat Britain, Introduction, p. Ixiv. 


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 w. 

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, 
Giblott, 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 



such thing as " perfect health " in man, and probably no such 
thing as absolute freedom from constitutional taint in animals. 
The experiments of Mr. Darwin, 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 Avill 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 docs 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 (Dafila 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 

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 arid goat is fertile. Breeds of this mixed race are 
numerous in the north of Europe." * Nothing appears to bo 
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, 175G) obtained one 
such hybrid in 1751 and eight in 1752. Sanson (La Culture^ 
vol. vi. p. 372, 18G5) mentions a case observed in the Vosges, 
France. Geoff. St. Hilaire (Hist. Nat. G6n. des rey. ory., vol. iii. p. 

1 Low's Domesticated Animals, p. 28. 


163) was the first to mention, T 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 
6 pellones ' of Chile are produced by the second and third 
generation of such hybrids (Guy, * 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 sc 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 arid five-eighths of she-iroat ; such beinuj the 

O O O ' O 

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 inter sc; 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 13. 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 ho 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 wo saw to be the case in 


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

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, 0. canaliculatum, or 
0. defixum all very distinct from it, produced perfectly 
fertile hybrids ; while 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. nyctanigemienora 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 Ehododendron 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 are freely 
1 Amaryllidacecc, by the Hon. and Kev. William Herbert, p. 379. 


crossed with each othe?', 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. Gartner, 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. ccerulca), 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 he 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 with 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 Species, p. 239. 


Parallelism between Crossing and 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 may 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 iriterchangeability of the beneficial effects 
of crossing and of new conditions, serves to explain some very 
puzzling phenomena in the forms and economy of flowers. 

Remarks on the Facts of Ilylridity. 

The facts that have now been adduced, though not very 
numerous, are 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 


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 arc, 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 two most distinct and 
widely 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 

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 


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 bo 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 nsnally correlated with 
other Characters, especially with Colour. 

The evidence already adduced as to the extreme suscep- 
tibility of the reproductive system, and the curious irregu- 
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 known 
to bo 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 species 
become fully differentiated. 

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


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 arc 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 Verbaseum 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 cany 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 

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 


distinctive marks, and they arc, 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 Pec.uHarilies. 

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. Tegctmeier, 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 sheep 
are riot 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 arc subject to cutaneous 
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 flies than arc 
those which are brown or black. The same law even extends 

1 Origin of Species, sixth edition, p. 9. 


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 

The Isolation of Varieties Inj 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 Mcdico-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 srnell 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. Wo 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 
rm 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. 


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 mouse-coloured variety seem 
to have already developed a physiological peculiarity in breed- 
ing a month 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; 
while in Circassia there are three races of hoi-ses 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 more 
than half a mile in diameter, the half-wild native black sheep 
do not readily mix with imported white shoe]). 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 Kev. W. D. Fox that his flocks of white 
and Chinese geese kept distinct. 1 

This constant preference of animals for their like, even in the 
case of slightly different varieties of the same species, is evidently 
1 Animals and Plants under Domestication, vol. ii. pp. 102, 303. 


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 bo 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 


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, will 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 
within 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 will be retarded, or perhaps 
entirely prevented ; for the offspring of the crossed unions 
will 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 certainly estab- 
lish a powerful antagonistic influence to the further differentia- 
tion of the two forms. 


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 will 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 will 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 will 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 were 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 


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 the 
forms in adaptation to their different conditions of life. 
Again, evidence has been adduced that change of conditions 
or of mode of life is a potent cause of disturbance of the 
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 also 
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 pan passu, and would ultimately 
lead to the production of two distinct forms having all the 
characteristics, physiological as well as structural, of true 

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 has 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 would 
have any tendency to increase, or would produce any but a 


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, would soon die out, 
while 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 bo eliminated, and could never rise much above 
the numbers which were produced by sporadic variation each 

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 


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. Each 
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 became 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 se, 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 


such infertility there is no proof. This is admitted ; hut 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 011 the subject about twenty years back. Some readers may find this 
easier to follow than the fuller discussion in the text : 

Can titcrility of Hybrids have been Produced Inj Natural Meet ion ? 

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 

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 hie, and lorm 
two slightly differing species. 

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

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 iirst 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 

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 
diminution of the production of hybrids. 


Physiological Selection. 

Another form of infertility has been suggested by Professor 
G. J. Komanes 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 arc 
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. 
liomanes 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 witli 
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 physiolor/ind variety of the two forms will be better suited to the 
conditions of existence than the remaining portion which has not varied 

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 
imperfectly adapted ; therefore, by natural selection the varieties which are 
sterile when crossed will become established as the only ones. 

10. Now let variations in the amount of sterility and in the disinclination 
to crossed 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 variations ; so that, just in propor- 
tion as the two forms diverged ami became better adapted to the conditions of 
existence, they would become more sterile when 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. 155. It must, however, be noted, 
that such sterility in first crosses appears to be equally rare between different 
species of the same genus as between individuals of the same species. Mules 
and other hybrids are freely produced between very distinct species, but are 


so, though wo 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. Ivomanes thinks that they would persist, and urges that 
"whenever this one kind of variation occurs it cannot escape 
Ihe preserving agency of physiological selection. Hence, even 
if it bo granted that the variation which affects the re- 
productive system in this particular way is a variation of 
comparatively rare occurrence, still, as it must always he 
preserved whenever it does occur, its influence in the manu- 
facture of specific types must Ic. cunwtatirc." The very positive 
statements which 1 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. .Romanes* 
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 ho the criterion 
of species, not the sterility of their first crosses. Hence we should not 
expect to lind 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. 
Homanes' theory of Physwlogwctl 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. 


form or colour, or with inherent peculiarities of likes or 
dislikes leading to any choice as to the pairing of the two sets 
of individuals. We 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 would 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 cither 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, wo 
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,V80 pairs of the normal variety to 1220 ] 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 99,984 of the 
normal, and only 1 6 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 are omitted for clearness. 


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 

5th i 16 + 1,220 H- 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 tho offspring of the individuals which 
possess it, in consequence of their superior numbers, a greater 


chance of survival in the buttle 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 where correlated as above, 
when they become useful, and therefore subject to be increased 
by natural selection. 

Summary and Concluding Remarks on llulridity. 

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 
which does not affect their general health, by captivit}^ 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 (arid 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 diversity 
of station or of habits, then, while each portion must be kept 
fertile inter se, 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 
colour both which circumstances we know to be either the 
cause of infertility or to be correlated with it, the fact of 


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 


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. 



Tho Darwinian theory throw 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. Poultou'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 ho 
believed that many of the most brilliant colours were developed 
by sexual choice ; while his great general principle, that all 


the fixed characters of organic beings have been developed 
under the action of the law of utility, led to the inevitable 
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 wellbeing 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 Kolccd. 

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, due to the 
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 rays 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 either by thin 
films or by very fine strise on the surfaces of bodies, which 
cause rays of certain wave-lengths to neutralise each other, 
leaving the remainder to produce the effects of colour. Such 


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 hero 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, arid 
minerals ; in the sky and in the ocean ; in sunset clouds arid in 
the many-tinted rainbow. Here we can have no question of 
use 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 nlgre and fungi, or even in the universal mantle of 
green which clothes so largo 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 ar*e 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 Aiwmnl (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 


great variability. We sec 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, shows 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 the 
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 
is, 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- 


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 white coloration in the arctic regions, which exceptions 
are, nevertheless, quite in harmony with the theory of pro- 

Whenever we find arctic animals which, from whatever 
cause, do riot 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 
riot 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 bo able to recognise its kind at a distance 
than to bo 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 order to approach its prey. The 
colour of the raven and of the musk -sheep are, therefore, 


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. Hero 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 -caters, turacos, 
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 on the upper surface, is a 
rusty brown of various shades, well corresponding with the 
bark, withered leaves, ferns, and bare thickets among which 


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 arc 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 colow, 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 with the colour of the sea as seen by hovering 
birds ; while they arc 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 nudibranchiato 
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 duo 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, 



two strong arguments against this theory. Wo 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 (Formicariidie), 
the tyrant-shrikes (Tyrannidte), the American creepers (Den- 
drocolaptida?), together with a large proportion of the wood- 
warblers (Mniotiltidae), the finches, the wrens, arid some other 
groups. In the eastern hemisphere, also, we have the babbling- 
thrushes (Timaliidse), the cuckoo-shrikes (Campephagida^), the 
honey-suckers (Meliphagidie), and several other smaller groups 
which are certainly riot 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 (Carabidao) attains its greatest 
brilliancy in the temperate zone ; while by far the larger 
proportion of the great families of the longicorris 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 Pieridse or 


whites and yellows, and the Satyridje 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 

This theory is founded on a number of very curious facts 
which prove, that such a change does sometime? 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, 


when they are not brought into action, is a dirty white. 
These animals are excessively sluggish arid 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 (My sis Chamoeleon) 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 pupae, which 
undergo changes of colour when exposed to differently 
coloured surroundings. This subject has been carefully 
investigated by Mr. E. B. Foul ton, 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 corre- 
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 Theory 
of Descent, Professor Meldola seemed disposed to think that 
the variations of colour of some of the species might be 
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 duo 
to the food but to the coloured light reflected from the leaves. 

1 Proceedings of the Royal Society, No. 243, 1886 ; Transactions of the Royal 
Society t vol. clxxviii. B. pp. 311-441. 


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 
larvoo which had been supplied with different food-plants or 
exposed to a different coloured light. 

An even more interesting scries of experiments was made 
on the colours of pupa3, which in many cases were known to 
bo 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 
brassier 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 
up several important points. Ho showed that the action 
of the coloured light did riot 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 tho common tortoise-shell butterfly from nearly 
black to pale, or to a brilliant golden ; and that of Pieris rapae 


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 oif 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 pupao 
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 when 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 are 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 the colours which more generally prevail in nature ; 


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 which cover the 
stones of the higher mountains ; wiiile 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 Walford, states in a letter, that the haunts 
of the tiger are invariably full of the long grass, dry and palo 
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 bo 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 


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, tiger-cats, and 
spotted deer as to afford them a very perfect concealment. 

Jri some cases the concealment is effected by colours and 
markings which are so striking and peculiar that no one who 
had riot 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 any 
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 NaturalisC s Wanderings in the Eastern Archipelago, p. 460. 


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 they frequent. The sun-birds are fully aware of this 
fact, for no sooner have they relinquished the ilowers 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 (Nectarinea anicthystina) is never absent 
from that magnificent forest-tree, the ' Kaffir Boom ' (Erythrina 
caifra) ; all day long the cheerful notes of these birds may bo 
heard amongst its spreading branches, yet the general aspect 
of the tree, which consists of a huge mass of scarlet and purple- 
black blossoms without a single green leaf, blends and har- 
monises with the colours of the black sun-bird to such an extent 
that a dozen of them may be feeding amongst its blossoms 
without being conspicuous, or even visible." l 

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 pi eta, 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. Zvol. Soc., 1862 p. 357. 


to Surinam (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 Phasmida?, or stick and leaf insects. The well- 

1 With reference to this general resemblance of insects to their environment 
the following remarks by Mr. Poulton are very instructive. He says : 
" Holding the larva of Sphinx ligustri in one hand and a twig of its food- 
plant in 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 larvce will bo 
passed over by those who are not accustomed to their appearance, although tho 
searcher may be told of the presence of a large caterpillar. An experienced 
entomologist may also fail to find the larva) 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-plant 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 arc apart." Numerous other examples arc 
given in the chapter on "Mimicry and other Protective Resemblances among 
Animals/' in my Contributions to the Theory of Natural Selection. 


known leaf-insects of Ceylon and of Java, species of Phy Ilium, 
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 minutiae 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 be 
alive. One species, which was brought me in Borneo, was 
covered with delicate semi transparent green foliations, exactly 
resembling the hepatic;*} 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 Lejridopterous Insects of Georgia, 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. Wo have, first, the larva of Sphinx fuciformis feeding 
1 The Naturalist in Nicaragua, p. 19. 


on a plant with 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 larvae 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 larvae 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, in Proc. Zool. Soc., 1873, p. 155. 


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 
was 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 docs not produce any movement externally. It was 
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 Jiave 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 arid 
their various kinds of enemies the need of protection arose, 
and was usually most easily met by modifications of colour. 
Hence, wo 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 Nature , vol. iii. p. 166. 


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. 
Weismann illustrate this progressive adaptation. The very 
young larvjB 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, arid 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 about. In many Iarva3 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. Weismann 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. Manscl 
Weale states that in South Africa there is a great prevalence 


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 withigUnall 
quartz pebbles ; while in other localities where the colours of 
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 arc rather large and showy butterflies, orange 
and bluish on the upper side, with a very rapid flight, arid 
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. Lon<L> 1878, p. 185. 
a Ibid. (Proceedings^ p. xhi.) 


which has just that amount of irregular curvature that is seen 
in dry arid 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 
me 1 / complete, and in Sumatra I have often seen one enter a 
bush D dnd 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 Moselcy 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, Scylla3a 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." 2 

1 Wallace's Malay Archipelago, vol. i. p. 204 (fifth edition, p. 130), with 

2 Moseley's Notes by a Naturalist on the Challenger. 


Professor E. S. Morso 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 red calcareous alga^, and Crepidula plaria, liv- 
ing within the apertures of the shells of larger species of 
Gasteropoda 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. George Brady. He 
says: "Amongst the Ntillipore which matted together the 
laminaria roots in the Firth of Clyde were living numerous 
small starfishes (Ophiocoma bellis) which, except when their 
writhing movements betrayed them, were quite undistinguish- 
able from the calcareous branches of the alga ; their i\ : d 
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/' 2 

These few 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 ly 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 Sphingida3, the forepart of the body 

1 Proceedings of the Boston Soc. of Nat. Hist.) vol, xiv, 1871. 

- Nature, 1870, p. 376. 



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 larvie 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 (Staphylinidre), 
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 zicmc, 
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, arid 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 


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. O. Forbes has described a 
most interesting example of this kind of simulation in 
Java. 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 Lycienidse 
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 was 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 
when 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 Wantlerings in the Eastern Archipelago, p. 63. 


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 labellum 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. 3 

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 sec 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, Hymenopus bicomis (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 Mantidte, of the genus 
Gongylus, 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 prothoracic 
shield, was found in Pegu by a botanist, and was for a moment mistaken by 
him for a flower. See Proc. Ent. Soc. Land., 1878, p. liii. 


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 pro- 
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 pale 
uniform coloration. Such is the case with kingfishers, bee- 
eaters, 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 bare soil ; but in these 
cases the birds themselves are protectively coloured, so that, 
when sitting, they are almost invisible ; arid 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 


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, where 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, in Seebohm's History nf British Rirds, vol. ii. Introduction, p. 
xxvi. Many of the other examples here cited are taken from the same valu- 
able work. 


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 largo 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 are 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 does. 

The beautiful blue or greenish eggs of the hed^e-sparrow, 
the song-thrush, and sometimes those of the blackbird, 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 riest 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, 


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 riest 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 lead to the 
destruction of the whole set. Those cuckoos, therefore, which 
most frequently placed their eggs among the kinds which 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. II. S. Lucas, in Proceedings of Royal Society of Victoria, 1887, 
p. 56. 


by some change in the colour of the eggs, or in the structure 
or position of the nest, or by 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. 

Colour as a Means of Recognition. 

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 easy 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 avoid 
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 Wm. II. Brewer of Yale College has shown that the white 
marks or the s^ojts of domesticated animals are rarely symmetrical, but have 
a tendency to ar^ear more frequently on the left side. This is the case with 
horses, cattle, dogs, 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 


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 servo 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 
when 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 
hcrbivora 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. 
Where 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. of the Am. Ass. for Advancement of Science^ vol. xxx. p. 246. 
1 Descent of Man, p. 542. 


of Africa we find many distinctive markings of a similar 
kind. Tho 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. IS. Gazella scemmcrringi. 

upturned white tail of the rabbit. In the pallnh 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 dillerentiated for the purpose of recogni- 
tion, rather than for any speciality of defence in species whose 
general habits arc so similar. 


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 duykcr-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 
mariner, the fine Speke's antelope, which lives entirely in the 
swamps and among reeds, has pale vertical stripes on the 
sides (protective), with 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 are 
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 bands, and 
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 
exposedto 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 inntility of any kind of coloration without a careful study of the 
habits of the species in its native country. 



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, while 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 wing, 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, 


"Fio. 20. (Edicneinus vcrniieulatus (above). CE. setiegaleusis (below). 



will serve to illustrate this difference ; which is frequently much 
greater and modified in an endless variety of ways (Fig. 22). 
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 chalcopteim 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, first stated in 
my article on " The Colours of Animals and Plants " in Macmillan's Magazine, 
and more fully in my volume on Tropical Nature. Subsequently Mrs, Barber 
gave a few examples under the head of " Indicative or Banner Colours," but 
she applied it to the distinctive colours of the males of birds, which I explain 
on another principle, though this may assist. 


J?io. 22. Scolopax megala (upper). B, stenura (lower). 



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 duo 
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, 
arid 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 13 
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 bib of paper on 
the ground, no doubt mistaking it for one of its own species ;" 
while, according to Mr. Collirigwood, in the Malay Archipelago, 
" a dead butterfly pinned upon a conspicuous twig will 'often 
arrest an insect of the same species in its headlong flight, and 


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 mollusca. But in marine animals it seems 
probable that the colours, hoAvevcr 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, \\ 317. 

2 In the American Naturalist of March 1SN8, Mr. J. E. Todd has an 
article on " Directive Coloration in Animals," 111 which he recognises many of 
the cases here referred to, and suggests a few others, though I think lie 
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 Ch((llen>jcr t and Dr. E, S. Morse ill Proc. of Host. 
Sue. of Nat. Hist., vol. xiv. 1871. 


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 colouis 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 sen-coast or on islands, and ho supposed that 
the more brilliant atmosphere of the inland stations was the 
explanation of the phenomenon. 1 Many American naturalists 
have observed similar facts, arid 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. 


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 docs also the protection afforded by clouds, the 
excessively humid regions being also regions of extreme 
cloudiness, while the dry regions are comparatively cloudless 
districts." 1 Almost identical changes occur in birds, and are 
imputed by Mr. Allen to similar causes. 

It will be seen that Mr. Gould arid Mr. Allen impute 
opposite effects to the 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 perpetual inconsistencies. 
At one time the brilliant colours of tropical birds and insects 
are imputed to the intensity of a tropical sun, while the same 
intensity of sunlight is now said to have a " bleaching " effect. 
The comparatively dull and sober hues of our northern fauna 
were once supposed to be the result of our cloudy skies ; but 
now wo are told that cloudy skies and a humid atmosphere 
intensify colour. 

In my Tropical Nature (pp. 257-264) I have called atten- 
tion to what is perhaps the most curious and decided relation 
of colour to locality which has yet been observed the preval- 
ence of white markings in the butterflies and birds of islands. 

1 The " Geographical Variation of North American Squirrels," Proc. flost. 
fioc. of Nat. Jfist., 1874, p. 284 ; and Mammals and Winter Birds o/Flvrida,\n>, 


So many cases are adduced from so many different islands, both 
in the eastern and western hemisphere, that it is impossible 
to doubt the existence of some common cause ; and it seems 
probable to me now, after a fuller consideration of the whole 
subjectof colour, that here too we have one of the almost innumer- 
able results of the principle of protective coloration. White is, 
as a rule, an uncommon colour in animals, but probably only 
because it is so conspicuous. Whenever it becomes pro- 
tective, as in the case of arctic animals and aquatic birds, it 
appears freely enough ; while we know that white varieties 
of many species occur occasionally in the wild state, and 
that, under domestication, white or parti-coloured breeds are 
freely produced. Now in all the islands in which exception- 
ally white-marked birds and butterflies have been observed, 
we find two features which would tend to render the con- 
spicuous white markings less injurious a luxuriant tropical 
vegetation, and a decided scarcity of rapacious mammals and 
birds. White colours, therefore, would not bo eliminated 
by natural selection ; but variations in this direction would 
bear their part in producing the recognition marks which 
are everywhere essential, and which, in these islands, need 
not be so small or so inconspicuous as elsewhere. 

Concluding Remarks. 

On a review of the whole subject, then, we must conclude 
that there is no evidence of the individual or prevalent colours 
of organisms being directly determined by the amount of light, 
or heat, or moisture, to which they are exposed ; while, on the 
other hand, the two great principles of the need of concealment 
from enemies or from their prey, and of recognition by their 
own kind, are so wide-reaching in their application that they 
appear at first sight to cover almost the whole ground of 
animal coloration. But, although they are indeed wonderfully 
general and have as yet been very imperfectly studied, we are 
acquainted with other modes of coloration which have a 
different origin. These chiefly appertain to the very singular class 
of warning colours, from which arise the yet more extraordinary 
phenomena of mimicry ; arid they open up so curious a field 
of inquiry and present so many interesting problems, that a 
chapter must be devoted to them. Yet another chapter will 


be required by the subject of sexual differentiation of colour 
and ornament, as to the origin and meaning of which I have 
arrived at different conclusions from Mr. Darwin. These vari- 
ous forms of coloration having been discussed and illustrated, 
wo shall be in a position to attempt a brief sketch of the funda- 
mental laws which have determined the general coloration of 
the animal world. 



The skunk as an example of warning coloration "Warning colours among 
insects Butteiilies Caterpillars Mimicry- -How mimicry has been 
produced Heliconidic Perfection of the imitation Other cases of 
mimicry among Lepidoptora Mimicry among protected groups Us 
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. 

WE have now to deal with a class of colours which are 
the very opposite of those we have hitherto considered, since, 
instead of serving to conceal the animals that possess them 
or as recognition marks to their associates, they are developed 
for the express purpose of rendering the species conspicuous. 
The reason of this is that the animals in question are either 
the possessors of some deadly weapons, as stings or poison 
fangs, or they are uneatable, and are thus so disagree- 
able to the usual enemies of their kind that they are never 
attacked when their peculiar powers or properties are known. 
It is, therefore, important that they should not be mis- 
taken for defenceless or eatable species of the same class or 
order, since in that case they might suffer injury, or even death, 
before their enemies discovered the danger or the uselessness 
of the attack. They require some signal or danger -flag 
which shall serve as a warning to would-be enemies not to 
attack them, arid they have usually obtained this in the 
form of conspicuous or brilliant coloration, very distinct 
from the protective tints of the defenceless animals allied to 


The Skunk as illustrating Warning Coloration. 

While staying a few days, in July 1887, at the Summit 
Hotel on the Central Pacific .Railway, I strolled out one evening 
after dinner, and on the road, not fifty yards from the house, 
I saw a pretty little white and black animal with a bushy tail 
coming towards me. As it came on at a slow pace and with- 
out any fear, although it evidently saw me, I thought at first 
that it must bo some tame creature, when it suddenly occurred 
to me that it was a skunk, it came on till within five or six 
yards of me, then quietly climbed over a dwarf wall and dis- 
appeared under a small outhouse, in search of chickens, as the 
landlord afterwards told me. This animal possesses, as is well 
known, a most offensive secretion, which it has the power of 
ejecting over its enemies, and which effectually protects it 
from attack. The odour of this substance is so penetrating 
that it taints, and renders useless, everything it touches, 
or in its vicinity. Provisions near it become uneatable, and 
clothes saturated with it will retain the smell for several 
weeks, even though they are repeatedly washed and dried. 
A drop of the liquid in the eyes will cause blindness, and 
Indians are said not unfrequently to lose their sight from this 
cause. Owing to this remarkable power of offence the skunk 
is rarely attacked by other animals, and its black and white 
fur, and the bushy white tail carried erect when disturbed, 
form the danger-signals by which it is easily distinguished in 
the twilight or moonlight from unprotected animals. Its 
consciousness that it needs only to be seen to be avoided gives 
it that slowness of motion and fearlessness of aspect which 
are, as we shall see, characteristic of most creatures so pro- 

Warning Colours among Insects. 

It is among insects that warning colours arc best developed, 
and most abundant. We all know how well marked and 
conspicuous arc the colours and forms of the stinging wasps 
and bees, no one of which in any part of the world is known 
to be protectively coloured like the majority of defenceless 
insects. Most of the great tribe of Malacoderms among 
beetles are distasteful to insect-eating animals. Our red and 


black Telephoriclge, commonly called "soldiers and sailors/' 
were found, by Mr. Jenncr Weir, to be refused by small 
birds. These and the allied Lampyridae (the fire -flies and 
glow-worms) in Nicaragua, were rejected by Mr. Belt's tame 
monkey and by his fowls, though most other insects were 
greedily eaten by them. The Coccinellidee or lady-birds are 
another uneatable group, and their conspicuous and singularly 
spotted bodies serve to distinguish them at a glance /from all 
other beetles. 

These uneatable insects are probably more numerous than 
is supposed, although we already know immense numbers 
that are so protected. The most remarkable are the three 
families of butterflies Heliconida3, Danaidas, and Acvandm 
comprising more than a thousand species, and characteristic re- 
spectively of the three great tropical regions South America, 
Southern Asia, and Africa. All these butterflies have 
peculiarities which serve to distinguish them from every 
other group in their respective regions. They all have ample 
but rather weak wings, and fly slowly ; they are always very 
abundant ; and they all have conspicuous colours or markings, 
so distinct from those of other families that, in conjunction 
with their peculiar outline and mode of flight, they can 
usually be recognised at a glance. Other distinctive features 
are, that their colours are always nearly the same on the 
under surface of their wings as on the upper ; they never try 
to conceal themselves, but rest on the upper surfaces of 
leaves or flowers ; and, lastly, they all have juices which 
exhale a powerful scent, so that when one kills them by 
pinching the body, the liquid that exudes stains the fingers 
yellow, and leaves an odour that can only be removed by 
repeated washings. 

Now, there is much direct evidence to show that this 
odour, though not very offensive to us, is so to most insect- 
eating creatures. Mr. Bates observed that, when set out to 
dry, specimens of Heliconidse were less subject to the attacks 
of vermin ; while both he and I noticed that they were not 
attacked by insect-eating birds or dragonflies, and that their 
wings were not found in the forest paths among the numerous 
wings of other butterflies whose bodies had been devoured. 
Mr. Belt once observed a pair of birds capturing insects for 


their young ; and although the Heliconidoe swarmed in the 
vicinity, and from their slow flight could have been easily 
caught, not one was ever pursued, although other butterflies 
did not escape. His tame monkey also, which would greedily 
munch up other butterflies, would never eat the Heliconicla>,. 
It would sometimes smell them, but always rolled them up in 
its hand and then dropped them. 

AVe have also some corresponding evidence as to the 
distastef illness of the Eastern Danaidrc. The Hon. Mr. 
Justice Newton, who assiduously collected and took notes 
upon the Lepidoptera of Bombay, informed Mr. Butler of the 
British Museum that the large and swift- flying butterfly 
Charaxes psaphon, was continually persecuted by the bulbul, 
so that he rarely caught a specimen of this species which had 
not a piece snipped out of the hind wings. He offered one to 
a bulbul which he had in a cage, and it was greedily devoured, 
whilst it was only by repeated persecution that he succeeded 
in inducing the bird to touch a Danais. 1 

Besides these three families of butterflies, there are certain 
groups of the great genus Papilio the true swallow -tailed 
butterflies which have all the characteristics of uneatable 
insects. They have a special coloration, usually red and 
black (at least in the females), they fly slowly, they are very 
abundant, and they possess a peculiar odour somewhat like 
that of the Helicoiii(Le. One of these groups is common in 
tropical America, another in tropical Asia., and it is curious 
that, although not very closely allied, they have each the same 
red and black colours, and are very distinct from all the other 
butterflies of their respective countries. There is reason to 
believe also that many of the brilliantly coloured and weak- 
flying diurnal moths, like the fine tropical Agaristidai and 
burnct- moths, are similarly protected, and that their con- 
spicuous colours serve as a warning of inedibility. The 
common burnet-moth (Anthrocera filipendula) and the equally 
conspicuous ragwort-moth (Euchelia jacobeaa) have been proved 
to bo distasteful to insect-eating creatures. 

1 Nature, vol. iii. p. 165. Professor Meldola observed that specimens of 
Danais and Eupla?a in collections were less subject to the attacks of mites 
(Proc. Knt. tfoe., 1877, p. xii.) ; and this was corroborated by Mr. .Tenner Weir. 
Entomologist, 1882, vol. xv. p. 100. 


The most interesting and most conclusive example of 
warning -coloration is, however, furnished by caterpillars, 
because in this case the facts have been carefully ascertained 
experimentally by competent observers. In the year 1866, 
when Mr. Darwin was collecting evidence as to the supposed 
effect of sexual selection in bringing about the brilliant 
coloration of the higher animals, he was struck by the fact 
that many caterpillars have brilliant and conspicuous colours, 
in the production of which sexual selection could have no 
place. We have numbers of such caterpillars in this country, 
arid they are characterised not only by their gay colours but 
by not concealing themselves. Such are the mullein and the 
gooseberry caterpillars, the larva? of the spurge hawk-moth, of 
the buff- tip, and many others. Some of these caterpillars are 
wonderfully conspicuous, as in the case of that noticed by 
Mr. Bates in South America, which was four inches long, 
banded across with black and yellow, and with bright red 
head, legs, and tail. Hence it caught the eye of any one who 
passed by, even at the distance of many yards. 

Mr. Darwin asked me to try and suggest some explanation 
of this coloration ; and, having been recently interested in 
the question of the warning coloration of butterflies, J 
suggested that this was probably a similar case, that these 
conspicuous caterpillars were distasteful to birds and other 
insect-eating creatures, and that their bright non- protective 
colours and habit of exposing themselves to view, enabled 
their enemies to distinguish them at a glance from the edible 
kinds and thus learn riot to touch them ; for it must bo 
remembered that the bodies of caterpillars while growing 
are so delicate, that a wound from a bird's beak would bo 
perhaps as fatal as if they were devoured. 1 At this time not 
a single experiment or observation had been made on the 
subject, but after I had brought the matter before the 
Entomological Society, two gentlemen, who kept birds and 
other tame animals, undertook to make experiments with a 
variety of caterpillars. 

Mr. Jenncr Weir was the first to experiment with ten 
species of small birds in his aviary, and he found that none of 
them would eat the following smooth-skinned conspicuous cater- 

1 See Darwin's Descent of Man, p. 325. 


pillars Abraxas grossulariata, Diloba cspruleocephala, An- 
throcera filipendula, and Cucullia verbasci. He also found that 
they would not touch any hairy or spiny larva?, and he was 
satisfied that it was not the hairs or the spines, but the un- 
pleasant taste that caused them to be rejected, because in one 
case a young smooth larva of a hairy species, and in another 
case the pupa of a spiny larva, were equally rejected. On 
the other hand, all green or brown caterpillars as well as 
those that resemble twigs were greedily devoured. 1 

Mr. A. G. Butler also made experiments with some green 
lizards (Lacerta viridis), which greedily ate all kinds of food, 
including flies of many kinds, spiders, bees, butterflies, and 
green caterpillars ; but they would not touch the caterpillnr of 
the gooseberry-moth (Abraxas grossulariata), or the imago of 
the burnet-moth (Anthroccra filipendula). The same thing 
happened with frogs. When the gooseberry caterpillars 
were first given to them, " they sprang forward and licked 
them eagerly into their mouths ; no sooner, however, had 
they done so, than they seemed to become aware of the 
mistake that they had made, and sat with gaping mouths, 
rolling their tongues about, until they had got quit of the 
nauseous morsels, which seemed perfectly uninjured, and 
walked off as briskly as ever." Spiders seemed equally to 
dislike them. This and another conspicuous caterpillar 
(Halia wavaria) were rejected by two species the geometrical 
garden spider (Plpeira diadema) and a hunting spider. 2 

Homo further experiments with lizards were made by 
Professor Wcisrnann, quite confirming the previous observa- 
tions ; and in 1886 Mr. K. B. Foulton of Oxford undertook 
a considerable series of experiments, with many other species of 
larvae and fresh kinds of lizards and frogs. Mr. Poulton then 
reviewed the whole subject, incorporating all recorded facts, as 
well as some additional observations made by Mr. .Tenner Weir 
in 1886. More than a hundred species of larvae or of perfect 
insects of various orders have now been made the subject of 
experiment, and the results completely confirm my original 
suggestion. In almost every case the protectively coloured 
larvae have been greedily eaten by all kinds of insectivorous 

1 Transactions of the Entomological Society of London, 1869, p. 21. 
- Ibid., p. 27. 


animals, while, in the immense majority of cases, the con- 
spicuous, hairy, or brightly coloured larva? have been rejected 
by some or all of them. In some instances the inedibility of 
the larva', extends to the perfect insect, but not in others. In 
the former cases the perfect insect is usually adorned with 
conspicuous colours, as the burnet and ragwort moths ; but 
in the case of the buff-tip, the moth resembles a broken piece 
of rotten stick, yet it is partly inedible, being refused by 
lizards. It is, however, very doubtful whether these are its 
chief enemies, and its protective form and colour may be 
needed against insectivorous birds or mammals. 

Mr. Samuel II. Scudder, who has largely bred North 
American butterflies, has found so many of the eggs and larvue 
destroyed by hymenopterous and dipterous parasites that he 
thinks at least nine-tenths, perhaps a greater proportion, never 
reach maturity. Yet he has never found any evidence that 
such parasites attack either the egg or the larva of the inedible 
Danais archippus, so that in this case the insect is distasteful 
to its most dangerous foes in all the stages of its existence, 
a fact which serves to explain its great abundance and its 
extension over almost the whole world. 1 

One case has been found of a protectively coloured larva, 
one, moreover, which in all its habits shows that it 
trusts to concealment to escape its enemies which was yet 
always rejected by lizards after they had seized it, evidently 
under the impression that from its colour it would be 
eatable. This is the caterpillar of the very common moth 
Mania typica ; and Mr. Poulton thinks that, in this case, the 
unpleasant taste is an incidental result of some physiological 
processes in the organism, and is itself a merely useless 
character. It is evident that the insect would not conceal 
itself so carefully as it docs if it had not some enemies, and 
these are probably birds or small mammals, as its food-plants 
are said to be dock and willow-herb, not suggestive of places 
frequented by lizards ; and it has been found by experiment 
that lizards and birds have not always the same likes and 
dislikes. The case is interesting, because it shows that 
nauseous fluids sometimes occur sporadically, and may thus be 
intensified by natural selection when required for the purpose 
1 Nature, vol. iii. p. 147. 


of protection. Another exceptional case is that of the very 
conspicuous caterpillar of the spurge hawk-moth (Deilephila 
euphorbia?), which was at once eaten by a lizard, although, as 
it exposes itself on its food-plant in the daytime and is very 
abundant in some localities, it must almost certainly be disliked 
by birds or by some animals who would otherwise devour it. 
If disturbed while feeding it is said to turn round with fury 
and eject a quantity of green liquid, of an acid and disagree- 
able smell similar to that of the spurge milk, only worse. 1 

These facts, and Mr. Poulton's evidence that some larva) 
rejected by lizards at first will be eaten if the lizards are very 
hungry, show that there are differences in the amount of the 
distastefulness, arid render it probable that if other food were 
wanting many of these conspicuous insects would be eaten. 
It is the abundance of the eatable kinds that gives value to 
the inedibility of the smaller number ; and this is probably 
the reason why so many insects rely on protective colouring 
rather than on the acquisition of any kind of defensive 
weapons. In the long run the powers of attack and defence 
must balance each other. Hence we see that even the power- 
ful stings of bees and wasps only protect them against some 
enemies, since a tribe of birds, the bee -eaters, have been 
developed which feed upon them, and some frogs and lizards 
do so occasionally 

The preceding outline will sufficiently explain the character- 
istics of " warning coloration " and the end it serves in nature. 
There are many other curious modifications of it, but these will 
be best appreciated after we have discussed the remarkable 
phenomenon of "mimicry," which is bound up with and 
altogether depends upon " warning colour," and is in some 
cases the chief indication we have of the possession of some 
offensive weapon to secure the safety of the species imitated. 


This term has been given to a form of protective resem- 
blance, in which one species so closely resembles another in 
external form and colouring as to be mistaken for it, although 
the two may not be really allied and often belong to distinct 

1 Stainton's Manual of Butterflies and Moths, vol. i. p. 93 ; E. B. 
Poulton, Proceedings of the Zool Soc. of London, 1887, pp. 191-274. 


families or orders. One creature seems disguised in order to 
be made like another; hence the terms "mimic" and mimicry, 
which imply no voluntary action on the part of the imitator. 
It has long been known that such resemblances do occur, as, for 
example, the clear-winged moths of the families Sesiida) and 
^Egeriida}, many of which resemble bees, wasps, ichneumons, 
or saw-flies, and have received names expressive of the re- 
semblance ; and the parasitic flies (Volucella) which closely 
resemble bees, on whose larva) the larva) of the flies feed. 

The great bulk of such cases remained, however, unnoticed, 
and the subject was looked upon as one of the inexplicable 
curiosities of nature, till Mr. Bates studied the phenomenon 
among the butterflies of the Amazon, and, on his return home, 
gave the first rational explanation of it. 1 The facts are, briefly, 
these. Everywhere in that fertile region for the entomologist 
the brilliantly coloured Hcliconidae abound, with all the char- 
acteristics which I have already referred to when describing 
them as illustrative of " warning coloration." But along 
with them other butterflies were occasionally captured, which, 
though often mistaken for them, on account of their close 
resemblance in form, colour, and mode of flight, were found 
on examination to belong to a very distinct family, the Pierida). 
Mr. Bates notices fifteen distinct species of Pierida), belonging 
to the genera Leptalis and Euterpe, each of which closely imitates 
some one species of Heliconidie, inhabiting the same region and 
frequenting the same localities. It must be remembered that 
the two families are altogether distinct in structure. The 
larva) of the Heliconidtc are tuberclcd or spined, the pupa) sus- 
pended head downwards, and the imago has imperfect fore- 
legs in the male ; while the larva) of the Pierida) are smooth, 
the pupa3 are suspended with a brace to keep the head erect, 
and the forefeet are fully developed in both sexes. These 
differences are as large and as important as those between pigs 
and sheep, or between swallows and sparrows ; while English 
entomologists will best understand the case by supposing that 
a species of Pieris in this country was coloured and shaped 
like a small tortoise-shell, while another species on the Con- 
tinent was equally like a Camberwell beauty so like in both 

1 See Transactions of the Linnean Society, vol. xxiii. pp. 495-566, coloured 
plate 5. 




cases as to be mistaken when on the wing, and the difference 
only to be detected by close examination. As an example of 
the resemblance, woodcuts are given of one pair in which the 
colours are simple, being olive, yellow, and black, while the 

FIG. 23. Methona psidii (Helicoiiidae). Leptalia orise (Pieridft). 

very distinct neuration of the wings and form of the head and 
body can be easily seen. 

Besides these Pieridse, Mr. Bates found four true Papilios, 
seven Erycinidae, three Castnias (a genus of day -fly ing moths), 
and fourteen species of diurnal Bombycidse, all imitating some 
species of Heliconidse which inhabited the same district; and 
it is to be especially noted that none of these insects were so 
abundant as the Heliconidse they resembled, generally they 



wore far less common, so that Mr. Bates estimated the pro- 
portion in some cases as not one to a thousand. Before 
giving an account of the numerous remarkable cases of mimicry 
in other parts of the world, and between various groups 
of inserts and of higher animals, it will be well to explain 
briefly the use and purport of the phenomenon, and also the 
mode by which it has been brought about. 

How Mimicry lias been Produced. 

The fact has been now established that the Heliconidne 
possess an offensive odour and taste, which lead to their 
being almost entirely free from attack by insectivorous 
creatures ; they possess a peculiar form and mode of flight, 
and do not seek concealment; while their colours although 
very varied, ranging from dee]) blue-black, with white, yellow, 
or vivid red bands and spots, to the most delicate semi trans- 
parent wings adorned with pale brown or yellow nun-kings 
arc yet always very distinctive, and unlike those of all the 
other families of butterflies in the same country. It is, 
therefore, clear that if any other butterflies in the same 
region, which are eatable and sutler great persecution from 
insectivorous animals, should come to resemble any of these 
uneatable species so closely as to be mistaken for them by 
their enemies, they will obtain thereby immunity from per- 
secution. This is the obvious and sufficient reason why the 
imitation is useful, and therefore why it occurs in nature. We 
have now to explain how it has probably been brought about, 
and also why a still larger number of persecuted groups have 
not availed themselves of this simple means of protection. 

From the great abundance of the HelicoirkUe 1 all over 
tropical America, the vast number of their genera and species, 
and their marked distinctions from all other butterflies, it 
follows that they constitute a group of high antiquity, which 
in the course of ages has become more and more specialised, 
and owing to its peculiar advantages has now become a 
dominant and aggressive race. But when they first arose 
from some ancestral species or group which, owing to the food 

1 These Butterflies are now divided into two sub-families, one of which is 
placed with the Danaida? ; but to avoid confusion 1 shall always speak of the 
American genera under the old term Heliconidie. 


of the larvae or some other cause, possessed disagreeable 
juices that caused them to be disliked by the usual enemies 
of their kind, they were in all probability not very different 
either in form or coloration from many other butterflies. They 
would at that time be subject to repeated attacks by insect- 
eaters, and, even if finally rejected, would often receive a 
fatal injury. Hence arose the necessity for some distinguish- 
ing mark, by which the dcvourers of butterflies in general 
might learn that these particular butterflies were uneatable ; 
and every variation leading to such distinction, whether by 
form, colour, or mode of flight, was preserved and accumulated 
by natural selection, till the ancestral Heliconoids became well 
distinguished from eatable butterflies, and thenceforth com- 
paratively free from persecution. Then they had a good 
time of it. They acquired lazy habits, and flew about slowly. 
They increased abundantly and spread all over the country, 
their larva) feeding on many plants and acquiring different 
habits ; while the butterflies themselves varied greatly, and 
colour being useful rather than injurious to them, gradually 
diverged into the many coloured and beautifully varied forms 
we now behold. 

But, during the early stages of this process, some of the 
Pierida 1 , inhabiting the same district, happened to be sufficiently 
like some of the Heliconidie to be occasionally mistaken for 
them. These, of course, survived while their companions were 
devoured. Those among their descendants that were still more 
like Ileliconida3 again survived, and at length the imitation 
would become tolerably perfect. Thereafter, as the protected 
group diverged into distinct species of many different colours, 
the imitative group would occasionally be able to follow it 
with similar variations, a process that is going on now, for 
Mr. Bates informs us that in each fresh district he visited he 
found closely allied representative species or varieties of 
Heliconidte, and along with them species of Leptalis 
(Pieridai), which had varied in the same way so as still to be 
exact imitations. But this process of imitation would be 
subject to check by the increasing acuteness of birds and other 
animals which, whenever the eatable Leptalis became numerous, 
would surely find them out, and would then probably attack 
both these and their friends the Heliconidae in order to devour 


the former and reject the latter. The Pierida) would, however, 
usually be less numerous, because their larvae are often pro- 
tectively coloured and therefore edible, while the larva) of the 
Heliconidre are adorned with warning colours, spines, or 
tubercles, and are uneatable. It seems probable that the 
larvse and pupae of the Heliconidre were the first to acquire 
the protective distastefulness, both because in this stage they 
are more defenceless and more liable to fatal injury, arid also 
because we now find many instances in which the Iarva3 are 
distasteful while the perfect insects are eatable, but I believe 
none in which the reverse is the case. The larva) of the 
Pieridas arc now beginning to acquire offensive juices, but 
have not yet obtained the corresponding conspicuous colours ; 
while the perfect insects remain eatable, except perhaps in 
some Eastern groups, the under sides of whose wings are 
brilliantly coloured although this is the part which is exposed 
when at rest. 

It is clear that if a large majority of the larvrc of Lepido- 
ptera, as well as the perfect insects, acquired these distaste- 
ful properties, so as seriously to diminish the food supply of 
insectivorous and nestling birds, these latter would be forced 
by necessity to acquire corresponding tastes, arid to eat with 
pleasure what some of them now eat only under pressure of 
hunger ; and variation and natural selection would soon bring 
about this change. 

Many writers have denied the possibility of such wonderful 
resemblances being produced by the accumulation of fortuitous 
variations, but if the reader will call to mi rid the large amount 
of variability that has been shown to exist in all organisms, 
the exceptional power of rapid increase possessed by insects, and 
the tremendous struggle for existence always going on, the 
difficulty will vanish, especially when we remember that 
nature has the same fundamental groundwork to act upon in 
the two groups, general similarity of forms, wings of similar 
texture and outline, and probably some original similarity of 
colour and marking. Yet there is evidently considerable 
difficulty in the process, or with these great resources at her 
command nature would have produced more of these mimicking 
forms than she has done. One reason of this deficiency prob- 
ably is, that the imitators, being always fewer in number, have 


not been able to keep pace with the variations of the much 
more numerous imitated form ; another reason may be the 
ever -increasing acuteness of the enemies, which have again 
and again detected the imposture and exterminated the 
feeble race before it has had time to become further modified. 
The result of this growing acuteness of enemies has been, 
that those mimics that now survive exhibit, as Mr. Bates well 
remarks, "a palpably intentional likeness that is perfectly 
staggering," and also " that those features of the portrait are 
most attended to by nature which produce the most effective 
deception when the insects are seen in nature." No one, in 
fact, can understand the perfection of the imitation who has 
not seen these species in their native wilds. So complete 
is it in general effect that in almost every box of butterflies, 
brought from tropical America by amateurs, are to be found 
some species of the mimicking Pierida}, Erycinida?, or moths, 
and the mimicked Heliconidse, placed together under tho 
impression that they are tho same species. Yet more ex- 
traordinary, it sometimes deceives the very insects themselves. 
Mr. Trimcn states that tho male Danais chrysippus is some- 
times deceived by the female Diadema bolina which mimics 
that species. Dr. Fritz Muller, writing from Brazil to Professor 
Meldola, says, " One of tho most interesting of our mimick- 
ing butterflies is Leptalis melite. Tho female alone of this 
species imitates one of our common white Pierida?, which she 
copies so well that even her own male is often deceived ; for 
I have repeatedly seen the male pursuing the mimicked 
species, till, after closely approaching and becoming aware of 
his error, he suddenly returned." 1 This is evidently not a 
case of true mimicry, since tho species imitated is riot pro- 
tected ; but it may be that tho less abundant Leptalis is able 
to mingle with the female Pierida) and thus obtain partial 
immunity from attack. Mr. Kirby of the insect department 
of the British Museum informs me that there are several 
species of South American Pieridae which the female Leptalis 
melite very nearly resembles. The case, however, is interest- 
ing as showing that the butterflies are themselves deceived by 
a resemblance which is not so great as that of some mimicking 

1 R. Meldola in Ann. and Mag. of Nat. Hist., Feb. 1878, p. 158. 


Other Examples of Mimicry among Lepidoptera. 

In tropical Asia, and eastward to the Pacific Islands, the 
Danaidae take the place of the Heliconidae of America, in their 
abundance, their conspicuousness, their slow flight, and their 
being the subjects of mimicry. They exist under three 
principal forms or genera. The genus Euploea is the most 
abundant both in species arid individuals, and consists of fine 
broad-winged butterflies of a glossy or metallic blue-black 
colour, adorned with pure white, or rich blue, or dusky mark- 
ings situated round the margins of the wings. Danais has 
generally more lengthened wings, of a semitransparcnt greenish 
or a rich brown colour, with radial or marginal pale spots ; 
while the fine Hestias are of enormous size, of a papery or 
semitransparcnt white colour, with dusky or black spots and 
markings. Each of these groups is mimicked by various 
species of the genus Papilio, usually with such accuracy that 
it is impossible to distinguish them on the wing. 1 Several 
species of Diadema, a genus of butterflies allied to our 
Vanessas, also mimic species of Danais, but in this case the 
females only are affected, a subject which will be discussed in 
another chapter. 

Another protected group in the Eastern tropics is that of 
the beautiful day-flying moths forming the family Agaristidue. 
These are usually adorned with the most brilliant colours or 
conspicuous markings, they fly slowly in forests among the 
butterflies and other diurnal insects, and their great abundance 
sufficiently indicates their possession of some distasteful ness 
which saves them from attack. Under these conditions we 
may expect to find other moths which are not so protected 
imitating them, and this is the case. One of the common and 
wide-ranging species (Opthalmis lincea), found in the islands 
from Amboyna to New Ireland, is mimicked in a wonderful 
manner by one of the LiparidtB (the family to which our 
common " tussock " and " vapourer " moths belong). This is 
a new species collected at Amboyna during the voyage of the 
Challenger, and has been named Artaxa simulans. Both 

i See Trans. Linn. Soc., vol. xxv. Wallace, on Variation of Malayan 
Papilionida; ; and, Wallace's Contributions to Natural Selection, chaps, iii. and 
iv., where full details are given. 


insects are black, with the apex of the fore wings ochre coloured, 
and the outer half of the hind wings bright orange. The 
accompanying woodcuts (for the use of which I am indebted 
to Mr. John Murray of the Challenger Office) well exhibit their 
striking resemblance to each other. 

FIG. 24. Optlialinis lincca (Agaristidac). Artaxa siinulans (Liparidae). 

In Africa exactly similar phenomena recur, species of Papilio 
arid of Diadema mimicking Danaidae or Acraeidae with the 
most curious accuracy. Mr. Trimen, who studied this subject 
in South Africa, has recorded eight species or varieties of 
Diadema, and eight of Papilio, which each mimic some 
species of Danais ; while eight species or varieties of Panopaea 
(another genus of Nymphalidaa), three of Melanitis (Eury- 
telidae), and two of Papilio, resemble with equal accuracy 
some species of Acraea. 1 Ho has also independently observed 
the main facts on which the explanation of the phenomenon 
rests, the unpleasant odour of the Danais and Acraea, extend- 
ing to their larvae and pupae; their great abundance, slow 
flight, and disregard of concealment ; and he states that while 
lizards, mantidae, and dragonflies all hunt butterflies, and the 
rejected wings are to be found abundantly at some of their 

1 See Trans. Linn. Soc. 9 vol. xxvi., with two coloured plates illustrating 
cases of mimicry. 


feeding-places, those of the two genera Danais and Acraea 
were never among them. 

The two groups of the great genus Papilio (the true swallow- 
tailed butterflies) which have been already referred to as 
having the special characteristics of uneatable insects, have also 
their imitators in other groups ; and thus, the belief in 
their inedibility derived mainly from their style of warning 
coloration and their peculiar habits is confirmed. In South 
America, several species of the " JEneas " group of these 
butterflies are mimicked by Pieridse and by day-flying moths 
of the genera Castnia and Pericopis. In the East, Papilio 
hector, P. diphilus, and P. liris, all belonging to the inedible 
group, are mimicked by the females of other species of Papilio 
belonging to very distinct groups ; while in Northern India 
and China, many fine day -flying moths (Epicopeia) have ac- 
quired the strange forms and peculiar colours of some of the 
large inedible Papilios of the same regions. 

In North America, the large and handsome Danais 
archippus, with rich reddish-brown wings, is very common ; 
and it is closely imitated by Limenitis misippus, a butterfly 
allied to our " white admiral," but which has acquired a colour 
quite distinct from that of the great bulk of its allies. In 
the same country there is a still more interesting case. The 
beautiful dark bronzy green butterfly, Papilio philenor, is 
inedible both in larva and perfect insect, and it is mimicked 
by the equally dark Limenitis Ursula. There is also in the 
Southern and Western States a dark female form of the yellow 
Papilio turnus, which in all probability obtains protection from 
its general resemblance to P. philenor. Mr. W. H. Edwards 
has found, by extensive experiment, that both the dark and 
yellow females produce their own kinds, with very few excep- 
tions ; and he thinks that the dark form has the advantage in 
the more open regions and in the prairies, where insectivorous 
birds abound. But in open country the dark form would 
be quite as conspicuous as the yellow form, if not more so, 
so that the resemblance to an inedible species would be there 
more needed. 1 

The only probable case of mimicry in this country is that 
of the moth, Diaphora mendica, whose female only is white, 
1 Edwards's Butterflies of North America, second series, part vi. 


while the larva is of protective colours, and therefore almost 
certainly edible. A much more abundant moth, of about the 
same size and appearing about the same time, is Spilosoma 
menthrasti, also white, but in this case both it and its larva 
have been proved to be inedible. The white colour of the 
female Diaphora, although it must be very conspicuous at 
night, may, therefore, have been acquired in order to re- 
semble the uneatable Spilosoma, and thus gain some pro- 
tection. 1 

Mimicry among Protected ( Uneatable) Genera. 

Before giving some account of the numerous other cases 
of warning colours and of mimicry that occur in the animal 
kingdom, it will be well to notice a curious phenomenon 
which long puzzled entomologists, but which has at length 
received a satisfactory explanation. 

We have hitherto considered, that mimicry could only occur 
when a comparatively scarce and much persecuted species 
obtained protection by its close external resemblance to a 
much more abundant uneatable species inhabiting its own dis- 
trict; and this rule undoubtedly prevails among the great 
majority of mimicking species all over the world. But Mr. 
Bates also found a number of pairs of species of different genera 
of Heliconidue, which resembled each other quite as closely as 
did the other mimicking species he has described ; and since 
all these insects appear to bo equally protected by their in- 
edibility, and to be equally free from persecution, it was not 
easy to see why this curious resemblance existed, or how it 
had been brought about. That it is not due to close affinity 
is shown by the fact that the resemblance occurs most fre- 
quently between the two distinct sub-families into which (as 
Mr. Bates first pointed out) the Heliconida3 are naturally 
divided on account of very important structural differences. 
One of these sub-families (the true Heliconinae) consists of two 
genera only, Heliconius and Eueides, the other (the Danaoid 
Heliconinse) of no less than sixteen genera ; and, in the in- 
stances of mimicry we are now discussing, one of the pairs or 

1 Professor Meltlola informs me that he has recorded another case of 
mimicry among British moths, in which Acidalia subsericata imitates Asthena 
candidata. See Ent, Mo. J\far/. t vol. iv. p. 163. 


triplets that resemble each other is usually a species of the la^cre 
and handsome genus Heliconius, the others being speci es V f 
the genera Mechanitis, Melinsea, or Tithorea, though s.j evcra i 
species of other Danaoid genera also imitate each other. The 
following lists will give some idea of the number j fj f these 
curious imitative forms, and of their presence in over^ p ltr t O f 
the Neotropical area. The bracketed species are th t p se that 
resemble each other so closely that the difference is iK t, p er _ 
ceptible when they are on the wing. 

In the Lower Amazon region are found 

( Heliconius sylvana. 
\ Melinrca cgina. 

f Ileliconius numata. 
-[ Melimea mnemc. 
^ Tithorea harmonia. 
( Methona psidii. 
I Thyridia ino. 

( Ccratina ninonia. 
( Meliiuwa mnasias. 

In Central America are found 

C Heliconius zuleika. 
Nicaragua ! Mclincca hezia. 
^Mechanitis sp. 

(Helicon ins formosus. 
Tithorea peiitluas. 


( Heliconius telcliina. 
| Melinsea imitata. 

In the Upper Amazon region 

Heliconius pardalinus. 
Meliuaia pardalis. 

( Heliconius aurora. 
( Melinaja lucifer. 

In New Grenada 

f Heliconius ismenius, 
\ Melinaea messatis. 
( Heliconius messene. 
-[ Melinaea mesenina. 
((?) Mechanitis sp. 
( Heliconius hecalcsia. 
f Tithorea hecalesina. 
( Heliconius hecuba. 
j Tithorea bonplandi. 




In Eastern Peru and Bolivia- 

( Helieonins aristoua. 
] Meliiiiua eydippe. 
\ (?) Mcchauitis mothone. 

In Pernambuco 

Hcliconius ethra. 
Median itis nesit-a. 

In Kio Janeiro 

Helieonins eucratc. 
Mechauitis lysimnia. 

In South Brazil 

( Thyridia mcgisto. 
\ Ituna ilioiie. 

A erica thalia. 

Eueides pavana. 

Besides these, a number of species of Ithomia and Napco 
genes, and of Napeogenes and Mechanitis, resemble each other 
Avith ecjual accuracy, so that they are liable to be mistaken 

PIG. 25. Wings of Ituna Ilione, 

Wings of Thyridia megisto, 

for each other when on the wing ; and no doubt many other 
equally remarkable cases are yet unnoticed. 

The figures above of the fore an'd hind wings of two of these 
mimicking species, from Dr. Fritz Miiller's original paper in 
Kosmos, will serve to show the considerable amount of 


difference, in the important character of the neuration of the 
wings, between these butterflies, which really belong to very 
distinct and not at all closely allied genera. Other important 
characters are (1) The existence of a small basal cell in the 
hind wings of Itima which is wanting in Thyridia ; (2) the 
division of the cell between the veins Ib and 2 of the 
hind wings in the former genus, while it is undivided in the 
latter ; and (3) the existence in Thyridia of scent-producing 
tufts of hair on the upper edge of the hind wing, while in 
Ituna these are wanting ; but in place of them are extensible 
processes at the end of the abdomen, also emitting a powerful 
scent. These differences characterise two marked subdivisions 
of the Danaoid Heliconinse, each containing several distinct 
genera; and these subdivisions are further distinguished by very 
different forms of larvae, that to which Ituna belongs having 
from two to four long threadlike tentacles on the back, while 
in that containing Thyridia these are always absent. The 
former usually feed on Asclepiadese, the latter on Solanace t f e 
or Scrophulariaceae. 

The two species figured, though belonging to such distinct 
and even remote genera, have acquired almost identical tints 
and markings so as to be deceptively alike. The surface of 
the wings is, in both, transparent yellowish, with black trans- 
verse bands and white marginal spots, while both have similar 
black- arid white -marked bodies and long yellow antennae. 
Dr. Miiller states that they both show a preference for the 
same flowers growing on the edges of the forest paths. 1 

We will now proceed to give the explanation of these 
curious similarities, which have remained a complete puzzle 
for twenty years. Mr. Bates, when first describing them, 
suggested that they might be due to some form of parallel 
variation dependent on climatic influences ; and I myself 
adduced other cases of coincident local modifications of 
colour, which did not appear to be explicable by any form 
of mimicry. 2 But we neither of us hit upon the simple 
explanation given by Dr. Fritz Miiller in 1879. 

His theory is founded on the assumed, but probable, 

1 From Professor Meldola's translation of Dr. F. Mailer's paper, in Proc. 
Ent. Soc. Lond., 1879, p. xx. 

2 Island Life, p. 265. 


fact, that insect-eating birds only learn by experience to 
distinguish the edible from the inedible butterflies, and in 
doing so necessarily sacrifice a certain number of the latter. 
The quantity of insectivorous birds in tropical America is 
enormous ; and the number of young birds which every year 
have to learn wisdom by experience, as regards the species of 
butterflies to be caught or to be avoided, is so great that the 
sacrifice of life of the inedible species must be considerable, 
arid, to a comparatively weak or scarce species, of vital im- 
portance. The number thus sacrificed will be fixed by the 
quantity of young birds, and by the number of experiences 
requisite to cause them to avoid the inedible species for the 
future, and not at all by the numbers of individuals of which 
each species consists. Hence, if two species are so much 
alike as to be mistaken for one another, the fixed number 
annually sacrificed by inexperienced birds will be divided be- 
tween them, and both will benefit. But if the two species are 
very unequal in numbers, the benefit will be comparatively 
slight for the more abundant species, but very great for the rare 
one. To the latter it may make all the difference between 
safety and destruction. 

To give a rough numerical example. Let us suppose that 
in a given limited district there are two species of Heliconidae, 
one consisting of only 1000, the other of 100,000 individuals, 
and that the quota required annually in the same district for 
the instruction of young insectivorous birds is 500. By the 
larger species this loss will be hardly felt ; to the smaller it 
will mean the most dreadful persecution resulting in a 
loss of half the total population. But, let the two species 
become superficially alike, so that the birds see no difference 
between them. The quota of 500 will now be taken from a 
combined population of 101,000 butterflies, and if propor- 
tionate numbers of each suffer, then the weak species will 
only lose five individuals instead of 500 as it did before. 
Now we know that the different species of Heliconidge are 
not equally abundant, some being quite rare ; so that the 
benefit to be derived in these latter cases would be very im- 
portant. A slight inferiority in rapidity of flight or in powers 
of eluding attack might also be a cause of danger to an in- 
edible species of scanty numbers, and in this case too the being 


merged in another much more abundant species, by similarity 
of external appearance, would be an advantage. 

The question of fact remains. Do young birds pursue and 
capture these distasteful butterflies till they have learned by 
bitter experience what species to avoid? On this point Dr. 
Miiller has fortunately been able to obtain some direct evi- 
dence, by capturing several Acrseas and Helicoriida^ which had 
evidently been seized by birds but had afterwards escaped, as 
they had pieces torn out of the wing, sometimes symmetri- 
cally out of both wings, showing that the insect had been 
seized when at rest and with the two pairs of wings in contact. 
There is, however, a general impression that this knowledge is 
hereditary, and does not need to be acquired by young birds ; 
in support of which view Mr. Jenner Weir states that his birds 
always disregarded inedible caterpillars. When, day by day, 
he threw into his aviary various larva*, those which were 
edible were eaten immediately, those which were inedible 
were no more noticed than if a pebble had been thrown 
before the birds. 

The cases, however, arc not strictly comparable. The 
birds were not young birds of the first year ; and, what 
is more important, edible larvae have a comparatively simple 
coloration, being always brown or green and smooth. Uneat- 
able larvae, on the other hand, comprise all that are of conspicu- 
ous colours and arc hairy or spiny. But with butterflies there 
is no such simplicity of contrast. The eatable butterflies com- 
prise not only brown or white species, but hundreds of 
Nymphalida?, Papilionidse, Lycujnidre, etc., which are gaily 
coloured and of an immense variety of patterns. The colours 
and patterns of the inedible kinds are also greatly varied, 
while they are often equally gay; and it is quite impossible 
to suppose that any amount of instinct or inherited habit 
(if such a thing exists) could enable young insectivorous 
birds to distinguish all the species of one kind from all 
those of the other. There is also some evidence to show 
that animals do learn by experience what to eat and what 
to avoid. Mr. Poulton was assured by Rev. G. J. Bursch 
that very young chickens peck at insects which they after- 
wards avoid. Lizards, too, often seized larva) which they were 
unable to eat and ultimately rejected. 


Although the Heliconidre present, on the whole, many 
varieties of coloration and pattern, yet, in proportion to the 
number of distinct species in each district, the types of 
coloration are few and very well marked, and thus it becomes 
easier for a bird or other animal to learn that all belonging to 
such types are uneatable. This must be a decided advantage to 
the family in question, because, not only do fewer individuals 
of each species need to be sacrificed in order that their enemies 
may learn the lesson of their inedibility, but they are more 
easily recognised at a distance, and thus escape even pursuit. 
There is thus a kind of mimicry between closely allied species 
as well as between species of distinct genera, all tending to the 
same beneficial end. This may be seen in the four or five 
distinct species of the genus Heliconius which all have the same 
peculiar type of coloration a yellow band across the upper 
wings and radiating red stripes on the lower, and are all found 
in the same forests of the Lower Amazon ; in the numerous 
very similar species of Ithomia with transparent wings, found 
in every locality of the same region ; and in the very numerous 
species of Papilio of the " ./Eneas " group, all having a similar 
style of marking, the resemblance being especially close in 
the females. The very uniform type of colouring of the 
blue -black Euplams and of the fulvous Acraeas is of the 
same character. 1 In all these cases the similarity of the 
allied species is so great, that, when they are on the wing 
at some distance oft', it is difficult to distinguish one species 
from another. But this close external resemblance is not 
always a sign of very near affinity ; for minute examination 
detects differences in the form and scalloping of the wings, in 
the markings on the body, and in those on the under surface 
of the wings, which do not usually characterise the closest allies. 
It is to be further noted, that the presence of groups of very 
similar species of the same genus, in one locality, is not at all 
a common phenomenon among unprotected groups. Usually 
the species of a genus found in one locality are each well 
marked and belong to somewhat distinct types, while the 

1 Tins extension of the theory of mimicry was pointed out by Professor 
Meldola in the paper already referred to ; and he has answered the objections 
to Dr. F. Muller's theory with great force in the Annals and May. of Nat. 
Hist., 1882, p. 417. 


closely allied forms those that require minute examination 
to discriminate them as distinct species are most generally 
found in separate areas, and are what are termed representative 

The extension we have now given to the theory of mimicry 
is important, since it enables us to explain a much wider 
range of colour phenomena than those which were first im- 
puted to mimicry. It is in the richest butterfly region in the 
world the Amazon valley that we find the most abundant 
evidence of the three distinct sets of facts, all depending on 
the same general principle. The form of mimicry first 
elucidated by Mr. Bates is characterised by the presence in 
each locality of certain butterflies, or other insects, themselves 
edible and belonging to edible groups, which derived protec- 
tion from having acquired a deceptive resemblance to some 
of the inedible butterflies in the same localities, which latter 
were believed to be wholly free from the attacks of in- 
sectivorous birds. Then came the extension of the principle, 
by Dr. F. Miiller, to the case of species of distinct genera 
of the inedible butterflies resembling each other quite as 
closely as in the former cases, and like them always found 
in the same localities. They derive mutual benefit from 
becoming, in appearance, one species, from which a certain 
toll is taken annually to teach the young insectivorous birds 
that they are uneatable. Even when the two or more species 
are approximately equal in numbers, they each derive a 
considerable benefit from thus combining their forces ; but 
when one of the species is scarce or verging on extinction, the 
benefit becomes exceedingly great, being, in fact, exactly appor- 
tioned to the need of the species. 

The third extension of the same principle explains the 
grouping of allied species of the same genera of inedible 
butterflies into sets, each having a distinct type of coloration, 
and each consisting of a number of species which can hardly 
be distinguished on the wing. This must be useful exactly 
in the same way as in the last case, since it divides the 
inevitable toll to insectivorous birds and other animals 
among a number of species. It also explains the fact of the 
great similarity of many species of inedible insects in the 
same locality a similarity which does not obtain to anything 


like the same extent among the edible species. The explana- 
tion of the various phenomena of resemblance and mimicry, 
presented by the distasteful butterflies, may now be considered 
tolerably complete. 

Mimicry in other Orders of Insects. 

A very brief sketch of these phenomena will be given, 
chiefly to show that the same principle prevails throughout 
nature, and that, wherever a rather extensive group is 
protected, either by distastefulness or offensive weapons, 
there are usually some species of edible arid inoffensive 
groups that gain protection by imitating them. It has been 
already stated that the Telephorida?, Lampyrida 1 , and other 
families of soft -winged beetles, are distasteful; and as they 
abound in all parts of the world, and especially in the tropics, 
it is not surprising that insects of many other groups should 
imitate them. This is especially the case with the Ion gi corn 
beetles, which are much persecuted by insectivorous birds ; and 
everywhere in tropical regions some of these are to be found 
so completely disguised as to be mistaken for species of the 
protected groups. Numbers of these imitations have been 
already recorded by Mr. Bates and myself, but I will hero 
refer to a few others. 

In the recently published volumes on the Longicorn and 
Malacoderm beetles of Central America l there are numbers of 
beautifully coloured figures of the new species ; and on looking 
over them we are struck by the curious resemblance of some 
of the Longicorns to species of the Malacoderm group. In 
some cases we discover perfect mimics, and on turning to the 
descriptions we always find these pairs to come from the 
same locality. Thus the Otheostethus melanurus, one of the 
Prionida?-, imitates the malacodcrm, Lucidota discolor, in 
form, peculiar coloration, and size, and both are found at 
Chontales in Nicaragua, the species mimicked having, how- 
ever, as is usual, a wider range. The curious and very rare 
little longicorn, Tethlimmena aliena, quite unlike its nearest 
allies in the same country, is an exact copy on a somewhat 
smaller scale of a malacoderm, Lygistopterus amabilis, both 

1 Godinan and Salvin's liiulogia Ceatrali- Americana, Insectii, 
vol. iii. part ii , and vol. v. 


found at Chontales. The pretty longicorn, Callia albicornis, 
closely resembles two species of malacoderms (Silis chaly- 
beiperinis and Colyphus signaticollis), all being small beetles 
with red head and thorax and bright blue elytra, and all 
three have been found at Panama. Many other species of 
Callia also resemble other malacoderms ; and the longicorn 
genus Lycidola has been named from its resemblance to 
various species of the Lycidrc, one of the species here figured 
(Lycidola belti) being a good mimic of Calopteron corrugatum 
and of several other allied species, all being of about the same 
size arid found at Chontales. In these cases, and in most 
others, the longicorn beetles have lost the general form and 
aspect of their allies to take on the appearance of a distinct 
tribe. Some other groups of beetles, as the Elateridso and 
Eucnemida?, also deceptively mimic malacoderms. 

Wasps and bees are often closely imitated by insects of 
other orders. Many longicorn beetles in the tropics exactly 
mimic wasps, bees, or ants. In Borneo a largo black wasp, 
whose wings have a broad white patch near the apex (Myg- 
nimia aviculus), is closely imitated by a heteromerous beetle 
(Coloborhombus fasciatipennis), which, contrary to the general 
habit of beetles, keeps its wings expanded in order to show 
the white patch on their apex, the wing-coverts being reduced 
to small oval scales, as shown in the figure. This is a most 
remarkable instance of mimicry, because the beetle has had to 
acquire so many characters which are unknown among its allies 
(except in another species from Java) the expanded wings, 
the white band on them, and the oval scale -like elytra. 1 
Another remarkable case has been noted by Mr. Neville 
Goodman, in Egypt, where a common hornet (Vespa orientalis) 
is exactly imitated in colour, size, shape, attitude when at 
rest, and mode of flight, by a beetle of the genus Laphria. 2 

The tiger -beetles (Ciciridclidae) are also the subjects of 
mimicry by more harmless insects. In the Malay Islands I 
found a heteromerous beetle which exactly resembled a 
Therates, both being found running on the trunks of trees. 
A longicorn (Collyrodes Lacordairei) mimics Collyris, another 
genus of the same family ; while in the Philippine Islands 

1 Trans. Ent. Snc , 1885, p. 369. 
2 jproc. Cambridge Phil. >S'oc., vol. iii. part ii., 1877. 




there is a cricket (Condylodeira tricondyloides), which so 
closely resembles a tiger-beetle of the genus Tricondyla 

Fio. 26. Mygnirnia aviculus (Wasp). Coloborhomlms fascia! ipennis (Beetle). 

that the experienced entomologist, Professor Westwood, at first 
placed it in his cabinet among those beetles. 




One of the characters by which some beetles are protected 
is excessive hardness of the elytra and integuments. Several 
genera of weevils (Curculionidoe) are thus saved from attack, 
and these are often mimicked by species of softer and more 

* k 


FIG. 27. 

a. Doliops sp. (Longicorn) mimics Pachyrhynchus orbifa>, (7j) (a hard curculio). 

c. Doliops curculionoides mimics (d) Pachyrhynohus sp. 

e. Scepastus pachyrhynchoides (a grasshopper) mimics (/) Apocyrtus sp. (a hard 


g. Doliops sp mimics (7i) Pachyrhynchus sp. 
L Phoraspis (grasshopper) mimics (k) a Coccinella. 

All the above are from the Philippines. The exact correspondence of the colours 
of the insects themselves renders the mimicry much moie complete in nature than it 
appears in the above figures. 

eatable groups. In South America, the genus Heilipus is one 
of these hard groups, and both Mr. Bates and M. "Roolofs, 
a Belgian entomologist, have noticed that species of other 
genera exactly mimic them. So, in the Philippines, there 


is a group of Curculionidoe, forming the genus Pachyrhynchus, 
in which all the species are adorned with the most brilliant 
metallic colours, banded and spotted in a curious manner, 
and are very smooth and hard. Other genera of Curculionida3 
(Desmidophorus, Alcides), which are usually very differently 
coloured, have species in the Philippines which mimic the 
Pachyrhynchi ; and there are also several longicorn beetles 
(Aprophata, Doliops, Acronia, and Agnia), which also mimic 
them. Besides these, there are some longicorns and cetonias 
which reproduce the same colours arid markings ; and there 
is even a cricket (Scepastus pachyrhynchoides), which has 
taken on the form and peculiar coloration of these beetles 
in order to escape from enemies, which then avoid them as 
uneatable. 1 The figures on the opposite page exhibit several 
other examples of these mimicking insects. 

Innumerable other cases of mimicry occur among tropical 
insects ; but we must now pass on to consider a few of the 
very remarkable, but much rarer instances, that are found 
among the higher animals. 

Mimicry among the F'ertebrata. 

Perhaps the most remarkable cases yet known are those of 
certain harmless snakes which mimic poisonous species. The 
genus Elaps, in tropical America, consists of poisonous snakes 
which do not belong to the viper family (in which are included 
the rattlesnakes and most of those which are poisonous), and 
which do riot possess the broad triangular head which charac- 
terises the latter. They have a peculiar stylo of coloration, 
consisting of alternate rings of red and black, or red, black, 
and yellow, of different widths and grouped in various ways 
in the different species ; and it is a style of coloration which 
does not occur in any other group of snakes in the world. 
But in the same regions are found three genera of harmless 
snakes, belonging to other families, some few species of which 
mimic the poisonous Elaps, often so exactly that it is with 
difficulty one can bo distinguished from the other. Thus 
Elaps fulvius in Guatemala is imitated by the harmless Plio- 
cerus equalis ; Elaps corallinus in Mexico is mimicked by the 

1 Compte- ftendu de la, Soci&tt Entomologique de Bdgaue> series ii., No. 59, 



harmless Homalocranium semicinctum ; and Elaps lemniscatus 
in Brazil is copied by Oxyrhopus trigeminus ; while in other 
parts of South America similar cases of mimicry occur, some- 
times two harmless species imitating the same poisonous 

A few other instances of mimicry in this group have been 
recorded. There is in South Africa an egg -eating snake 
(Dasypeltis scaber), which has neither fangs nor teeth, yet it is 
very like the Berg adder (Clothos atropos), and when alarmed 
renders itself still more like by flattening out its head and 
darting forward with a hiss as if to strike a foe. 1 Dr. A. B. 
Meyer has also discovered that, while some species of the 
genus Callophis (belonging to the same family as the American 
Elaps) have large poison fangs, other species of the same genus 
have none ; and that one of the latter (C. gracilis) resembles 
a poisonous species (C. intestinalis) so closely, that only an 
exact comparison will discover the difference of colour and 
marking. A similar kind of resemblance is said to exist 
between another harmless snake, Megserophis flaviceps, and 
the poisonous Callophis bivirgatus ; and in both these cases 
the harmless snake is less abundant than the poisonous one, 
as occurs in all examples of true mimicry. 2 

In the genus Elaps, above referred to, the very peculiar 
style of colour and marking is evidently a " warning colour " 
for the purpose of indicating to snake-eating birds and mam- 
mals that these species are poisonous ; and this throws light on 
the long -disputed question of the use of the rattle of the 
rattlesnake. This reptile is really both sluggish and timid, 
and is very easily captured by those who know its habits. If 
gently tapped on the head with a stick, it will coil itself up 
and lie still, only raising its tail and rattling. It may then 
be easily caught. This shows that the rattle is a warning to 
its enemies that it is dangerous to proceed to extremities ; 
and the creature has probably acquired this structure and 
habit because it frequents open or rocky districts where 
protective colour is needful to save it from being pounced 
upon by buzzards or other snake -eaters. Quite parallel 
in function is the expanded hood of the Indian cobra, a 

1 Nature, vol. xxxiv. p. 547. 
2 Proceedings oftheZool. Soc. qf London, 1870, p. 369. 


poisonous snake which belongs also to the Elapida?. This 
is, no doubt, a warning to its foes, not an attempt to 
terrify its prey ; and the hood has been acquired, as in the 
case of the rattlesnake, because, protective coloration being 
on the whole useful, some mark was required to distinguish 
it from other protectively coloured, but harmless, snakes. 
Both these species feed on active creatures capable of escaping 
if their enemy were visible at a moderate distance. 

Mimicry among Birds. 

The varied forms and habits of birds do not favour the 
production among them of the phenomena of warning colours 
or of mimicry ; and the extreme development of their instincts 
and reasoning powers, as well as their activity and their 
power of flight, usually afford them other means of evading 
their enemies. Yet there are a few imperfect, arid one or 
two very perfect cases of true mimicry to be found among 
them. The less perfect examples are those presented by 
several species of cuckoos, an exceedingly weak and de- 
fenceless group of birds. Our own cuckoo is, in colour and 
markings, very like a sparrow-hawk. In the East, several 
of the small black cuckoos closely resemble the aggressive 
drorigo- shrikes of the same country, and the small metallic 
cuckoos are like glossy starlings ; while a large ground- 
cuckoo of Borneo (Carpococcyx radiatus) resembles one of 
the fine pheasants (Euplocamus) of the same country, both in 
form and in its rich metallic colours. 

More perfect cases of mimicry occur between some of the 
dull-coloured orioles in the Malay Archipelago and a genus of 
large honey -suckers the Tropidorhynchi or "Friar -birds." 
These latter are powerful and noisy birds which go in small 
flocks. They have long, curved, and sharp beaks, and power- 
ful grasping claws ; and they are quite able to defend them- 
selves, often driving away crows and hawks which venture to 
approach them too nearly. The orioles, on the other hand, 
are weak and timid birds, and trust chiefly to concealment 
and to their retiring habits to escape persecution. In each 
of the great islands of the Austro-Malayan region there is a 
distinct species of Tropidorhynchus, and there is always along 
with it an oriole that exactly mimics it. All the Tropidorhynchi 


have a patch of bare black skin round the eyes, and a ruff 
of curious pale recurved feathers on the nape, whence their 
name of Friar-birds, the ruff being supposed to resemble the 
cowl of a friar. These peculiarities are imitated in the orioles 
by patches of feathers of corresponding colours ; while the dif- 
ferent tints of the two species in each island are exactly the 
same. Thus in Bouru both are earthy brown ; in Ceram they are 
both washed with yellow ochre ; in Timor the under surface 
is pale and the throat nearly white, and Mr. H. 0. Forbes has 
recently discovered another pair in the island of Timor Laut. 
The close resemblance of these several pairs of birds, of widely 
different families, is quite comparable with that of many of the 
insects already described. It is so close that the preserved 
specimens have even deceived naturalists \ for, in the great 
French work, Voyage de I' Astrolabe, the oriole of Bouru is 
actually described and figured as a honey-sucker ; and Mr. 
Forbes tells us that, when his birds were submitted to Dr. 
Sclater for description, the oriole and the honey-sucker were, 
previous to close examination, considered to be the same 

Objections to the Theory of Mimicry. 

To set forth adequately the varied and surprising facts of 
mimicry would need a large and copiously illustrated volume; 
and no more interesting subject could be taken up by a 
naturalist who has access to our great collections and can de- 
vote the necessary time to search out the many examples of 
mimicry that lie hidden in our museums. The brief sketch of 
the subject that has been here given will, however, serve to 
indicate its nature, and to show the weakness of the objections 
that were at first made to it. It was urged that the action 
of "like conditions/' with "accidental resemblances " and 
" reversion to ancestral typos," would account for the facts. If, 
however, we consider the actual phenomena as here set forth, 
and the very constant conditions under which they occur, we 
shall see how utterly inadequate are these causes, either 
singly or combined. These constant conditions are 

1. That the imitative species occur in the same area and 

occupy the very same station as the imitated. 

2. That the imitators are always the more defenceless. 


3. That the imitators are always less numerous in in- 


4. That the imitators differ from the bulk of their allies. 

5. That the imitation, however minute, is external and 

visible only, never extending to internal characters or 
to such as do not affect the external appearance. 

These five characteristic features of mimicry show us that 
it is really an exceptional form of protective resemblance. 
Different species in the same group of organisms may obtain 
protection in different ways : some by a general resemblance 
to their environment ; some by more exactly imitating the 
objects that surround them bark, or leaf, or flower ; while 
others again gain an equal protection by resembling some 
species which, from whatever cause, is almost as free from 
attack as if it were a leaf or a flower. This immunity may 
depend on its being uneatable, or dangerous, or merely strong ; 
and it is the resemblance to such creatures for the purpose 
of sharing in their safety that constitutes mimicry. 

Concluding Jlenwrks on Warning Colours and Mimicry. 

Colours which have been acquired for the purpose of serv- 
ing as a warning of inedibility, or of the possession of 
dangerous offensive weapons, are probably more numerous than 
have been hitherto supposed ; and, if so, we shall be able to 
explain a considerable amount of colour in nature for which 
no use has hitherto been conjectured. The brilliant and 
varied colours of sea-anemones and of many coral animals 
will probably come under this head, since wo know that 
many of them possess the power of ejecting stinging threads 
from various parts of their bodies which render them quite 
uneatable to most animals. Mr. Gosso describes how, on 
putting an Anthea into a tank containing a half -grown 
bullhead (Cottus bubalis) which had not been fed for some 
time, the fish opened his mouth and sucked in the morsel, 
but instantly shot it out again. He then seized it a second 
time, and after rolling it about in his mouth for a moment 
shot it out again, and then darted away to hide himself 
in a hole. Some tropical fishes, however, of the genera 
Tetrodon, Pseudoscarus, Astracion, and a few others, seem 


to have acquired the power of feeding on corals and medusae ; 
and the beautiful bands and spots and bright colours with 
which they are frequently adorned, may be either protective 
when feeding in the submarine coral groves, or may, in some 
cases, be warning colours to show that they themselves are 
poisonous and uneatable. 

A remarkable illustration of the wide extension of warning 
colours, and their very definite purpose in nature, is afforded 
by what may now be termed " Mr. Belt's frog." Frogs in all 
parts of the world are, usually, protectively coloured with 
greens or browns ; and the little tree-frogs are either green 
like the leaves they rest upon, or curiously mottled to imitate 
bark or dead leaves. But there are a certain number of very 
gaily coloured frogs, and these do not conceal themselves as 
frogs usually do. Such was the small toad found by Darwin 
at Bahia Blanca, which was intense black and bright vermilion, 
and crawled about in the sunshine over dry sand-hills and 
arid plains. And in Nicaragua, Mr. Belt found a little frog 
gorgeously dressed in a livery of red and blue, which did 
not attempt concealment and was very abundant, a combina- 
tion of characters which convinced him that it was uneatable. 
He, therefore, took a few specimens home with him and 
gave them to his fowls and ducks, but none would touch 
them. At last, by throwing down pieces ot meat, for 
which there was a great competition among the poultry, 
he managed to entice a young duck into snatching up 
one of the little frogs. Instead of swallowing it, however, 
the duck instantly threw it out of its mouth, and went 
about jerking its head as if trying to get rid of some un- 
pleasant taste. 1 

The power of predicting what will happen in a given case 
is always considered to be a crucial test of a true theory, 
and if so, the theory of warning colours, and with it that of 
mimicry, must be held to be well established. Among the 
creatures which probably have warning colours as a sign of 
inedibility are, the brilliantly coloured nudibranchiato molluscs, 
those curious annelids the Nereis arid the Aphrodite or sea- 
mouse, and many other marine animals. The brilliant colours 
of the scallops (Pecten)and some other bivalve shells are perhaps 
1 The Natwalist in Nicaragua, p. 321. 


an indication of their hardness and consequent inedibility, as 
in the case of the hard beetles ; and it is not improbable that 
some of the phosphorescent fishes and other marine organisms 
may, like the glow- worm, hold out their lamp as a warning to 
enemies. 1 In Queensland there is an exceedingly poisonous 
spider, whose bite will kill a dog, and cause severe illness with 
excruciating pain in man. It is black, with a bright vermilion 
patch on the middle of the body ; and it is so well recognised 
by this conspicuous coloration that even the spider-hunting 
wasps avoid it. 2 

Locusts and grasshoppers are generally of green protective 
tints, but there are many tropical species most gaudily 
decorated with red, blue, and black colours. On the same 
general grounds as those by which Mr. Belt predicted the in- 
edibility of his conspicuous frog, we might safely predict the 
same for these insects ; but wo have fortunately a proof that 
they are so protected, since Mr. Charles Home states that 
one of the bright coloured Indian locusts was invariably 
rejected when offered to birds and lizards. 3 

The examples now given lead us to the conclusion that 
colours acquired for the purpose of serving as a danger-signal 
to enemies are very widespread in nature, and, with the 
corresponding colours of the species which mimic them, 
furnish us with a rational explanation of a considerable 
portion of the coloration of animals which is outside the 
limits of those colours that have been acquired for either 
protection or recognition. There remains, however, another 
set of colours, chiefly among the higher animals, which, being 
connected with some of the most interesting and most 
disputed questions in natural history, must be discussed in a 
separate chapter. 

1 Mr. Belt first suggested this use of the light of the Lampyridrc (fireflies 
and glow-worms) Naturalist in Nicaragua, p. 320. Mr. Verrill and 
Professor Meldola made the same suggestion in the case of modus and other 
phosphorescent marine organisms (Nature, vol. xxx. pp. 281, 289). 

2 \V. E. Armit, in Nature, vol. xviii. p. 642. 
8 Proc. Ent. Sec., 1869, p. xiii. 



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 
plumes 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. 

IN the preceding chapters we have dealt chiefly with the 
coloration of animals as distinctive of the several species; 
and we have seen that, in an enormous number of cases, the 
colours can be shown to have a definite purpose, and to be 
useful either as a means of protection or concealment, of 
warning to enemies, or of recognition by their own kind. We 
have now to consider a subordinate but very widespread 
phenomenon the differences of colour or of ornamental 
appendages in the two sexes. These differences are found to 
have special relations with the three classes of coloration 
above referred to, in many cases confirming the explanation 
already given of their purport and use, arid furnishing us with 
important aid in formulating a general theory of animal 

In comparing the colours of the two sexes we find a perfect 
gradation, from absolute identity of colour up to such extreme 
difference that it is difficult to believe that the two forms 
can belong to the same species ; and this diversity in the 


colours of the sexes does not bear any constant relation 
to affinity or systematic position. In both insects and birds 
we find examples of complete identity and extreme diversity 
of the sexes ; and these differences occur sometimes in the 
same tribe or family, and sometimes even in the same 

It is only among the higher and more active animals that 
sexual differences of colour acquire any prominence. In the 
mollusca the two sexes, when separated, arc always alike in 
colour, and only very rarely present slight differences in the 
form of the shell. In the extensive group of Crustacea the 
two sexes as a rule are identical in colour, though there are 
often differences in the form of the prehensile organs ; but in 
a very few cases there are differences of colour also. Thus, in 
a Brazilian species of shore-crab (Gelasimus) the female is 
grayish-brown, while in the male the posterior part of the 
cephalo- thorax is pure white, with the anterior part of a rich 
green. This colour is only acquired by the males when they 
become mature, and is liable to rapid change in a few 
minutes to dusky tints. 1 In some of the fresh-water fleas 
(Daphnoidse) the males are ornamented with red and blue 
spots, while in others similar colours occur in both sexes. In 
spiders also, though as a rule the two sexes are alike in colour, 
there are a few exceptions, the males being ornamented with 
brilliant colours on the abdomen, while the female is dull 

Sexual Coloration in Insects. 

It is only when we come to the winged insects that we find 
any large amount of peculiarity in sexual coloration, and 
even here it is only developed in certain orders. Flies (Dip- 
tera), field-bugs (Hemiptera), cicadas (Homoptera), and the 
grasshoppers, locusts, and crickets (Orthoptera) present very 
few and unimportant sexual differences of colour ; but the last 
two groups have special musical organs very fully developed 
in the males of some of the species, and these no doubt enable 
the sexes to discover and recognise each other. In some cases, 
however, when the female is protectively coloured, as in the 
well-known leaf -insects already referred to (p. 207), the male 

1 Darwin's Descent of Afar?, p. 271. 


is smaller and much less protectively formed and c6loured. 
In the bees and wasps (Hymenoptera) it is also the rule that 
the sexes are alike in colour, though there are several cases 
among solitary bees where they differ ; the female being 
black, and the male brown in Anthophora rctusa, while in 
Andrajna fulva the female is more brightly coloured than the 
male. Of the great order of beetles (Coleoptera) the same 
thing may be said. Though often so rich and varied in their 
colours the sexes are usually alike, and Mr. Darwin was only 
able to find about a dozen cases in which there was any con- 
spicuous difference between them. 1 They exhibit, however, 
numerous sexual characters, in the length of the antennae, and 
in horns, legs, or jaws remarkably enlarged or curiously modi- 
fied in the male sex. 

It is in the family of dragonflies (order Neuroptera) that 
we first meet with numerous cases of distinctive sexual 
coloration. In some of the Agrionidae the males have the 
bodies rich blue and the wings black, while the females have 
the bodies green and the wings transparent. In the North 
American genus Heta3rina the males alone have a carmine 
spot at the base of each wing ; but in some other genera the 
sexes hardly differ at all. 

The great order of Lepidoptera, including the butterflies and 
moths, affords us the most numerous and striking examples of 
diversity of sexual colouring. Among the moths the differ- 
ence is usually but slight, being manifested in a greater inten- 
sity of the colour of the smaller winged male ; but in a few 
cases there is a decided difference, as in the ghost -moth 
(Hepialus humuli), in which the male is pure white, while the 
female is yellow with darker markings. This may be a 
recognition colour, enabling the female more readily to discover 
her mate ; and this view receives some support from the fact 
that in the Shetland Islands the male is almost as yellow as 
the female, since it has been suggested that at midsummer, 
when this moth appears, there is in that high latitude sufficient 
twilight all night to render any special coloration unneces- 
sary. 2 

Butterflies present us with a wonderful amount of sexual 

1 Darwin's Descent of Man, p. 294, and footnote. 
8 Nature, 1871, p. 489. 


differ en co of colour, in many cases so remarkable that the two 
sexes of the same species remained for many years under 
different names and were thought to be quite distinct species. 
We find, however, every gradation from perfect identity to 
complete diversity, and in some cases we are able to sec a 
reason for this difference. Beginning with the most extra- 
ordinary cases of diversity as in Diadcma misippus, where the 
male is black, ornamented with a large white spot on each 
wing margined with rich changeable blue, while the female is 
orange -brown with black spots and stripes we find the 
explanation in, the fact that the female mimics an uneatable 
Danais, and thus gains protection while laying its eggs on low 
plants in company with that insect. In the allied species, 
Diadema bolina, the females are also very different from the 
males, but are of dusky brown tints, evidently protective and 
very variable, some specimens having a general resemblance 
to the uneatable Euplseas ; so that we see here some of the 
earlier stages of both forms of protection. The remarkable 
differences in some South American Pieridse are similarly 
explained. The males of Pieris pyrrha, P. lorena, and 
several others, are white with a few black hands and marginal 
spots like so many of their allies, while the females are 
gaily coloured with yellow and brown, and exactly resemble 
some species of the uneatable Heliconidaa of the same 
district. Similarly, in the Malay Archipelago, the female 
of Diadema anomala is glossy metallic blue, while the 
male is brown; the reason for this reversal of the usual 
rule being, that the female exactly mimics the brilliant 
colouring of the common and uneatable Euplsea midamus, 
and thus secures protection. In the fine Adolias dirtea, the 
male is black with a few specks of ochre-yellow and a broad 
marginal band of rich metallic greenish-blue, while the female 
is brownish-black entirely covered with rows of ochre-yellow 
spots. This latter coloration does not appear to be protective 
when the insect is seen in the cabinet, but it really is so. 
I have observed the female of this butterfly in Sumatra, where 
it settles on the ground in the forest, and its yellow spots 
so harmonise with the flickering gleams of sunlight on the 
dead leaves that it can only bo detected with the greatest 


A hundred other cases might be quoted in which the 
female is either more obscurely coloured than the male, or 
gains protection by imitating some inedible species ; and any 
one who has watched these female insects flying slowly 
along in search of the plants on which to deposit their 
eggs, will understand how important it must be to them 
not to attract the attention of insect- eating birds by too 
conspicuous colours. The number of birds which capture 
insects on the wing is much greater in tropical regions 
than in Europe ; and this is perhaps the reason why many 
of our showy species are alike, or almost alike, in both 
sexes, while they are protectively coloured on the under side 
which is exposed to view when they are at rest. Such are 
our peacock, tortoise-shell, arid red admiral butterflies ; while 
in the tropics we more commonly find that the females are 
less conspicuous on the upper surface even when protectively 
coloured beneath. 

We may here remark, that the cases already quoted prove 
clearly that either male or female may be modified in colour 
apart from the opposite sex. In Pieris pyrrha and its allies 
the male retains the usual type of coloration of the whole 
genus, while the female has acquired a distinct and peculiar 
stylo of colouring. In Adolias dirtea, on the other hand, 
the female appears to retain something like the primitive 
colour and markings of the two sexes, modified perhaps for 
more perfect protection ; while the male has acquired more and 
more intense and brilliant colours, only showing his original 
markings by the few small yellow spots that remain near the 
base of the wings. In the more gaily coloured Pieridse, of 
which our orange-tip butterfly may be taken as a type, we see in 
the female the plain ancestral colours of the group, while the 
male has acquired the brilliant orange tip to its wings, prob- 
ably as a recognition mark. 

In those species in which the under surface is protectively 
coloured, we often find the upper surface alike in both sexes, 
the tint of colour being usually more intense in the male. But 
in some cases this leads to the female being more conspicuous, 
as in some of the Lycsenidao, where the female is bright blue 
and the male of a blue so much deeper and soberer in tint as 
to appear the less brilliantly coloured of the two. 


Probable Causes of these Colours. 

In tho production of these varied results there have prob- 
ably beon several causes at work. There seems to be 
a constant tendency in the male of most animals but 
especially of birds and insects to develop more and more 
intensity of colo ^, often culminating in brilliant metallic blues 
or greens or the *^ost splendid iridescent hues ; while, at 
the same time, natural selection is constantly at work, pre- 
venting the female from acquiring these same tints, or 
modifying her colours in various directions to secure pro- 
tection by assimilating her to her surroundings, or by pro- 
ducing mimicry of some protected form. At the same 
time, the need for recognition must be satisfied ; and this 
seems to have led to diversities of colour in allied species, 
sometimes the female, sometimes the male undergoing 
the greatest change according as one or other could be 
modified with the greatest ease, and so as to interfere least 
with the welfare of the race. Hence it is that sometimes 
the males of allied species vary most, as in the different 
species of Epicalia; sometimes the females, as in the magnifi- 
cent green species of Ornithoptera and the " ^neas " group 
of Papilio. 

The importance of the two principles the need of pro- 
tection and recognition in modifying the comparative colora- 
tion of the sexes among butterflies, is beautifully illustrated 
in the case of the groups which are protected by their dis- 
tasteful ness, and whose females do riot, therefore, need the 
protection afforded by sober colours. 

In the great families, Ileliconidre and Acroeidoe, we find 
that the two sexes are almost always alike ; and, in the very 
few exceptions, that the female, though differently, is not less 
gaily or less conspicuously coloured. In the Danaidre the same 
general rule prevails, but the cases in which the male exhibits 
greater intensity of colour than the female are perhaps more 
numerous than in the other two families. There is, however, a 
curious difference in this respect betjveen the Oriental and 
the American groups of distasteful Papilios with warning 
colours, both of which are the subjects of mimicry. In the 
Eastern groups of which P. hector and P. coon may be token 



as typos the two sexes are nearly alike, the male being 
sometimes more intensely coloured and with fe i er pale 
markings ; hut in the American groups represented hy P. 
seneas, P. sesostris, and allies there is a wonderful diversity, 
the males having a rich green or bluish patch on the fo ^e wings, 
while the females have a band or spots of pure white, not 
always corresponding in position to the green spot of the 
males. There are, however, transitional forms, by which a 
complete series can be traced, from close similarity to great 
diversity of colouring between the sexes ; and this may perhaps 
be only an extreme example of the interiser colour and more 
concentrated markings which are a very prevalent character- 
istic of male butterflies. 

There are, in fact, many indications of a regular succession 
of tints in which colour development has occurred in the 
various groups of butterflies, from an original grayish or 
brownish neutral tint. Thus in the " yEneas " group of 
Papilios We have the patch on the upper wings yellowish in 
P. triopas, olivaceous in P. bolivar, bronzy-gray with a white 
spot in P. erlaces, more greenish and buff in P. iphidamas, 
gradually changing to the fine blue of P. brissonius, and the 
magnificent green of P. sesostris. In like manner, the intense 
crimson spots of the lower wings can be traced step by step 
from a yellow or buff tint, which is one of the most wide- 
spread colours in the whole order. The greater purity and 
intensity of colour seem to be usually associated with more 
pointed wings, indicating greater vigour and more rapid flight. 

Sexual Selection as a supposed Cause of Colour Development. 

Mr. Darwin, as is well known, imputed most of the 
brilliant colours and varied patterns of butterflies' wings to 
sexual selection that is, to a constant preference, by female 
butterflies, for the more brilliant males ; the colours thus 
produced being sometimes transmitted to the males alone, 
sometimes to both sexes. This view has always seemed to 
me to be unsupported by evidence, while it is also quite 
inadequate to account for the facts. The only direct evidence, 
as set forth with his usual fairness by Mr. Darwin himself, is 
opposed to his views. Several entomologists assured him 
that, in moths, the females evince not the least choice of their 


partners ; and Dr. Wallace of Colchester, who has largely bred 
the fine Bombyx cynthia, confirmed this statement. Among 
butterflies, several males often' pursue one female, and Mr. 
Darwin says, that, unless the female exerts a choice the 
pairing must be left to chance. But, surely, it may be the 
most vigorous or most persevering male that is chosen, not 
necessarily one more brightly or differently coloured, and 
this will be true " natural selection." Butterflies have been 
noticed to prefer some coloured flowers to others ; but that 
does not prove, or even render probable, any preference for 
the colour itself, but only for flowers of certain colours, on 
account of the more agreeable or more abundant nectar 
obtained from them. Dr. Schulte called Mr. Darwin's atten- 
tion to the fact, that in the Diadema bolina the brilliant blue 
colour surrounding the white spots is only visible when we 
look towards the insect's head, and this is true of many of 
the iridescent colours of butterflies, and probably depends 
upon the direction of the stria) on the scales. It is suggested, 
however, that this display of colour will bo seen by the 
female as the male is approaching her, and that it has been 
developed by sexual selection. 1 But in the majority of cases 
the males follow the female, hovering over her in a position 
which would render it almost impossible for her to see the 
particular colours or patterns on his upper surface ; to do so 
the female should mount higher than the male, and fly 
towards him being the seeker instead of the sought, and this 
is quite opposed to the actual facts. I cannot, therefore, 
think that this suggestion adds anything whatever to the 
evidence for sexual selection of colour by female butterflies. 
This question will, however, be again touched upon after we 
have considered the phenomena of sexual colour among the 

Semal Coloration of Birds. 

The general rule among vertebrates, as regards colour, is, 
for the two sexes to bo alike. This prevails, with only a few 
exceptions, in fishes, reptiles, and mammalia; but in birds 
diversity of sexual colouring is exceedingly frequent, and is, 
not improbably, present in a greater or less degree in more 

1 Darwin in Nature, 1880, p, 237. 


than half of the known species. It is this class, therefore, that 
will afford us the best materials for a discussion of the problem, 
and that may perhaps lead us to a satisfactory explanation of 
the causes to which sexual colour is due. 

The most fundamental characteristic of birds, from our 
present point of view, is a greater intensity of colour in the 
male. This is the case in hawks and falcons ; in many 
thrushes, warblers, and finches ; in pigeons, partridges, rails, 
plovers, and many others. When the plumage is highly 
protective or of dull uniform tints, as in many of the 
thrushes and warblers, the sexes are almost or quite identical 
in colour ; but when any rich markings or bright tints are 
acquired, they are almost always wanting or much fainter in 
the female, as we see in the black -cap among warblers, and 
the chaffinch among finches. 

It is in tropical regions, where from a variety of causes 
colour has been developed to its fullest extent, that we find 
the most remarkable examples of sexual divergence of colour. 
The most 'gorgeously coloured birds known are the birds 
of paradise, the chatterers, the tanagers, the humming-birds, 
and the pheasant -tribe, including the peacocks. In all these 
the females are much less brilliant, and, in the great majority 
of cases, exceptionally plain and dull coloured birds. Not 
only are the remarkable plumes, crests, and gorgets of the 
birds of paradise entirely wanting in the females, but these 
latter are usually without any bright colour at all, and rank no 
higher than our thrushes in ornamental plumage. Of the 
humming-birds the same may bo said, except that the females 
are often green, and sometimes slightly metallic, but from 
their small size and uniform tints are never conspicuous. 
The glorious blues and purples, the pure whites and intense 
crimsons of the male chatterers are represented in the females 
by olive- greens or dull browns, as are the infinitely varied 
tints of the male tanagers. And in pheasants, the splendour 
of plumage which characterises the males is entirely absent 
in the females, which, though often ornamental, have always 
comparatively sober and protective tints. The same thing 
occurs with many other groups. In the Eastern tropics 
are many brilliant birds belonging to the families of the 
warblers, flycatchers, shrikes, etc., but the female is always 


much less brilliant than the male and often quite dull 

Cause of Dull Colours of Female Birds. 

The reason of this phenomenon is not difficult to find, if 
we consider the essential conditions of a bird's existence, and 
the most important function it has to fulfil. In order 
that the species may be continued, young birds must be pro- 
duced, and the female birds have to sit assiduously on their 
eggs. While doing this they are exposed to observation and 
attack by the numerous devourers of eggs and birds, and it is 
of vital importance that they should be protectively coloured 
in all those parts of the body which are exposed during in- 
cubation. To secure this end all the bright colours and 
showy ornaments which decorate the male have not been 
acquired by the female, who often remains clothed in the 
sober hues which were probably once common to the whole 
order to which she belongs. The different amounts of colour 
acquired by the females have no doubt depended on 
peculiarities of habits and of environment, and on the 
powers of defence or of concealment possessed by the species. 
Mr. Darwin has taught us that natural selection cannot 
produce absolute, but only relative perfection; and as a 
protective colour is only one out of many means by which 
the female birds are able to provide for the safety of their 
young, those which are best endowed in other respects will 
have been allowed to acquire more colour than those with 
whom the struggle for existence is more severe. 

Relation of Sex Colour to Nesting Habits. 

This principle is strikingly illustrated by the existence of 
considerable numbers of birds in which both sexes are 
similarly and brilliantly coloured, in some cases as brilliantly 
as the males of many of the groups above referred to. Such 
are the extensive families of the kingfishers, the woodpeckers, 
the toucans, the parrots, the turacos, the hangnests, the 
starlings, and many other smaller groups, all the species of 
which are conspicuously or brilliantly coloured, while in all 
of them the females are cither coloured exactly like the males, 
or, when differently coloured, are equally conspicuous. When 


searching for some cause for this singular apparent exception 
to the rule of female protective colouring, I came upon a fact 
which beautifully explains it ; for in all these cases, without 
exception, the species either nests in holes in the ground or in 
trees, or builds a domed or covered nest, so as completely to 
conceal the sitting - bird. We have here a case exactly 
parallel to that of the butterflies protected by distasteful- 
ness, whoso females are either exactly like the males, or, if 
different, are equally conspicuous. We can hardly believe 
that so exact a parallel should exist between such remote 
classes of animals, except under the influence of a general 
law y and, in the need of protection by all defenceless animals, 
and especially by most female insects and birds, we have such 
a law, which has been proved to have influenced the colours 
of a considerable proportion of the animal kingdom. 1 

The general relation which exists between the mode of 
nesting and the coloration of the sexes in those groups of 
birds which need protection from enemies, may be thus 
expressed : When both sexes are brilliant or conspicuous, 
the nest is such as to conceal the sitting-bird ; but when the 
male is brightly coloured and the female sits exposed on the 
nest, she is always less brilliant and generally of quite sober 
and protective hues. 

It must be understood that the mode of nesting has in- 
fluenced the colour, not that the colour has determined the 
mode of nesting ; and this, I believe, has been generally, though 
not perhaps universally, the case. For we know that colour 
varies more rapidly, and can be more easily modified and 
fixed by selection, than any other character ; whereas habits, 
especially when connected with structure, and when they 
pervade a whole group, are much more persistent and more 
difficult to change, as shown by the habit of the dog turning 
round two or three times before lying down, believed to be 
that of the wild ancestral form which thus smoothed down 
the herbage so as to form a comfortable bed. We see, too, 
that the general mode of nesting is characteristic of whole 
families differing widely in size, form, and colours. Thus, all 
the kingfishers and their allies in every part of the world nest 

1 See the author's Contributions to Natural Selection, chap. vii. in which 
these facts were first brought forward. 


in holes, usually in banks, but sometimes in trees. The 
motmots and the puff-birds (Bucconidafi) build in similar 
places ; while the toucans, burbots, trogons, woodpeckers, and 
parrots all make their nests in hollow trees. This habit, 
pervading all the members of extensive families, must there- 
fore be extremely ancient, more especially as it evidently 
depends in some degree on the structure of the birds, the 
bills, and especially the feet, of all these groups being unfitted 
for the construction of woven arboreal nests. 1 But in all 
these families the colour varies greatly from species to species, 
being constant only in the one character of the similarity of 
the sexes, or, at all events, in their being equally conspicuous 
even though differently coloured. 

When I first put forward this view of the connection 
between the mode of nesting and the coloration of female 
birds, I expressed the law in somewhat different terms, which 
gave rise to some misunderstanding, and led to numerous 
criticisms and objections. Several cases were brought forward 
in which the females were far less brilliant than the males, 
although tho nest was covered. This is the caso with the 
Maluridje, or superb warblers of Australia, in which the males 
are very brilliant during the pairing season and the females 
quite plain, yet they build domed nests. Here, there can be 
little doubt, the covered nest is a protection from rain or from 
some special enemies to the eggs ; while the birds themselves 
are protectively coloured in both sexes, except for a short 
time during the breeding season when the male acquires 
brilliant colours ; and this is probably connected with the fact 
of their inhabiting the open plains and thin scrub of Australia, 
where protective colours are as generally advantageous as 
they arc in our north-temperate zones. 

As 1 have now stated the law, 1 do not think there are 
any exceptions to it, while there are an overwhelming number 
of cases which give it a strong support. It has been objected 
that tho domed nests of many birds are as conspicuous as tho 
birds themselves would be, and would, therefore, bo of no use 
as a protection to the birds and young. But, as a matter of 
fact, they do protect from attack, for hawks or crows do not 
pluck such nests to pieces, as in doing so they would be 

1 On this point sew the author's Cond'ifadioHs (o Natural Selection t chap. v. i. 


exposed to the attack of the whole colony ; whereas a hawk or 
falcon could carry off a sitting-bird or the young at a swoop, 
and entirely avoid attack. Moreover, each kind of covered 
nest is doubtless directed against the attacks of the most 
dangerous enemies of the species, the purse-like nests, often a 
yard long, suspended from the extremity of thin twigs, being 
useful against the attacks of snakes, which, if they attempted 
to enter them, would be easily made to lose their hold and 
fall to the ground. Such birds as jays, crows, magpies, 
hawks, and other birds of prey, have also been urged as an 
exception ; but these are all aggressive birds, able to protect 
themselves, and thus do not need any special protection for 
their females during nidification. Some birds which build in 
covered nests are comparatively dull coloured, like many of 
the weaver birds, but in others the colours are more showy, 
and in all the sexes are alike ; so that none of these are in any 
way opposed to the rule. The golden orioles have, however, 
been adduced as a decided exception, since the females are 
showy and build in an open nest. But even here the females 
are less brilliant than the males, and are sometimes greenish 
or olivaceous on the upper surface ; while they very carefully 
conceal their nests among dense foliage, and the male is 
sufficiently watchful and pugnacious to drive off most in- 

On the other hand, how remarkable it is that the only small 
and brightly coloured birds of our own country in which the 
male and female are alike the tits and starlings either 
build in holes or construct covered nests ; while the beautiful 
hangnests (Icterida?) of South America, which always build 
covered or purse-shaped nests, are equally showy in both 
sexes, in striking contrast with the chatterers and tanagers of 
the same country, whose females are invariably less conspicuous 
than the males. On a rough estimate, there are about 1200 
species of birds in the class of showy males and females, with 
concealed nidification; while there are probably, from an 
equally rough estimate, about the same number in the con- 
trasted class of showy males and dull females, with open 
nests. This will leave the great bulk of known birds in the 
classes of those which are more or less protectively coloured 
in both sexes ; or which, from their organisation and habits, do 


not require special protective coloration, such as many of 
the birds of prey, the larger waders, and the oceanic birds. 

There are a few very curious cases in which the female 
bird is actually more brilliant than the male, and which yet 
have open nests. Such are the dotterel (Eudromias morinel- 
lus), several species of phalarope, an Australian creeper 
(Climacteris erythropus), and a few others ; but in every one 
of these cases the relation of the sexes in regard to nidification 
is reversed, the male performing the duties of incubation, 
while the female is the stronger and more pugnacious. This 
curious case, therefore, quite accords with the general law of 
coloration. 1 

Sexual Colours of other Vertebrates. 

We may consider a few of the cases of sexual colouring of 
other classes of vertebrates, as given by Mr. Darwin. In 
fishes, though the sexes are usually alike, there are several 
species in which the males are more brightly coloured, and 
have more elongated fins, spines, or other appendages, and in 
some few cases the colours are decidedly different. The males 
often fight together, and are altogether more vivacious and 
excitable than the females during the breeding season ; and 
with this we may connect a greater intensity of coloration. 

In frogs and toads the colours are usually alike, or a little 
more intense in the males, and the same may be said of most 
snakes. It is in lizards that we first meet with considerable 
sexual differences, many of the species having gular pouches, 
frills, dorsal crests, or horns, cither confined to the males, or 
more developed in them than in the females, and these orna- 
ments are often brightly coloured. In most cases, however, 
the tints of lizards are protective, the male being usually a 
little more intense in coloration ; and the difference in extreme 
cases may be partly due to the need of protection for the 
female, which, when laden with eggs, must be less active and 
less able to escape from enemies than the male, and may, 
therefore, have retained more protective colours, as so many 
insects and birds have certainly done. 2 

In mammalia there is often a somewhat greater intensity 

1 Sccbohrn's IKsturj/ of Jlritish Birds, vol. ii., introduction, p. xiii. 
2 For details see Darwin's Descent of Man , chap. xii. 


of colour in the male, but rarely a decided difference. The 
female of the great red kangaroo, however, is a delicate gray ; 
while in the Lemur macaco of Madagascar the male is jet- 
black and the female brown. In many monkeys also there are 
some differences of colour, especially on the face. The sexual 
weapons and ornaments of male mammalia, as horns, crests, 
manes, and dewlaps, are well known, and are very numerous 
and remarkable. Having thus briefly reviewed the facts, we 
will now consider the theories to which they have given rise. 

Sexual Selection ly the Struggles of Males. 

Among the higher animals it is a very general fact that 
the males fight together for the possession of the females. 
This leads, in polygamous animals especially, to the stronger 
or better armed males becoming the parents of the next 
generation, which inherits the peculiarities of the parents ; 
and thus vigour and offensive weapons are continually 
increased in the males, resulting in the strength and horns 
of the bull, the tusks of the boar, the antlers of the stag, 
and the spurs and fighting instinct of the gamecock. But 
almost all male animals fight together, though not specially 
armed ; oven hares, moles, squirrels, and beavers fight to the 
death, and are often found to be scarred and wounded. The 
same rule applies to almost all male birds ; and these battles 
have been observed in such different groups as humming- 
birds, finches, goatsuckers, woodpeckers, ducks, and waders. 
Among reptiles, battles of the males are known to occur in 
the cases of crocodiles, lizards, and tortoises ; among fishes, 
in those of salmon and sticklebats. Even among insects the 
same law prevails ; arid male spiders, beetles of many groups, 
crickets, and butterflies often fight together. 

From this very general phenomenon there necessarily 
results a form of natural selection which increases the vigour 
and fighting power of the male animal, since, in every case, 
the weaker are either killed, wounded, or driven away. This 
selection would be more powerful if males were always in 
excess of females, but after much research Mr. Darwin could 
not obtain any satisfactory evidence that this was the case. 
The same effect, however, is produced in some cases by con- 
stitution or habits ; thus male insects usually emerge first from 


tho pupa, and among migrating birds the males arrive first 
both in this country and in North America. The struggle 
is thus intensified, and the most vigorous males are the 
first to have offspring. This in all probability is a great 
advantage, as the early breeders have tho start in securing 
food, and the young are strong enough to protect themselves 
while the later broods are being produced. 

It is to this form of male rivalry that Mr. Darwin first 
applied the term " sexual selection." It is evidently a real 
power in nature ; and to it we must impute the development of 
the exceptional strength, size, arid activity of the male, together 
with the possession of special offensive and defensive weapons, 
and of all other characters which arise from the development 
of these or are correlated with them. But he has extended 
the principle into a totally different field of action, which 
has none of that character of constancy and of inevitable 
result that attaches to natural selection, including male 
rivalry; for by far the larger portion of the phenomena, 
which he endeavours to explain by the direct action of 
sexual selection, can only be so explained on the hypothesis 
that the immediate agency is female choice or preference. 
It is to this that ho imputes the origin of all secondary 
sexual characters other than weapons of offence and defence, 
of all the ornamental crests and accessory plumes of birds, 
the stridulatirig sounds of insects, the crests and beards 
of monkeys and other mammals, and the brilliant colours and 
patterns of male birds and butterflies. He even goes further, 
and imputes to it a largo portion of the brilliant colour that 
occurs in both sexes, on the principle that variations occurring 
in one sex are sometimes transmitted to the same sex only, 
sometimes to both, owing to peculiarities in the laws of inherit- 
ance. In this extension of sexual selection to include the 
action of female choice or preference, and in tho attempt to 
give to that choice such wide-reaching effects, I am unable 
to follow him more than a very little way ; and 1 will now 
state some of the reasons why I think his views are unsound. 

Sexual Characters dm to Natural Selection. 

Besides the acquisition of weapons by the male for tho 
purpose of fighting with other males, there are some other 


sexual characters which may have been produced by natural 
selection. Such are the various sounds and odours which are 
peculiar to the male, and which serve as a call to the female 
or as an indication of his presence. These are evidently a 
valuable addition to the means of recognition of the two sexes, 
and are a further indication that the pairing season has 
arrived ; and the production, intensification, and differentiation 
of these sounds and odours are clearly within the power of 
natural selection. The same remark will apply to the peculiar 
calls of birds, and even to the singing of the males. These 
may well have originated merely as a means of recognition 
between the two sexes of a species, and as an invitation from 
the male to the female bird. When the individuals of a 
species are widely scattered, such a call must be of great 
importance in enabling pairing to take place as early as 
possible, and thus the clearness, loudncss, and individuality of 
the song becomes a useful character, and therefore the subject 
of natural selection. Such is especially the case with the 
cuckoo, and with all solitary birds, and it may have been 
equally important at some period of the development of all 
birds. The act of singing is evidently a pleasurable one ; and 
it probably serves as an outlet for superabundant nervous 
energy and excitement, just as dancing, singing, and field 
sports do with us. It is suggestive of this view that the 
exercise of the vocal power seems to be complementary 
to the development of accessory plumes and ornaments, 
all our finest singing birds being plainly coloured, and 
with no crests, neck or tail plumes to display ; while the 
gorgeously ornamented birds of the tropics have no song, 
and those which expend much energy in display of plumage, 
as the turkey, peacocks, birds of paradise, and humming- 
birds, have comparatively an insignificant development of 
voice. Some birds have, in the wings or tail, peculiarly 
developed feathers which produce special sounds. In some of 
the little manakins of Brazil, two or three of the wing- 
feathers are curiously shaped and stiffened in the male, so 
that the bird is able to produce with them a peculiar 
snapping or cracking sound ; and the tail-feathers of several 
species of snipe are so narrowed as to produce distinct 
drumming, whistling, or switching sounds when the birds 


descend rapidly from a great height. All these are probably 
recognition and call notes, useful to each species in relation 
to the most important function of their lives, and thus capable 
of being developed by the agency of natural selection. 

Decorative Plumage of Kurds and its Display. 

Mr. Darwin has devoted four chapters of his Descent of 
Man to the colours of birds, their decorative plumage, and 
its display at the pairing season ; and it is on this latter 
circumstance that he founds his theory, that both the 
plumage and the colours have been developed by the prefer- 
ence of the females, the more ornamented males becoming the 
parents of each successive generation. Any one who reads 
these most interesting chapters will admit, that the fact of the 
display is demonstrated ; and it may also be admitted, as highly 
probable, that the female is pleased or excited by the display. 
But it by no means follows that slight differences in the shape, 
pattern, or colours of the ornamental plumes are what lead a 
female to give the preference to one male over another ; still 
less that all the females of a species, or the great majority of 
them, over a wide area of country, and for many successive 
generations, prefer exactly the same modification of the colour 
or ornament. 

The evidence on this matter is very scanty, and in most 
cases not at all to the point. Some peahens preferred an old 
pied peacock ; albino birds in a state of nature have never 
been seen paired with other birds ; a Canada goose paired with 
a Berniclo gander ; a male widgeon preferred a pintail duck 
to its own species ; a hen canary preferred a male greenfinch 
to either linnet, goldfinch, siskin, or chaffinch. These cases 
are evidently exceptional, and are not such as generally occur 
in nature ; and they only prove that the female does exert 
some choice between very different males, and some observa- 
tions on birds in a state of nature prove the same thing ; but 
there is no evidence that slight variations in the colour or 
plumes, in the way of increased intensity or complexity, are 
what determines the choice. On the other hand, Mr. Darwin 
gives much evidence that it is not so determined. He tells us 
that Messrs. Hewitt, Tegetmeier, and Brent, three of the 
highest authorities and best observers, " do not believe that 


tho females prefer certain males on account of the beauty of 
their plumage." Mr. Hewitt was convinced "that the female 
almost invariably prefers the most vigorous, defiant, and 
mettlesome male;" and Mr. Tegetmeier, "that a gamecock, 
though disfigured by being dubbed, and with his hackles 
trimmed, would be accepted as readily as a male retaining all 
his natural ornaments." 1 Evidence is adduced that a female 
pigeon will sometimes take an antipathy to a particular male 
without any assignable cause ; or, in other cases, will take a 
strong fancy to some one bird, and will desert her own mate 
for him ; but it is not stated that superiority or inferiority 
of plumage has anything to do with these fancies. Two 
instances are indeed given, of male birds being rejected, which 
had lost their ornamental plumage but in both cases (a 
widow- finch and a silver pheasant) the long tail-plumes are 
the indication of sexual maturity. Such cases do not support 
the idea that males with the tail-feathers a trifle longer, or 
the colours a trifle brighter, are generally preferred, and 
that those which are only a little inferior are as generally 
rejected, and this is what is absolutely needed to establish 
the theory of the development of these plumes by means of 
the choice of the female. 

It will be seen, that female birds have unaccountable likes 
and dislikes in the matter of their partners, just as we have 
ourselves, and this may afford us an illustration. A young 
man, when courting, brushes or curls his hair, and has his 
moustache, beard, or whiskers in perfect order, and no doubt 
his sweetheart admires them ; but this does not prove that 
she marries him on account of these ornaments, still less that 
hair, beard, whiskers, and moustache were developed by the 
continued preferences of the female sex. So, a girl likes to see 
her lover well and fashionably dressed, and he always dresses 
as well as he can when he visits her ] but we cannot conclude 
from this that tho whole series of male costumes, from the 
brilliantly coloured, puffed, and slashed doublet and hose of 
the Elizabethan period, through the gorgeous coats, long 
waistcoats, and pigtails of the early Georgian era, down to 
the funereal dress-suit of the present day, are the direct result 
of female preference. In like manner, female birds may be 
1 Descent of Man, pp. 417, 418, 420. 


charmed or excited by the fine display of plumage by the 
males ; but there is no proof whatever that slight differences 
in that display have any effect in determining their choice of 
a partner. 

Display of Decorative Plumage. 

The extraordinary mariner in which most birds display 
their plumage at the time of courtship, apparently with the 
full knowledge that it is beautiful, constitutes one of Mr. 
Darwin's strongest arguments. It is, no doubt, a very curious 
and interesting phenomenon, and indicates a connection be- 
tween the exertion of particular muscles and the develop- 
ment of colour and ornament ; but, for the reasons just given, 
it does not prove that the ornament has been developed by 
female choice. During excitement, and when the organism 
develops superabundant energy, many animals find it pleasur- 
able to exercise their various muscles, often in fantastic ways, 
as seen in the gambols of kittens, lambs, and other young 
animals. But at the time of pairing, male birds are in a 
state of the most perfect development, and possess an 
enormous store of vitality ; and under the excitement of the 
sexual passion they perform strange antics or rapid flights, as 
much probably from an internal impulse to motion and exertion 
as with any desire to please their mates. Such are the rapid 
descent of the snipe, the soaring and singing of the lark, and 
the dances of the cock-of-the-rock and of many other birds. 

It is very suggestive that similar strange movements are 
performed by many birds which have no ornamental plumage 
to display. Goatsuckers, geese, carrion vultures, and many 
other birds of plain plumage have been observed to dance, 
spread their wings or tails, and perform strange love-antics. 
The courtship of the great albatross, a most unwieldy and 
dull coloured bird, has been thus described by Professor 
Moseley : " The male, standing by the female on the nest, 
raises his wings, spreads his tail and elevates it, throws up his 
head with the bill in the air, or stretches it straight out, or 
forwards, as far as he can, and then utters a curious cry." 1 Mr. 
Jenner Weir informs me that " the male blackbird is full of 
action, spreads out his glossy wing and tail, turns his rich golden 

1 Notes of a Naturalist on the Challenger. 


beak towards the female, and chuckles with delight," while he 
has never seen the more plain coloured thrush demonstrative 
to the female. The linnet distends his rosy breast, and 
slightly expands his brown wings and tail ; while the various 
gay coloured Australian finches adopt such attitudes and 
postures as, in every case, to show off their variously coloured 
plumage to the best advantage. 1 

A Theory of Animal Coloration. 

Having rejected Mr. Darwin's theory of female choice as 
incompetent to account for the brilliant colours and markings 
of the higher animals, the preponderance of these colours and 
markings in the male sex, and their display during periods 
of activity or excitement, I may be asked what explanation 
I have to offer as a preferable substitute. In my Tropical 
Nature I have already indicated such a theory, which I will 
now briefly explain, supporting it by some additional facts 
and arguments, which appear to me to have great weight, and 
for which I am mainly indebted to a most interesting and 
suggestive posthumous work by Mr. Alfred Tylor. 2 

The fundamental or ground colours of animals are, as has 
been shown in preceding chapters, very largely protective, 
and it is not improbable that the primitive colours of all 
animals were so. During the long course of animal develop- 
ment other modes of protection than concealment by harmony 
of colour arose, and thenceforth the normal development of 
colour due to the complex chemical arid structural changes 
ever going on in the organism, had full play ; arid the colours 
thus produced were again and again modified by natural selection 
for purposes of warning, recognition, mimicry, or special pro- 
tection, as has been already fully explained in the preceding 

Mr. Tylor has, however, called attention to an important 
principle which underlies the various patterns or ornamental 
markings of animals namely, that diversified coloration 
follows the chief lines of structure, and changes at points, such 
as the joints, where function changes. He says, "If we 
take highly decorated species that is, animals marked by 

1 Descent of Man, pp. 401, 402. 
2 Coloration in Animals and Plants, London, 1886. 


alternate dark or light hands or spots, such as the zebra, some 
deer, or the carnivora, we find, first, that the region of the 
spinal column is marked by a dark stripe ; secondly, that the 
regions of the appendages, or limbs, are differently marked ; 
thirdly, that the flanks are striped or spotted, along or be- 
tween the regions of the lines of the ribs ; fourthly, that the 
shoulder and hip regions are marked by curved lines ; fifthly, 
that the pattern changes, and the direction of the lines, or 
spots, at the head, neck, and every joint of the limbs ; and 
lastly, that the tips of the ears, nose, tail, and feet, and the 
eye are emphasised in colour. In spotted animals the greatest 
length of the spot is generally in the direction of the largest 
development of the skeleton." 

This structural decoration is well seen in many insects. In 
caterpillars, similar spots and markings are repeated in each 
segment, except where modified for some form of protection. 
In butterflies, the spots and bands usually have reference to 
the form of the wing and the arrangement of the ncrvures ; 
and there is much evidence to show that the primitive mark- 
ings are always spots in the cells, or between the nervures, or 
at the junctions of ncrvures, the extension and coalescence of 
these spots forming borders, bands, or blotches, which have 
become modified in infinitely varied ways for protection, 
warning, or recognition. Even in birds, the distribution of 
colours and markings follows generally the same law. The 
crown of the head, the throat, the ear-coverts, and the eyes 
have usually distinct tints in all highly coloured birds ; the 
region of the furcula has often a distinct patch of colour, 
as have the pectoral muscles, the uropygium or root of the tail, 
and the under tail-coverts. 1 

Mr. Tylor was of opinion that the primitive form of 
ornamentation consisted of spots, the confluence of these in 
certain directions forminglines or bands; and, these again, some- 
times coalescing into blotches, or into more or less uniform 
tints covering a large portion of the surface of the body. The 
young lion and tiger are both spotted ; and in the Java hog 
(Sus vittatus) very young animals are banded, but have spots 
over the shoulders and thighs. These spots run into stripes 

1 Coloration of Animal^ PI. X, p. 90 ; and Pis. II, III, and IV, pp. 30, 
40, 42. 



as the animal grows older ; then the stripes expand, and 
at last, meeting together, the adult animal becomes of a 
uniform dark brown colour. So many of the species of 
deer are spotted when young, that Darwin concludes the 
ancestral form, from which all deer are derived, must have 
been spotted. Pigs and tapirs are banded or spotted when 
young ; an imported young specimen of Tapirus Bairdi 
was covered with white spots in longitudinal rows, here 
and there forming short stripes. 1 Even the horse, which 
Darwin supposes to be descended from a striped animal, 
is often spotted, as in dappled horses ; and great numbers 
show a tendency to spottincss, especially on the haunches. 

Ocelli may also be developed from spots, or from bars, as 
pointed out by Mr. Darwin. Spots arc an ordinary form of 
marking in disease, and these spots sometimes run together, 
forming blotches. There is evidence that colour markings are 
in some way dependent on nerve distribution. In the disease 
known as frontal herpes, an eruption occurs which corresponds 
exactly to the distribution of the ophthalmic division of the 
fifth cranial nerve, mapping out all its little branches even 
to the one which goes to the tip of the nose. In a Hindoo 
suffering from herpes the pigment was destroyed in the arm 
along the course of the ulnar nerve, with its branches along 
both sides of one finger and the half of another. In the leg 
the sciatic and scaphcnous nerves were partly mapped out, 
giving to the patient the appearance of an anatomical 
diagram. 2 

These facts are very interesting, because they help to 
explain the general dependence of marking on structure which 
has been already pointed out. For, as the nerves everywhere 
follow the muscles, and these are attached to the various bones, 
we see how it happens, that the tracts in which distinct 
developments of colour appear, should so often be marked out 
by the chief divisions of the bony structure in vertebrates, and 
by the segments in the annulosa. There is, however, another 
correspondence of even greater interest and importance. 
Brilliant colours usually appear just in proportion to the 

1 See coloured Fig. in Proc. Zool. For., 1871, ]>. 626. 
- A. Tylor's Coloration, p. 40 ; and Photograph in Hutchinson's Illustra- 
tions of Clinical Surgery, quoted by Tylor. 


development of tegumentary appendages. Among birds the 
most brilliant colours are possessed by those which have 
developed frills, crests, and elongated tails like the humming- 
birds; immense tail-coverts like the peacock; enormously 
expanded wing-feathers, as in the argus-pheasant ; or magnifi- 
cent plumes from the region of the coracoids in many of 
the birds of paradise. It is to be noted, also, that all these 
accessory plumes spring from parts of the body which, in 
other species, are distinguished by patches of colour ; so that 
we may probably impute the development of colour and of 
accessory plumage to the same fundamental cause. 

Among insects, the most brilliant and varied coloration 
occurs in the butterflies and moths, groups in which the wing- 
membranes have received their greatest expansion, and whose 
specialisation has been carried furthest in the marvellous scaly 
covering which is the seat of the colour. It is suggestive, that 
the only other group in which functional wings are much 
coloured is that of the dragonflies, where the membrane is 
exceedingly expanded. In like manner, the colours of beetles, 
though greatly inferior to those of the lepidoptera,, occur in a 
group in which the anterior pair of wings has been thickened 
and modified in order to protect the vital parts, arid in which 
these wing-covers (elytra), in the course of development in the 
different groups, must have undergone great changes, and have 
been the seat of very active growth. 

The Origin of Accessory Plumes. 

Mr. Darwin supposes, that these have in almost every case 
been developed by the preference of female birds for such 
males as possessed them in a higher degree than others ; but 
this theory docs not account for the fact that these plumes 
usually appear in a few definite parts of the body. We 
require some cause to initiate the development in one part 
rather than in another. Now, the view that colour has arisen 
over surfaces where muscular and nervous development is 
considerable, and the fact that it appears especially upon the 
accessory or highly developed plumes, leads us to inquire whether 
the same cause has not primarily determined the development 
of these plumes. The immense tuft of golden plumage in the 
best known birds of paradise (Paradisea apoda and P. minor) 


springs from a very small area on the side of the breast. Mr. 
Frank E. Beddard, who has kindly examined a specimen for 
me, says that "this area lies upon the pectoral muscles, and 
near to the point where the fibres of the muscle converge 
towards their attachment to the humerus. The plumes arise, 
therefore, close to the most powerful muscle of the body, and 
near to where the activities of that muscle would be at a 
maximum. Furthermore, the area of attachment of the plumes 
is just above the point where the arteries and nerves for the 
supply of the pectoral muscles, and neighbouring regions, 
leave the interior of the body. The area of attachment of 
the plume is, also, as you say in your letter, just above the 
junction of the coracoid and sternum." Ornamental plumes 
of considerable size rise from the same part in many other 
species of paradise birds, sometimes extending laterally in front, 
so as to form breast shields. They also occur in many humming- 
birds, and in some sun-birds and honey-suckers; and in all these 
cases there is a wonderful amount of activity and rapid move- 
ment, indicating a surplus of vitality, which is able to manifest 
itself in the development of these accessory plumes. 1 

In a quite distinct set of birds, the gallinaccae, we find the 
ornamental plumage usually arising from very different parts, in 
the form of elongated tail-feathers or tail-coverts, and of ruffs 
or hackles from the neck. Here the wings are comparatively 
little used, the most constant activities depending on the legs, 
since the gallinaccoo are pre-eminently walking, running, and 
scratching birds. Now the magnificent train of the peacock 
the grandest development of accessory plumes in this order 
springs from an oval or circular area, a-bout three inches in 
diameter, just above the base of the tail, and, therefore, 
situated over the lower part of the spinal column near the 
insertion of the powerful muscles which move the hind limbs 
and elevate the tail. The very frequent presence of neck-ruffs 
or breast-shields in the males of birds with accessory plumes 
may be partly due to selection, because they must serve as a 
protection in their mutual combats, just as does the lion's or the 
horse's mane. The enormously lengthened plumes of the bird 
of paradise and of the peacock can, however, have no such use, 

1 For activity and pugnacity of humming-birds, see Tropical Nature, pp. 
130, 213. 


but must be rather injurious than beneficial in the bird's ordi- 
nary life. The fact that they have been developed to so great 
an extent in a few species is an indication of such perfect adapta- 
tion to the conditions of existence, such complete success in 
the battle for life, that there is, in the adult male at all events, 
a surplus of strength, vitality, and growth-power which is able 
to expend itself in this way without injury. That such is the 
case is shown by the great abundance of most of the species 
which possess these wonderful superfluities of plumage. Birds 
of paradise arc among the commonest birds in New Guinea, 
and their loud voices can be often heard when the birds them- 
selves are invisible in the depths of the forest ; while Indian 
sportsmen have described the peafowl as being so abundant, 
that from twelve to fifteen hundred have been seen within 
an hour at one spot ; and they range over the whole country 
from the Himalayas to Ceylon. Why, in allied species, the 
development of accessory plumes has taken different forms, we 
are unable to say, except that it may be due to that individual 
variability which has served as the starting-point for so much 
of what seems to us strange in form, or fantastic in colour, 
both in the animal and vegetable world. 

Development of ^Iccessury Plumes ami their Display. 

If we have found a rcm causa for tho origin of ornamental 
appendages of birds and other animals in a surplus of vital 
energy, leading to abnormal growths in those parts of the 
integument where muscular and nervous action are greatest, 
the continuous development of these appendages will result 
from the ordinary action of natural selection in preserving the 
most healthy and vigorous individuals, and the still further 
selective agency of sexual struggle in giving to tho very 
strongest and most energetic the parentage of the next genera- 
tion. And, as all the evidence goes to show that, so far as 
female birds exercise any choice, it is of " the most vigorous, 
defiant, and mettlesome male," this form of sexual selection 
will act in the same direction, and help to carry on the process 
of plume development to its culmination. That culmination 
will be reached when the excessive length or abundance of tho 
plumes begins to be injurious to tho bearer of them ; and it 
may be this check to tho further lengthening of tho peacock's 


train that has led to the broadening of tho feathers at the 
ends, and the consequent production of the magnificent eye- 
spots which now form its crowning ornament. 

The display of these plumes will result from tho same 
causes which led to their production. Just in proportion as 
the feathers themselves increased in length and abundance, the 
skin-muscles which serve to elevate them would increase also ; 
and the nervous development as well as the supply of blood 
to these parts being at a maximum, the erection of the plumes 
would become a habit at all periods of nervous or sexual 
excitement. The display of the plumes, like the existence of 
the plumes themselves, would be the chief external indication 
of the maturity and vigour of the male, and would, therefore, 
be necessarily attractive to the female. We have, thus, no 
reason for imputing to her any of those aesthetic emotions 
which are excited in us, by the beauty of form, colour, and 
pattern of these plumes ; or the still more improbable a3Sthetic 
tastes, which would cause her to choose her mate on account 
of minute differences in their forms, colours, or patterns. 

As co-operating causes in tho production of accessory 
ornamental plumes, I have elsewhere suggested 1 that crests 
and other erectile feathers may have been useful in making 
the bird more formidable in appearance, and thus serving to 
frighten away enemies ; while long tail or wing feathers might 
serve to distract the aim of a bird of prey. But though this 
might be of some use in the earlier stages of their develop- 
ment, it is probably of little importance compared with the 
vigour and pugnacity of which the plumes are the indication, 
and which enable most of their possessors to defend them- 
selves against the enemies which are dangerous to weaker 
and more timid birds. Even the tiny humming-birds are said 
to attack birds of prey that approach too near to their nests. 

The Effect of Female Preference will be Neutralised by 
Natural Selection. 

The various facts and arguments now briefly set forth, 
afford an explanation of the phenomena of male ornament, 

1 Tropical Nature^ p. 209. In Chapter V of this work tho vieww hero 
advocated were first set forth, and the reader is referred there for further 


as being duo to the general laws of growth and develop- 
ment, and make it unnecessary to call to our aid so hypo- 
thetical a cause as the cumulative action of female prefer- 
ence. Theic remains, however, a general argument, arising 
from the action of natural selection itself, which renders it 
almost inconceivable that female preference could have been 
effective in the way suggested ; while the same argument 
strongly supports the view here set forth. Natural selec- 
tion, as we have seen in our earlier chapters, acts per- 
petually and on an enormous scale in weeding out the 
" unfit " at every stage of existence, and preserving only 
those which are in all respects the very best. Each year, only 
a small percentage of young birds survive to take the place of 
the old birds which die ; and the survivors will be those which 
are best able to maintain existence from the egg onwards, an 
important factor being that their parents should be well able 
to feed and protect them, while they themselves must in turn 
l>e equally able to feed and protect their own offspring. Now 
this extremely rigid action of natural selection must render 
any attempt to select mere ornament utterly nugatory, unless 
the most ornamented always coincide with " the fittest " in 
every other respect ; while, if they do so coincide, then any 
selection of ornament is altogether superfluous. If the most 
brightly coloured and fullest plumaged males are not the most 
healthy and vigorous, have 'not the best instincts for the proper 
construction and concealment of the nest, and for the care 
and protection of the young, they are certainly not the fittest, 
and will not survive, or be the parents of survivors. If, on 
the other hand, there is generally this correlation if, as has 
been here argued, ornament is the natural product and direct 
outcome of superabundant health and vigour, then no other 
mode of selection is needed to account for the presence of 
such ornament. The action of natural selection does not 
indeed disprove the existence of female selection of ornament 
as ornament, but it renders it entirely ineffective ; and as 
the direct evidence for any such female selection is almost 
nil, while the objections to it are certainly weighty, there can 
bo no longer any reason for upholding a theory which was 
provisionally useful in calling attention to a most curious and 
suggestive body of facts, but which is now no longer tenable. 


The term " sexual selection " must, therefore, be restricted 
to the direct results of male struggle and combat. This is 
really a form of natural selection, and is a matter of direct 
observation ; while its results are as clearly deducible as those 
of any of the other modes in which selection acts. And if 
this restriction of the term is needful in the case of the higher 
animals it is much more so with the lower. In butterflies the 
weeding out by natural selection takes place to an enormous 
extent in the egg, larva, and pupa states ; and perhaps not 
more than one in a hundred of the eggs laid produces a perfect 
insect which lives to breed. Here, then, the impotence of 
female selection, if it exist, must be complete ; for, unless the 
most brilliantly coloured males are those which produce the 
best protected eggs, larva}, and pupa?, and unless the particular 
eggs, larvae, and pupae, which are able to survive, are those 
which produce the most brilliantly coloured butterflies, any 
choice the female might make must be completely swamped. 
If, on the other hand, there is this correlation between colour 
development and perfect adaptation to conditions in all stages, 
then this development will necessarily proceed by the agency 
of natural selection and the general laws which determine 
the production of colour and of ornamental appendages, 1 

General Laws of Animal Coloration. 

The condensed account which has now been given of the 
phenomena of colour in the animal world will sufficiently show 
the wonderful complexity and extreme interest of the subject ; 
while it affords an admirable illustration of the importance of 
the great principle of utility, and of the effect of the theories 
of natural selection and development in giving a new interest 

1 The Rev. 0. Pickard-C'arnbridge, \vho lias devoted himself to the .study 
of spiders, has kindly .sent me the following extract from a letter, written 
in 1869, in which lie states his views on this question : 

" I myself doubt that particular application of the Darwinian theory 
which attributes male peculiarities of form, structure, colour, and ornament 
to female appetency or predilection. There is, it seems to me, undoubtedly 
something in the male organisation of a special, and sexual nature, which, 
of its own vital force, develops the remarkable male peculiarities so 
commonly seen, and of no imaginable use to that sex. In as far as 
these peculiarities show a great vital power, they point out to us the finest 
and strongest individuals of the sex, and show us which of them would 
most certainly appropriate to themselves the best and greatest number of 
females, arid leave behind them the strongest and greatest number of 


to the most familiar facts of nature. Much yet remains to bo 
done, both in the observation of new facts as to the relations 
between the colours of animals and their habits or economy, 
and, more especially, in the elucidation of the laws of growth 
which determine changes of colour in the various groups; but 
so much is already known that we are a!>le, with some 
confidence, to formulate the general principles which have 
brought about all the beauty and variety of colour which 
everywhere delight us in our contemplation of animated 
nature. A brief statement of these principles will fitly con- 
clude our exposition of the subject. 

1. Colour may be looked upon as a necessary result of the 
highly complex chemical constitution of animal tissues and 
fluids. The blood, the bile, the bones, the fat, and other 
tissues have characteristic, and often brilliant colours, which 
we cannot suppose to have been determined for any special 
purpose, as colours, since they are usually concealed. The 
external organs, with their various appendages and integu- 
ments, would, by the same general laws, naturally give rise to 
a greater variety of colour. 

2. We find it to be the fact that colour increases in variety 
and intensity as external structures and dermal appendages 
become more differentiated and developed. It is on scales, 
hair, and especially on the more highly specialised feathers, 
that colour is most varied and beautiful ; while among insects 
colour is most fully developed in those whose wing membranes 
are most expanded, and, as in the lepidoptera, are clothed 
with highly specialised scales. Here, too, we find an additional 
mode of colour production in transparent lamella 4 - or in fine 
surface stria; which, by tho laws of interference, produce the 
wonderful metallic hues of so many birds and insects. 

progeny. And here would come in, us it appears to me, the proper 
application of Darwin's theory of Natural Selection ; for tho possessors 
of greatest vital power being those most frequently produced and repro- 
duced, the external signs of it would go on developing in an ever-increasing 
exaggeration, only to be cheeked where it became really detrimental in some 
respect or other to the individual." 

This passage, giving tho independent views of a close observer one, 
moreover, who has studied the species of au extensive group of animals 
both in the field and in the laboratory very nearly accords with my own 
conclusions above given ; and, so far as the matured opinions of a competent 
naturalist have any weight, allbrd them an important support. 


3. There are indications of a progressive change of colour, 
perhaps in some definite order, accompanying the development 
of tissues or appendages. Thus spots spread and fuse into 
bands, and when a lateral or centrifugal expansion has 
occurred as in the termination of the peacocks' train feathers, 
the outer we!) of the secondary quills of the Argus pheasant, 
or the broad and rounded wings of many butterflies into 
variously shaded or coloured ocelli. The fact that we find 
gradations of colour in many of the more extensive groups, 
from comparatively dull or simple to brilliant and varied hues, 
is an indication of some such law of development, due 
probably to progressive local segregation in the tissues of 
identical chemical or organic molecules, and dependent on 
laws of growth yet to be investigated. 

4. The colours thus produced, and subject to much in- 
dividual variation, have been modified in innumerable ways 
for the benefit of each species. The most general modifica- 
tion has been in such directions as to favour concealment 
when at rest in the usual surroundings of the species, some- 
times carried on by successive steps till it has resulted in the 
most minute imitation of some inanimate object or exact 
mimicry of some other animal. In other cases bright colours 
or striking contrasts have been preserved, to serve as a warning 
of inedibility or of dangerous powers of attack. Most frequent 
of all has been the specialisation of each distinct form by some 
tint or marking for purposes of easy recognition, especially in 
the case of gregarious animals whose safety largely depends 
upon association and mutual defence. 

5. As a general rule the colours of the two sexes are alike; 
but in the higher animals there appears a tendency to deeper 
or more intense colouring in the male, due probably to his 
greater vigour and excitability. In many groups in which 
this superabundant vitality is at a maximum, the develop- 
ment of dermal appendages and brilliant colours has gone on 
increasing till it has resulted in a great diversity between the 
sexes ; and in most of these cases there is evidence to show 
that natural selection has caused the female to retain the 
primitive arid more sober colours of the group for purposes of 


Concluding Eemarks. 

The general principles of colour development now sketched 
out enable us to give some rational explanation of the 
wonderful amount of brilliant colour which occurs among 
tropical animals. Looking on colour as a normal product of 
organisation, which has either been allowed free play, or has 
been checked and modified for the benefit of the species, we 
can see at once that the luxuriant and perennial vegetation of 
the tropics, by affording much more constant means of con- 
cealment, has rendered brilliant colour less hurtful there than 
in the temperate and colder regions. Again, this perennial 
vegetation supplies abundance of both vegetable and insect 
food throughout the year, and thus a greater abundance and 
greater variety of the forms of life are rendered possible, than 
where recurrent seasons of cold and scarcity reduce the 
possibilities of life to a minimum. Geology furnishes us with 
another reason, in the fact, that throughout the tertiary period 
tropical conditions prevailed far into the temperate regions, so 
that the possibilities of colour development were still greater 
than they are at the present time. The tropics, therefore, 
present to us the results of animal development in a much 
larger area and under more favourable conditions than 
prevail to-day. We see in them samples of the productions of 
an earlier and a better world, from an animal point of view ; 
and this probably gives a greater variety and a finer display of 
colour than would have been produced, had conditions always 
been what they are now. The temperate zones, on the other 
hand, have recently suffered the effects of a glacial period of 
extreme severity, with the result that almost the only gay 
coloured birds they now possess arc summer visitors from 
tropical or sub -tropical lands. It is to the unbroken and 
almost unchecked course of development from remote geo- 
logical times that has prevailed in the tropics, favoured by 
abundant food and perennial shelter, that we owe such superb 
developments a the frills and crests and jewelled shields of 
the humming-birds, the golden plumes of the birds of paradise, 
and the resplendent train of the peacock. This last exhibits to 
us the culmination of that marvel and mystery of animal colour 
which is so well expressed by a poet -artist in the following 


lines. The marvel will ever remain to tho sympathetic 
student of nature, but I venture to hope that in the pre- 
ceding chapters I have succeeded in lifting if only by one 
of its corners the veil of mystery which has for long 
shrouded this department of nature. 

0)i a Peacock's Feather. 

In Nature's workshop but a shaving, 

Of her poem but a word, 
But a tint brushed from her palette, 

This feather of a bird ! 
Yet set it in the sun glance, 

Display it in the shine, 
Take graver's lens, explore it, 

Note filament and line, 
Mark amethyst to sapphire, 

And sapphire to gold, 
And gold to emerald changing 

The archetype unfold ! 
Tone, tint, thread, tissue, texture, 

Through every atom scan, 
Conforming still, developing, 

Obedient to plan. 
This but to form a pattern 

On the garment of a bird ! 
What then must be the poem, 

This but its lightest word ! 
Sit before it ; ponder o'er it, 

'Twill thy mind advantage more, 
Than a treatise, than a sermon, 

Than a library of lore. 



The general colour relations of plants -Colours of fruits The moaning of 
nuts Edible or attractive fruits The colours of ilowcrs Modes of 
securing cross-fertilisation The interpretation of the farts Summary 
of additional facts bearing on insect fertilisation Fertilisation of 
flowers by birds Self-fertilisation of Jlowers Dilliculties and con- 
tradictions Intercrossing not necessarily advantageous Supposed 
evil results of close interbreeding How the struggle for existence 
acts among llowers Flowers the product of insect agency Concluding 
remarks on colour in nature. 

THE colours of plants are both less definite and less complex 
than arc those of animals, and their interpretation on the 
principle of utility is, on the whole, more direct and more 
easy. Yet here, too, we find that in our investigation of the 
uses of the various colours of fruits and flowers, we are 
introduced to some of the most obscure recesses of nature's 
workshop, and are confronted with problems of the deepest 
interest and of the utmost complexity. 

So much has been written on this interesting subject 
since Mr. Darwin first called attention to it, and its main 
facts have become so generally known by means of lectures, 
articles, and popular books, that I shall give here a mere 
outline sketch, for the purpose of leading up to a discussion 
of some of the more fundamental problems which arise out of 
the facts, and which have hitherto received less attention than 
they deserve. 


The General Colour Relations of Plants. l 

The green colour of the foliage of leafy plants is due ".to 
the existence of a substance called chlorophyll, which dp 
almost universally developed in the leaves under the action 
of light. It is subject to definite chemical changes during 
the processes of growth and of decay, and it is owing to 
these changes that we have the delicate tints of spring 
foliage, and the more varied, intense, and gorgeous hues of 
autumn. But these all belong to the class of intrinsic or 
normal colours, due to the chemical constitution of the 
organism ; as colours they are unadaptive, and appear to 
have no more relation to the wellbeing of the plants them- 
selves than have the colours of gems and minerals. We may 
also include in the same category those alga? and fungi 
which have bright colours the "red snow" of the arctic 
regions, the red, green, or purple seaweeds, the brilliant 
scarlet, yellow, white, or black agarics, and other fungi. 
All these colours are probably the direct results of chemical 
composition or molecular structure, and, being thus normal 
products of the vegetable organism, need no special explana- 
tion from our present point of view ; and the same remark 
will apply to the varied tints of the bark of trunks, branches, 
and twigs, which are often of various shades of brown and 
green, or even vivid reds or yellows. 

There are, however, a few cases in which the need of 
protection, which we have found to be so important an 
agency in modifying the colours of animals, has also deter- 
mined those of some of the smaller members of the vegetable 
kingdom. Dr. Burchell found a mesembryanthcmum in 
South Africa like a curiously shaped pebble, closely resem- 
bling the stones among which it grew j 1 and Mr. J. P. Mansel 
Weale states that in the same country one of the Asclepi- 
adeae has tubers growing above ground among stones which 
they exactly resemble, and that, when not in leaf, they 
are for this reason quite invisible. 2 It is clear that such 
resemblances must be highly useful to these plants, inhabiting 
an arid country abounding in herbivorous mammalia, which, 

1 Burcliell's Travels, vol. i. p. 10. 
2 Nature, vol. iii. p. 507. 


in times of drought or scarcity, will devour everything in the 
shape of a fleshy stein or tuber. 

True mimicry is very rare in plants, though adaptation to 
like conditions often produces in foliage and habit a similarity 
that is deceiving. Euphorbias growing in deserts often closely 
resemble cacti. Seaside plants and high alpine plants of 
different orders are often much alike ; and innumerable 
resemblances of this kind are recorded in the names of 
plants, as Veronica epacridea (the veronica like an epacris), 
Limnanthemuin nymphivoides (the limnanthemum like a 
nymphsea), the resembling species in each case belonging to 
totally distinct families. 1'ut in these cases, and in most others 
that have been observed, the essential features of true mimicry 
i "0 absent, inasmuch as the one plant cannot be supposed to 
derive any benefit from its close resemblance to the other, 
and this is still more certain from the fact that the two 
species usually inhabit different localities. A few cases exist, 
however, in which there does seem to be the necessary 
accordance and utility. Mr. Mansel AVealc mentions a labiate 
plant (Ajuga ophrydis), the only species of the genus Ajuga in 
South Africa, which is strikingly like an orchid of the same 
country ; while a balsam (Impatiens capensis), also a solitary 
species of the genus in that country, is equally like an orchid, 
growing in the same locality and visited by the same insects. 
As both these genera of plants are specialised for insect 
fertilisation, and both of the plants in question are isolated 
species of their respective genera, we may suppose that, 
when they first reached South Africa they were neglected 
by the insects of the country ; but, being both remotely like 
orchids in form of flower, those varieties that approached 
nearest to the familiar species of the country were visited 
by insects and cross-fertilised, and thus a closer resemblance 
would at length be brought about. Another case of close 
general resemblance, is that of our common white dead- 
nettle (Lamium album) to the stinging-nettle (Urtica dioica) ; 
and Sir John Lubbock thinks that this is a case of true 
mimicry, the dead-nettle being benefited by being mistaken 
by grazing animals for the stinging-nettle. 1 

Flowers, FrtnlSt and Leaves, p. 128 (Fig. 79). 


Colours of Fruits. 

It is when we come to the essential parts of plants on 
which their perpetuation and distribution depends, that we 
find colour largely utilised for a distinct purpose in flowers 
arid fruits. In the former we find attractive colours and 
guiding marks to secure cross - fertilisation by insects ; in 
the latter attractive or protective coloration, the first to 
attract birds or other animals when the fruits are intended to be 
eaten, the second to enable them to escape being eaten when 
it would be injurious to the species. The colour phenomena 
of fruits being much the most simple will be considered first. 

The perpetuation and therefore the very existence of each 
species of ilowering plant depend upon its seeds being pre- 
served from destruction and more or less effectually dispersed 
over a considerable area. The dispersal is e (Footed either 
mechanically or by the agency of animals. Mechanical dis- 
persal is chiefly by means of air-currents, and large numbers 
of seeds are specially adapted to be so carried, either by being 
clothed with down or pappus, as in the well-known thistle and 
dandelion seeds ; by having wings or other appendages, as in 
the sycamore, birch, and many other trees ; by being thrown 
to a considerable distance by the splitting of the seed-vessel, 
and by many other curious devices. 1 Very large numbers of 
seeds, however, are so small and light that they can be carried 
enormous distances by gales of wind, more especially as most 
of this kind are flattened or curved, so as to expose a large 
surface in proportion to their weight. Those which are 
carried by animals have their surfaces, or that of the seed- 
vessel, armed with minute hooks, or some prickly covering 
which attaches itself to the hair of mammalia or the feathers 
of birds, as in the burdock, cleavers, and many other species. 
Others again are sticky, as in Plumbago curopaca, mistletoe, 
and many foreign plants. 

All the seeds or seed-vessels which are adapted to bo 
dispersed in any of these ways are of dull protective tints, so 
that when they fall on the ground they are almost indis- 
tinguishable ; besides which, they aro usually small, hard, and 

1 For a popular sketch of these, see Sir J. LuLbock's Flowers, Fruits, and 
Leaves, or any general botanical work. 


altogether unattractive, never having any soft, juicy pulp ; 
while the edible seeds often bear such a small proportion 
to the hard, dry envelopes or appendages, that few animals 
would care to eat them. 

The Meaning of Nuts. 

There is, however, another class of fruits or seeds, usually 
termed nuts, in which there is a large amount of edible matter, 
often very agreeable to the taste, and especially attractive 
and nourishing to a large number of animals. But when 
eaten, the seed is destroyed and the existence of the species 
endangered. It is evident, therefore, that it is by a kind of 
accident that these nuts are eatable ; and that they are not 
ii/ ended to be eaten is shown by the special care nature seems 
to have taken to conceal or to protect them. Wo see that all 
our common nuts are green when on the tree, so as not easily 
to be distinguished from the leaves ; but when ripe they turn 
brown, so that when they fall on to the ground they are equally 
indistinguishable among the dead leaves and twigs, or on the 
brown earth. Then they are almost always protected by hard 
coverings, as in hazel-nuts, which are concealed by the enlarged 
leafy involucre, and in the large tropical brazil-nuts and cocoa- 
nuts by such a hard and tough case as to be safe from almost 
every animal. Others have an external bitter rind, as in the 
walnut; while in the chestnuts and beechnuts two or three 
fruits arc enclosed in a prickly involucre. 

Notwithstanding all these precautions, nuts are largely 
devoured by mammalia and birds ; but as they are chiefly 
the product of trees or shrubs of considerable longevity, 
and are generally produced in great profusion, the perpetua- 
tion of the species is not endangered. In some cases the 
devourers of nuts may aid in their dispersal, as they probably 
now and then swallow the seed whole, or not sufficiently 
crushed to prevent germination ; while squirrels have been 
observed to bury nuts, many of which are forgotten and 
afterwards grow in places they could not have otherwise 
reached. 1 Nuts, especially the larger kinds which are so 
well protected by their hard, nearly globular cases, have their 
dispersal facilitated by rolling down hill, and more especially 
1 Nature, vol. xv. p. 117, 


by floating in rivers and lakes, and thus reaching other locali- 
ties. During the elevation of land areas this method would 
bo very effective, as the new land would always be at a lower 
level than that already covered with vegetation, and therefore 
in the best position for being stocked with plants from it. 

The other modes of dispersal of seeds are so clearly adapted 
to their special wants, that we feel sure they must have been 
acquired by the process of variation and natural selection. 
The hooked and sticky seeds are always those of such her- 
baceous plants as are likely, from their size, to come in 
contact with the wool of sheep or the hair of cattle ; while 
seeds of this kind never occur on forest trees, on aquatic 
plants, or even on very dwarf creepers or trailers. The 
winged seed-vessels or seeds, on the other hand, mostly belong 
to trees and to tall shrubs or climbers. We have, therefore, a 
very exact adaptation to conditions in these different modes of 
dispersal ; while, when we come to consider individual cases, 
we find innumerable other adaptations, some of which the 
reader will find described in the little work by Sir John 
Lubbock already referred to. 

Edible or Attractive Fruits. 

It is, however, when we come to true fruits (in a popular 
sense) that we find varied colours evidently intended to 
attract animals, in order that the fruits may be eaten, while 
the seeds pass through the body undigested and arc then in 
the fittest state for germination. This end has been gained in 
a gieat variety of ways, and with so many corresponding 
adaptations as to leave no doubt as to the value of the result. 
Fruits are pulpy or juicy, and usually sweet, and form the 
favourite food of innumerable birds and some mammals. They 
are always coloured so as to contrast with, the foliage or 
surroundings, red being the most common as it is certainly the 
most conspicuous colour, but yellow, purple, black, or white 
being not uncommon. The edible portion of fruits is developed 
from different parts of the floral envelopes, or of the ovary, in 
the various orders and genera. Sometimes the calyx becomes 
enlarged and fleshy, as in the apple and pear tribe ; more 
often the integuments of the ovary itself are enlarged, as in 
the plum, peach, grape, etc. ; the receptacle is enlarged and 


forms the fruit of the strawberry ; while the mulberry, pine- 
apple, and fig are examples of compound fruits formed in 
various ways from a dense mass of flowers. 

In all cases the seeds themselves are protected from injury 
by various devices. They are small and hard in the straw- 
berry, raspberry, currant, etc., and are readily swallowed 
among the copious pulp. In the grape they are hard and 
bitter; in the rose (hip) disagreeably hairy ; in the orange 
tribe very bitter; and all these have a smooth, glutinous 
exterior which facilitates their being swallowed. When the 
seeds are larger and arc eatable, they are enclosed in an 
excessively hard and thick covering, as in the various kinds 
of " stone " fruit (plums, peaches, etc.), or in a very tough core, 
as in the apple. In the nutmeg of the Eastern Archipelago 
we have a curious adaptation to a single group of birds. The 
fruit is yellow, somewhat like an oval peach, but firm and 
hardly eatable. This splits open and shows the glossy 
black covering of the seed or nutmeg, over which spreads 
the bright scarlet arillus or " mace," an adventitious growth 
of no use to the plant except to attract attention. Large 
fruit pigeons pluck out this seed and swallow it entire 
for the sake of the mace, while the large nutmeg passes 
through their bodies and germinates ; and this has led to 
the wide distribution of wild nutmegs over New Guinea 
and the surrounding islands. 

In the restriction of bright colour to those edible fruits the 
eating of which is beneficial to the plant, we see the undoubted 
result of natural selection ; and this is the more evident when 
we find that the colour never appears till the fruit is ripe 
that is, till the seeds within it are fully matured and in the 
best state for germination. Some brilliantly coloured fruits 
are poisonous, as in our bitter-sweet (Solanum dulcamara), 
cuckoo-pint (Arum) and the West Indian manchinccl. Many 
of these are, no doubt, eaten by animals to whom they are 
harmless ; arid it has been suggested that even if some 
animals are poisoned by them the plant is benefited, since it 
not only gets dispersed, but finds, in the decaying body 
of its victim, a rich manure heap. 1 The particular colours 
of fruits are not, so far as we know, of any use to them other 
1 Grant Allen's Colour Sense, p. 113. 


than as regards conspicuousness, hence a tendency to any 
decided colour has been preserved and accumulated as serving 
to render the fruit easily visible among its surroundings of 
leaves or herbage. Out of 134 fnr't- bearing plants in 
Mongredien's Trees and Shrubs, and Hooker's British Flora, 
the fruits of no less than sixty-eight, or rather more than half, 
are red, forty-five are black, fourteen yellow, and seven white. 
The great prevalence of red fruits is almost certainly due to 
their greater conspicuousness having favoured their dispersal, 
though it may also have arisen in part from the chemical 
changes of chlorophyll during ripening and decay producing 
red tints as in many fading leaves. Yet the comparative 
scarcity of yellow in fruits, while it is the most common tint 
of fading leaves, is against this supposition. 

There are, however, a few instances of coloured fruits which 
do not seem to be intended to be eaten ; such are the colo- 
cynth plant (Cucumis colocynthus), which has a beautiful fruit 
the size and colour of an orange, but nauseous beyond descrip- 
tion to the taste. It has a hard rind, and may perhaps be dis- 
persed by being blown along the ground, the colour being an 
adventitious product ; but it is quite possible, notwithstanding 
its repulsiveness to us, that it may be eaten by some animals. 
With regard to the fruit of another plant, Calotropis 
procera, there is less doubt, as it is dry and full 'of thin, 
flat-winged seeds, with fine silky filaments, eminently adapted 
for wind-dispersal; yet it is of a bright yellow colour, as 
large as an apple, arid therefore very conspicuous. Here, 
therefore, we seem to have colour which is a mere by- 
product of the organism and of no use to it; but such 
cases are exceedingly rare, and this rarity, when compared 
with the great abundance of cases in which there is an 
obvious purpose in the colour, adds weight to the evidence 
in favour of the theory of the attractive coloration of edible 
fruits in order that birds and other animals may assist in 
their dispersal. Both the above-named plants are natives of 
Palestine and the adjacent arid countries. 1 

The Colours of Flowers. 

Flowers are much more varied in their colours than fruits, 
1 Canon Tristram's Natural History of the Bible, pp. 483, 484. 


as they are more complex and more varied in form and 
structure ; yet there is some parallelism between them in both 
respects. Flowers are frequently adapted to attract insects 
as fruits are to attract birds, the object being in the former to 
secure cross -fertilisation, in the latter dispersal; while just 
as colour is an index of the edibility of fruits which supply 
pulp or juice to birds, so are the colours of flowers an indica- 
tion of the presence of nectar or of pollen which are devoured 
by insects. 

The main facts and many of the details, as to the relation 
of insects to flowers, were discovered by Sprengel in 1793. 
He noticed the curious adaptation of the structure of many 
flowers to the particular insects which visit them ; he proved 
that insects do cross-fertilise flowers, and he believed that this 
was the object of the adaptations, while the presence of nectar 
and pollen ensured the continuance of their visits ; yet he 
missed discovering the use of this cross-fertilisation. Several 
writers at a later period obtained evidence that cross-fertilisa- 
tion of plants was a benefit to them ; but the wide generality 
of this fact and its intimate connection with the numerous 
and curious adaptations discovered by Sprengel, was first 
shown by Mr. Darwin, and has since been demonstrated by a 
vast mass of observations, foremost among which are his own 
researches on orchids, primulas, and other plants. 1 

By an elaborate series of experiments carried on for many 
years Mr. Darwin demonstrated the great value of cross- 
fertilisation in increasing the rapidity of growth, the strength 
and vigour of the plant, and in adding to its fertility. This 
effect is produced immediately, not as he expected would be 
the case, after several generations of crosses. He planted seeds 
from cross-fertilised and self-fertilised plants on two sides of 
the same pot exposed to exactly similar conditions, and in 
most cases the difference in size and vigour was amazing, 
while the plants from cross -fertilised parents also produced 
more and finer seeds. These experiments entirely confirmed 
the experience of breeders of animals already referred to 
(p. 160), and led him to enunciate his famous aphorism, 

1 For a complete historical .account of this subject with full references to 
all the works upon it, see the Introduction to Hermann Miiller's Fertilisation 
of Flowers, translated by D'Arcy W. Thompson. 


" Nature abhors perpetual self -fertilisation. 1 In this principle 
we appear to have a sufficient reason for the various con- 
trivances by which so many flowers secure cross-fertilisation, 
either constantly or occasionally. These contrivances are so 
numerous, so varied, and often so highly complex and extra- 
ordinary, that they have formed the subject of many elaborate 
treatises, and have also been amply popularised in lectures 
and handbooks. It will be unnecessary, therefore, to give 
details here, but the main facts will be summarised in order 
to call attention to some difficulties of the theory which seem 
to require further elucidation. 

Modes of securing Cross-Fertilisation. 

When we examine the various modes in which the cross- 
fertilisation of flowers is brought about, we find that some are 
comparatively simple in their operation and needful adjust- 
ments, others highly complex. The simple methods belong to 
four principal classes : (1) By dichogamy that is, by the 
anthers and the stigma becoming mature or in a fit state for 
fertilisation at slightly different times on the same plant. The 
result of this is that, as plants in different stations, on different 
soils, or exposed to different aspects flower earlier or later, the 
mature pollen of one plant can only fertilise some plant 
exposed to somewhat different conditions or of different con- 
stitution, whose stigma will be mature at the same time ; and 
this difference has been shown by Darwin to be that which is 
adapted to secure the fullest benefit of cross-fertilisation. 
This occurs in Geranium pratense, Thymus serpyllum, Arum 
maculatum, and many others. (2) By the flower being 
self-sterile with its own pollen, as in the crimson flax. This 
absolutely prevents self -fertilisation. (3) By the stamens and 
anthers being so placed that the pollen cannot fall upon the 
stigma, while it does fall upon a visiting insect which carries 
it to the stigma of another flower. This effect is produced in 
a variety of very simple ways, and is often aided by the 
motion of the stamens which bend down out of the way of 
the stigmas before the pollen is ripe, as in Malva sylvcstris 
(see Fig. 28). (4) By the male and female flowers being on 

1 For the full detail of his experiments, see Cross- and fielf- Fertilisation 
of Plants, 1876. 




different plants, forming the class Dioccia of Linnaeus. In these 
cases the pollen may bo carried to the stigmas either by the 
wind or by the agency of insects. 

Now these four methods are all apparently very simple, 
and easily produced by varia- 
tion and selection. They are 
applicable to flowers of any 
shape, requiring only such size 
and colour as to attract insects, 
and some secretion of nectar 
to ensure their repeated visits, 
characters common to the great 
majority of flowers. All these 
methods are common, except 
perhaps the second ; but there 
arc many flowers in which the 
pollen from another plant is 
prepotent over the pollen from 
the same flower, and this has nearly the same effect as self- 
sterility if the flowers are frequently crossed by insects. We 
cannot help asking, therefore, why have other and much 
more elaborate methods been needed ? And how have the 
more complex arrangements of so many flowers been brought 
about ? Before attempting to answer these questions, and in 
order that the reader may appreciate the difficulty of the 
problem and the nature of the facts to be explained, it will be 
necessary to give a summary of the more elaborate modes of 

Malva sylvostris, 
adapted for insect- 

Maha rot undi folia, 
adapted lor selt- 

securing cross-fertilisation. 

(1) We first have dimorphism and heteromorphism, the 
phenomena of which have been already sketched in our 
seventh chapter. 

Here we have both a mechanical and a physiological 
modification, the stamens and pistil being variously modified 
in length and position, while the different stamens in the same 
flower have widely different degrees of fertility when applied 
to the same stigma, a phenomenon which, if it were not so 
well established, would have appeared in the highest degree 
improbable. The most remarkable case is that of the three 
different forms of the loosestrife (Ly thrum salicaria) here 
figured (Fig. 29 on next page). 




(2) Some flowers have irritable stamens which, when their 

bases are touched by an insect, spring up and dust it with 
pollen. This occurs in our common berberry. 


(3) In others there are levers or processes by which the 
anthers are mechanically brought down on to the head or 
back of an insect entering the flower, in such a position as to 
be carried to the stigma of the next flower it visits. This 
may be well seen in many species of Sal via and Erica. 

(4) In some there is a sticky secretion which, getting on 
to the proboscis of an insect, carries away the pollen, and 
applies it to the stigma of another flower. This occurs in our 
common milkwort (Poly gala vulgaria). 

(5) In papilionaceous plants there are many complex ad- 
justments, such as the squeezing out of pollen from a 
receptacle on to an insect, as in Lotus corniculatus, or the 
sudden springing out and exploding of the anthers so as 
thoroughly to dust the insect, as in Medicago falcata, this 
occurring after the stigma has touched the insect and taken 
off some pollen from the last flower. 

(G) Some flowers or spathes form closed boxes in which 
insects find themselves entrapped, arid when they have fertilised 
the flower, the fringe of hairs opens and allows them to escape. 
This occurs in many species of Arum and Aristolochia. 

(7) Still more remarkable are the traps in the flower of 
Asclepias which catch flies, butterflies, and wasps by the legs, 
and the wonderfully complex arrangements of the orchids. 
One of these, our common Orchis pyramidalis, may be briefly 
described to show how varied and beautiful are the arrange- 
ments to secure cross-fertilisation. The broad trifid lip of 
the flower offers a support to the moth which is attracted 
by its sweet odour, and two ridges at the base guide the 
proboscis with certainty to the narrow entrance of the 
nectary. When the proboscis has reached the end of the 
spur, its basal portion depresses the little hinged rostcllum 
that covers the saddle -shaped sticky glands to which the 
pollen masses (pollinia) are attached. On the proboscis 
being withdrawn, the two pollinia stand erect and parallel, 
firmly attached to the proboscis. In this position, however, 
they would bo useless, as they would miss the stigmatic 
surface of the next flower visited by the moth. But as 
soon as the proboscis is withdrawn, the two pollen masses 
begin to diverge till they are exactly as far apart as are the 
stigmas of tho flower ; and then commences a second move- 




A. Fi-nnl, 

FlO. 30.- Orchis pyramidal is. 

miller. I r . . rostellum I I' . guiding ridges on labcllum. 

stigma. I I . . labcllum or lip. | n . nectary. 

with all the sepals and petals removed, except the labellnm. 
Side view, with all the, sepals and petals removed and the upper part <>!' 
The two pollinia attached to the saddle-shaped viscid disc. 
The disc after the first act of contraction. 
The disc seen from above with one polliniutn removed. 
The pollinia removed by the insertion of a needle into the nectary. 
The same pollinia alter d''pivssion has la]] place. 

Hie llnuvr 


merit which brings them clown till they project straight for- 
ward nearly at right angles to their first position, so as exactly 
to hit against the stigmatic surfaces of the next flower visited 
on which they leave a portion of their pollen. The whole of 
these motions take about half a minute, and in that time the 
moth will usually have flown to another plant, and thus effect 
the most beneficial kind of cross-fertilisation. 1 This descrip- 
tion will be better understood by referring to the' illustration 
opposite, from Darwin's Fertilisation of Orchids (Fig. 30). 

The Intnyvctaium of these Farts. 

Having thus briefly indicated the general character of the 
more complex adaptations for cross-fertilisation, the details of 
which are to be found in any of the numerous works on the 
subject, 2 we find ourselves confronted with the very puzzling 
question Why were these innumerable highly complex 
adaptations produced, when the very same result may be 
effected and often is effected by extremely simple means ? 
Supposing, as we must do, that all flowers were once of 
simple and regular forms, like a buttercup or a rose, how 
did such irregular and often complicated flowers as the 
papilionaceous or pea family, the labiates or sage family, and 
the infinitely varied and fantastic orchids ever come into ex- 
istence ? No cause has yet been suggested but the need of 
attracting insects to cross-fertilise them ; yet the attractive- 
ness of regular flowers with bright colours and an ample 
supply of nectar is equally great, and cross-fertilisation can be 
quite as effectively secured in these by any of the four simple 
methods already described. Before attempting to suggest a 
possible solution of this difficult problem, we have yet to pass 
in review a large body of curious adaptations connected with 
insect fertilisation, and will first call attention to that portion 
of the phenomena which throw some light upon the special 
colours of flowers in their relation to the various kinds of 
insects which visit them. For these facts we are largely in- 

1 See Darwin's Fertilisation of OrcMds for the many extraordinary and 
complex arrangements in these plants. 

2 The English reader may consult Sir John Lubbock's British Wild 
Flmfiers in Relation to Insects, and If. Muller's great and original work, The 
Fertilisation of Flowers. 


debted to the exact and long-continued researches of Professor 
Hermann Miiller. 

Summary of Additional Facts bearing on Insect Fertilisation. 

1. That the size and colour of a flower are importi.nt 
factors in determining the visits of insects, is shown by the 
general fact of more insects visiting conspicuous than incon- 
spicuous flowers. As a single instance, the handsome Geranium 
palustre was observed by Professor Miiller to be visited by 
sixteen different species of insects, the equally showy G. 
pratense by thirteen species, while the smaller and much 
less conspicuous G. molle was visited by eight species, and 
G. pusillum by only one. In many cases, however, a flower 
may be very attractive to only a few species of insects ; and 
Professor Miiller states, as the result of many years' assiduous 
observation, that " a species of flower is the more visited by 
insects the more conspicuous it is." 

2. Sweet odour is usually supplementary to the attraction of 
colour. Thus it is rarely present in the largest and most gaudily 
coloured flowers which inhabit open places, such as poppies, 
pneonies, sunflowers, and many others; while it is often the 
accompaniment of inconspicuous flowers, as the mignonette ; of 
such as grow in shady places, as the violet and primrose ; and 
especially of white or yellowish flowers, as the white jasmine, 
clematis, stephanotis, etc. 

3. White flowers are often fertilised by moths, and very 
frequently give out their scent only by night, as in our butterfly- 
orchis (Habenaria chlorantha); arid they sometimes open only at 
night, as do many of the evening primroses and other flowers. 
These flowers are often long tubed in accordance with the 
length of the moths' probosces, as in the genus Pancratium, 
our butterfly orchis, white jasmine, and a host of others. 

4. Bright red flowers are very attractive to butterflies, and 
are sometimes specially adapted to be fertilised by them, as 
in many pinks (Dianthus deltoides, D. superbus, D. atrorubens), 
the corn-cockle (Lychnis Githago), and many others. Blue 
flowers are especially attractive to bees and other hymenoptera 
(though they frequent flowers of all colours), no less than sixty- 
seven species of this order having been observed to visit the 
common "sheep's -bit" (Jasione montana). Dull yellow or 


brownish flowers, some of which smell like carrion, are 
attractive to flies, as the Arum and Aristolochia ; while the 
dull purplish flowers of the Scrophularia are specially attrac- 
tive to wasps. 

5. Some flowers have neither scent nor nectar, and yet 
attract insects by sham nectaries ! In the herb-paris (Paris 
quadrifolia) the ovary glistens as if moist, and flies alight on it 
and carry away pollen to another flower ; while in grass of 
parnassus (Parnassia palustris) there are a number of small 
stalked yellow balls near the base of the flower, which look 
like drops of honey but are really dry. In this case there is 
a little nectar lower down, but the special attraction is a 
sham ; and as there are fresh broods of insects every year, it 
takes time for them to learn by experience, and thus enough 
are always deceived to effect cross -fertilisation. 1 This is 
analogous to the case of the young birds, which have to learn 
by experience the insects that are inedible, as explained at 
page 253. 

6. Many flowers change their colour as soon as fertilised ; 
and this is beneficial, as it enables bees to avoid wasting time 
in visiting those blossoms which have been already fertilised 
and their nectar exhausted. The common lungwort (Pul- 
monaria officinalis), is at first red, but later turns blue ; and 
II. Miiller observed bees visiting many red flowers in 
succession, but neglecting the blue. In South Brazil there 
is a species of Lantana, whose flowers are yellow the first day, 
orange the second, and purple the third ; and Dr. Fritz 
Miiller observed that many butterflies visited the yellow 
flowers only, some both the yellow and the orange flowers, 
but none the purple. 

7. Many flowers have markings which serve as guides to 
insects ; in some cases a bright central eye, as in the borage 
and forget-me-not ; or lines or spots converging to the centre, 
as in geraniums, pinks, and many others. This enables 
insects to go quickly and directly to the opening of the 
flower, and is equally important in aiding them to obtain a 
better supply of food, and to fertilise a larger number of 

8. Flowers have been specially adapted to the kinds of 

1 Miiller's Fertilisation of Flmvers, p. 248. 


insects that most abound where they grow. Thus the gentians 
of the lowlands are adapted to bees, those of the high alps to 
butterflies only ; and while most species of Khinanthus (a 
genus to which our common " yellow rattle " belongs) are bee- 
flowers, one high alpine species (K. alpinus) has been also 
adapted for fertilisation by butterflies only. The reason of 
this is, that in the high alps butterflies are immensely more 
plentiful than bees, and flowers adapted to bo fertilised by 
bees can often have their nectar extracted by butterflies 
without effecting cross -fertilisation. It is, therefore, im- 
portant to have a modification of structure which shall make 
butterflies the fertilisers, and this in many cases has been done. 1 
9. Economy of time is very important both to the insects 
and the flowers, because the fine working days are com- 
paratively few, and if no time is wasted the bees will get 
more honey, and in doing so will fertilise more flowers. Now, 
it has been ascertained by several observers that many insects, 
bees especially, keep to one kind of flower at a time, visiting 
hundreds of blossoms in succession, and passing over other 
species that may be mixed with them. They thus acquire 
quickness in going at once to the nectar, and the change of 
colour in the flower, or incipient withering when fertilised, 
enables them to avoid those flowers that have already had 
their honey exhausted. It is probably to assist the insects in 
keeping to one flower at a time, which is of vital importance 
to the perpetuation of the species, that the flowers which 
bloom intermingled at the same season are usually very dis- 
tinct both in form and colour. In the sandy districts of 
Surrey, in the early spring, the copses are gay with three 
flowers the primrose, the wood -anemone, and the lesser 
celandine, forming a beautiful contrast, while at the same 
time the purple and the white dead-nettles abound on hedge 
banks. A little later, in the same copses, we have the blue 
wild hyacinth (Scilla nutans), the red campion (Lychnis 
dioica), the pure white great starwort (Stellaria Holosteum), 
and the yellow dead-nettle (Lamium Galeobdolon), all distinct 
and well-contrasted flowers. In damp meadows in summer 
we have the ragged robin (Lychnis Floscuculi), the spotted 
orchis (0. maculata), and the yellow rattle (lihinanthus 

1 " Alpenblumen," by D. H. Miiller. See Nature, vol. xxiii. p. 333. 


Crista-galli) ; while in drier meadows we have cowslips, 
ox-eye daisies, and buttercups, all very distinct both in form 
and colour. So in cornfields we have the scarlet poppies, the 
purple corn-cockle, the yellow corn -mary gold, and the blue 
cornflower; while on our moors the purple heath and the 
dwarf gorse make a gorgeous contrast. Thus the difference 
of colour which enables the insect to visit with rapidity and 
unerring aim a number of flowers of the same kind in suc- 
cession, serves to adorn our meadows, banks, woods, and 
heaths with a charming variety of floral colour and form at 
each season of the year. 1 

Fertilisation of Flowers ly Lirch. 

In the temperate regions of the Northern Hemisphere, 
insects are the chief agents in cross-fertilisation when this is 
not effected by the wind ; but in warmer regions, and in the 
Southern hemisphere, birds are found to take a considerable 
part in the operation, and have in many cases led to modifi- 
cations in the form and colour of flowers. Each part of the 
globe has special groups of birds which are flower-haunters. 
America has the humming-birds (Trochilid^e), and the smaller 
group of the sugar-birds (Carebi(he). In the Eastern tropics 
the sun-birds (Nectarineida3) take the place of the humming- 
birds, and another small group, the flower-peckers (Dicrcidrc), 
assist them. In the Australian region there are also two 
flower-feeding groups, the Meliphagidjr, or honey -suckers, 
and the brush -tongued lories (Trichoglossidie). Recent re- 
searches by American naturalists have shown that many 
flowers are fertilised by humming -birds, such as passion- 
flowers, trumpet -flowers, fuchsias, and lobelias ; while some, 
as the Sal via splendens of Mexico, are specially adapted to 
their visits. Wo may thus perhaps explain the number of 
very large tubular flowers in the tropics, such as the huge 
brugmansias and bignonias ; while in the Andes and in 

1 Tliis peculiarity of local distribution of colour in flowers may be com- 
pared, as regards its purpose, with the recognition colours of animals. Just 
as these latter colours enable the sexes to recognise each other, and thus avoid 
sterile unions of distinct species, so the distinctive form and colour of each 
species of flower, as compared with those that usually grow around it, enables 
the fertilising insects to avoid carrying the pollen of one flower to the stigma 
of a distinct species. 



Chile, where humming-birds are especially plentiful, we find 
great numbers of red tubular flowers, often of large size and 
apparently adapted to these little creatures. Such are the 
beautiful Lapageria and Philesia, the grand Pitcairneas, and 
the genera Fuchsia, Mitraria, Embothrium, Escalloriia, Desfon- 
tainea, Eccremocarpus, and many Gesneracese. Among the 
most extraordinary modifications of flower structure adapted 

FIG. 31. Humming-bird fertilising Marcgravia nepenthoides. 

to bird fertilisation are the species of Marcgravia, in which the 
pedicels and bracts of the terminal portion of a pendent bunch 
of flowers have been modified into pitchers which secrete 
nectar and attract insects, while birds feeding on the nectar, 
or insects, have the pollen of the overhanging flowers dusted 
on their backs, and, carrying it to other flowers, thus cross- 
fertilise them (see Illustration). 

In Australia and New Zealand the fine " glory peas " 
(Clianthus), the Sophora, Lorarithus, many Kpacride.'w smd 
Myrtacese, and the large flowers of the New Zealand flax 


(Phorrnium tenax), are cross-fertilised by birds ; while in Natal 
the fine trumpet -creeper (Tecoma capensis) is fertilised by 

The great extent to which insect and bird agency is 
necessary to flowers is well shown by the case of New 
Zealand. The entire country is comparatively poor in species 
of insects, especially in bees and butterflies which are the 
chief flower fertilisers ; yet according to the researches of 
local botanists no less than one -fourth of all the flowering 
plants are incapable of self-fertilisation, and, therefore, wholly 
dependent on insect or bird agency for the continuance of 
the species. 

The facts as to the cross-fertilisation of flowers which have 
now been very briefly summarised, taken in connection with 
Darwin's experiments proving the increased vigour and fer- 
tility given by cross -fertilisation, seem amply to justify his 
aphorism that " Nature abhors self-fertilisation," and his more 
precise statement, that, "No plant is perpetually self -fertil- 
ised ; " and this view has been upheld by Hildebrand, Delpino, 
and other botanists. 1 

Self -Fertilisation of Flowers. 

But all this time we have been only looking at one side of 
the question, for there exists an abundance of facts which 
seem to imply, just as surely, tjje utter uselcssncss of cross- 
fertilisation. Let us, then, see what these facts are before pro- 
ceeding further. 

1. An immense variety of plants are habitually self-fer- 
tilised, and their numbers probably far exceed those which 
are habitually cross-fertilised by insects. Almost all the very 
small or obscure flowered plants with hermaphrodite flowers 
are of this kind. Most of these, however, may be insect 
fertilised occasionally, and may, therefore, come under the rule 
that no species are perpetually self-fertilised. 

2. There are many plants, however, in which special 
arrangements exist to secure self-fertilisation. Sometimes the 
corolla closes and brings the anthers and stigma into contact ; 
in others the anthers cluster round the stigmas, both maturing 
together, as in many buttercups, stitch wort (Stellaria media), 

1 See II. Muller's Fertilisation of Flowers, p. 18. 


sandwort (Spergula), and some willow-herbs (Epilobium) ; or 
they arch over the pistil, as in Galium aparine and Alisma 
Plantago. The style is also modified to bring it into contact 
with the anthers, as in the dandelion, groundsel, and many 
other plants. 1 All these, however, may be occasionally cross- 

3. In other cases precautions are taken to prevent cross- 
fertilisation, as in the numerous cleistogamous or closed flowers. 
These occur in no less than fifty-five different genera, belonging 
to twenty-four natural orders, and in thirty- two of these genera 
the normal flowers are irregular, and have therefore been 
specially modified for insect fertilisation. 2 These flowers appear 
to be degradations of the normal flowers, arid are closed up by 
various modifications of the petals or other parts, so that it is 
impossible for insects to reach the interior, yet they produce 
seed in abundance, and are often the chief means by which 
the species is continued. Thus, in our common dog-violet the 
perfect flowers rarely produce seed, while the rudimentary 
cleistogamic flowers do so in abundance. The sweet violet also 
produces abundance of seed from its cleistogamic flowers, and 
few from its perfect flowers ; but in Liguria it produces only 
perfect flowers which seed abundantly. No case appears to 
be known of a plant which has cleistogamic flowers only, but 
a small rush (Juncus bufonius) is in this condition in some 
parts of Russia, while in other parts perfect flowers are also 
produced. 3 Our common henbit dead-nettle (Lamium amplex- 
icaule) produces cleistogamic flowers, as do also some orchids. 
The advantage gained by the plant is great economy of 
specialised material, since with very small flowers and very 
little expenditure of pollen an abundance of seed is produced. 

4. A considerable number of plants which have evidently 
been specially modified for insect fertilisation have, by further 

1 The above examples are taken from Rev. G. Henslow's paper on " Self- 
Fertilisation of Plants," in Trans. Linn. Soc. Second series, Botany, vol. i. 
pp. 317-398, with plate. Mr. H. 0. Forbes has shown that the same thing 
occurs among tropical orchids, in his paper " On the Contrivances for insuring 
Self- Fertilisation in some Tropical Orchids, " Journ. Linn. Soc., xxi. p. 538. 

a These are the numbers given by Darwin, but I am informed by Mr. 
Hemsley that many additions have been since made to the list, and that 
cleistogamic flowers probably occur in nearly all the natural orders. 

8 For a full account of cleistogamic flowers, see Darwin's Forms of Flower a % 
chap. viii. 


modification, become quite self-fertile. This is the case with 
the garden-pea, and also with our beautiful bee-orchis, in which 
the pollen-masses constantly fall on to the stigmas, and the 
flower, being thus self -fertilised, produces abundance of capsules 
and of seed. Yet in many of its close allies insect agency is 
absolutely required ; but in one of these, the fly-orchis, com- 
paratively very little seed is produced, and self-fertilisation 
would therefore be advantageous to it. When garden-peas 
were artificially cross-fertilised by Mr. Darwin, it seemed to do 
them no good, as the seeds from these crosses produced less 
vigorous plants than seed from those which were self -fertilised ; 
a fact directly opposed to what usually occurs in cross-fer- 
tilised plants. 

5. As opposed to the theory that there is any absolute need 
for cross-fertilisation, it has been urged by Mr. Henslow and 
others that many self -fertilised plants are exceptionally vigorous, 
such as groundsel, chickweed, sow-thistle, buttercups, and other 
common weeds ; while most plants of world- wide distribution 
are self -fertilised, and these have proved themselves to be best 
fitted to survive in the battle of life. More than fifty species 
of common British plants are very widely distributed, and all 
are habitually self-fertilised. 1 That self -fertilisation has some 
great advantage is shown by the fact that it is usually the 
species which have the smallest and least conspicuous flowers 
which have spread widely, while the large and showy flowered 
species of the same genera or families, which require insects to 
cross-fertilise them, have a much more limited distribution. 

6. It is now believed by some botanists that many in- 
conspicuous and imperfect flowers, including those that are 
wind-fertilised, such as plantains, nettles, sedges, and grasses, 
do not represent primitive or undeveloped forms, but are 
degradations from more perfect flowers which were once 
adapted to insect fertilisation. In almost every order we find 
some plants which have become thus reduced or degraded for 
wind or self -fertilisation, as Poterium and Sanguisorba among 
the Eosacese ; while this has certainly been the case in the 
cleistogamic flowers. In most of the above-mentioned plants 
there are distinct rudiments of petals or other floral organs, 

1 Henslow's "Self- Fertilisation," Tran*. Linn. Soc. Second series, Botany, 
vol. i. p. 391. 


and as the chief use of these is to attract insects, they could 
hardly have existed in primitive flowers. 1 We know, moreover, 
that when the petals cease to be required for the attraction of 

1 The Rev. George Henslow, in his Origin of Floral Structures, says : 
" There is little doubt but that all wind-fertilised angiosperms are degradations 
from insect - fertilised flowers. . . . Poterium sanguisorba is anemophilous ; 
and Sanguisorba qfficinalis presumably was so formerly, but has reacquired 
an entomophilous habit ; the whole tribe Poteriese being, in fact, a degraded 
group which has descended from Potentillese. Plantains retain their corolla 
but in a degraded form. Juncese are degraded Lilies ; while Cyperacete and 
Gramineae among monocotyledons may be ranked with Amentifcra among 
dicotyledons, as representing orders which have retrograded very far from 
the entomophilous forms from which they were possibly and probably de- 
scended" (p. 266). 

"The genus Plantago, like Thcdictrum minus, Poterium, and others, well 
illustrate the change from an entomophilous to the anemophilous state. 
P. lanceolate has polymorphic flowers, and is visited by pollen-seeking insects, 
so that it can be fertilised either by insects or the wind. P. media illustrates 
transitions in point of structure, as the filaments are pink, the anthers 
motionless, and the pollen grains aggregated, and it is regularly visited by 
Bombus Urrestris. On the other hand, the slender filaments, versatile anthers, 
powdery pollen, and elongated protogynous style are features of other species 
indicating anemophily ; while the presence of a degraded corolla shows its 
ancestors to have been entomophilous. P. media, therefore, illustrates, not 
a primitive entomophilous condition, but a return to it ; just as is the case 
with Sanguisorba ojficinalis and Salix Caprea ; but these show no capacity of 
restoring the corolla, the attractive features having to be borne by the calyx, 
which is purplish in Sanguisorba, by the pink filaments of Plantago, and by 
the yellow anthers in the Sallow willow " (p. 271). 

" The interpretation, then, I would offer of inconspicuousness and all kinds 
of degradations is the exact opposite to that of conspicuousness and great 
differentiations ; namely, that species with miwite flowers, rarely or never 
visited by insects, and habitually self-fertilised, have primarily arisen through 
the neglect of insects, and have in consequence assumed their present floral 
structures " (p. 282). 

In a letter just received from Mr. Henslow, h<? gives a few additional^ 
illustrations of his views, of which the following are the most important : 
"Passing to Incomplete, the orders known collectively as ' Cyclospermeas ' 
are related to Caryophyllere ; and to my. mind are degradations from it, of 
which Orache is anemophilous. Cupuliferae have an inferior ovary and rudi- 
mentary calyx-limb on the top. These, 'as far as I know, cannot be inter- 
preted except as degradations. The whole of Monocotyledons appear to me 
(from anatomical reasons especially) to be degradations from Dicotyledons, 
and primarily through the agency of growth in water. Many subsequently 
became terrestrial, but retained the effects of their primitive habitat through 
heredity. The 3-merous perianth of grasses, the parts of the flower being in 
whorls, point to a degradation from a sub-liliaceous condition." 

Mr. Henslow informs me that he has long held these views, but, as far as 
he knows, alone. Mr. Grant Allen, however, set forth a similar theory in his 
Vignettes from Nature (p. 15) and more fully in The Colours of Flowers 
(chap, v.), where he develops it fully and uses similar arguments to those of 
Mr. Henslow. 


insects, they rapidly diminish in size, lose their "bright colour 
or almost wholly disappear 1 

Difficulties and Contradictions. 

The very bare summary that has now been given of the 
main facts relating to the fertilisation of flowers, will have 
served to show the vast extent and complexity of the inquiry, 
and the extraordinary contradictions and difficulties which it 
presents. We have direct proof of the beneficial results of 
intercrossing in a great number of cases ; we have an over- 
whelming mass of facts as to the varied and complex structure 
of flowers evidently adapted to secure this intercrossing by 
insect agency ; yet we see many of the most vigorous plants 
which spread widely over the globe, with none of these 
adaptations, and evidently depending on self-fertilisation for 
their continued existence and success in the battle of life. 
Yet more extraordinary is it to find numerous cases in which 
the special arrangements for cross-fertilisation appear to have 
been a failure, since they have either been supplemented by 
special means for self-fertilisation, or have reverted back in 
various degrees to simpler forms in which self-fertilisation 
becomes the rule. There is also a further difficulty in the 
highly complex modes by which cross-fertilisation is often 
brought about ; for we have seen that there are several very 
effective yet very simple modes of securing intercrossing, 
involving a minimum of change in the form and structure of 
the flower; and when we consider that the result attained 
with so much cost of structural modification is by no means 
an unmixed good, and is far less certain in securing the per- 
petuation of the species than is self-fertilisation, it is most 
puzzling to find such complex methods resorted to, some- 
times to the extent of special precautions against the possi- 
bility of self-fertilisation ever taking place. Let us now see 
whether any light can be thrown on these various anomalies 
and contradictions. 

Intercrossing not necessarily Advantageous. 

No one was more fully impressed than Mr. Darwin with 
the beneficial effects of intercrossing on the vigour and fertility 

1 H. Mhller crives amnle tiroof of this in his FfrfiJiJtatimt nf Wlnitvrx. 


of the species or race, yet he clearly saw that it was not 
always and necessarily advantageous. He says : " The most 
important conclusion at which I have arrived is, that the mere 
act of intercrossing by itself does no good. The good 
depends on the individuals which are crossed differing slightly 
in constitution, owing to their progenitors having been sub- 
jected during several generations to slightly different con- 
ditions. This conclusion, as we shall hereafter see, is closely 
connected with various important physiological problems, such 
as the benefit derived from slight changes in the conditions of 
life." 1 Mr. Darwin has also adduced much direct evidence 
proving that slight changes in the conditions of life are 
beneficial to both animals and plants, maintaining or restoring 
their vigour arid fertility in the same way as a favourable 
cross seems to restore it. 2 It is, I believe, by a careful 
consideration of these two classes of facts that we shall find 
the clue to the labyrinth in which this subject has appeared 
to involve us. 

Supposed Evil Results of Close Interbreeding. 

Just as we have seen that intercrossing is not necessarily 
good, we shall be forced to admit that close interbreeding is 
not necessarily bad. Our finest breeds of domestic animals 
have been thus produced, and by a careful statistical inquiry 
Mr. George Darwin has shown that the most constant and 
long- continued intermarriages among the British aristocracy 
have produced no prejudicial results. The rabbits on Porto 
Santo are all the produce of a single female ; they have lived 
on the same small island for 470 years, and they still abound 
there and appear to be vigorous and healthy (see p. 161). 

We have, however, on the other hand, overwhelming 
evidence that in many cases, among our domestic animals and 
cultivated plants, close interbreeding does produce bad results, 
and the apparent contradiction may perhaps be explained on 
the same general principles, and under similar limitations, as 
were found to be necessary in defining the value of inter- 
crossing. It appears probable, then, that it is not inter- 
breeding in itself that is hurtful, but interbreeding without 

1 Cross- and Self-Fertilisation, p. 27. 

2 Animals and Plants, vol. ii. p. 145. 


rigid selection or some change of conditions. Under nature, 
as in the case of the Porto Santo rabbits, the rapid increase of 
these animals would in a very few years stock the island with 
a full population, and thereafter natural selection would act 
powerfully in the preservation only of the healthiest and the 
most fertile, and under these conditions no deterioration 
would occur. Among the aristocracy there has been a 
constant selection of beauty, which is generally synonymous 
with health, while any constitutional infertility has led to the 
extinction of the family. With domestic animals the selec- 
tion practised is usually neither severe enough nor of the 
right kind. There is no natural struggle for existence, but 
certain points of form and colour characteristic of the breed 
are considered essential, and thus the most vigorous or the 
most fertile are riot always those which are selected to 
continue the stock. In nature, too, the species always extends 
over a larger area and consists of much greater numbers, and 
thus a difference of constitution soon arises in different parts 
of the area, which is wanting in the limited numbers of pure 
bred domestic animals. From a consideration of these varied 
facts we conclude that an occasional disturbance of the organic 
equilibrium is what is essential to keep up the vigour and 
fertility of any organism, and that this disturbance may be 
equally well produced cither by a cross between individuals 
of somewhat different constitutions, or by occasional slight 
changes in the conditions of life. Now plants which have 
great powers of dispersal enjoy a constant change of con- 
ditions, and can, therefore, exist permanently, or at all events, 
for very long periods, without intercrossing; while those 
which have limited powers of dispersal, and are restricted to 
a comparatively small and uniform area, need an occasional 
cross to keep up their fertility and . general vigour. Wo 
should, therefore, expect that those groups of plants which are 
adapted both for cross- and self-fertilisation, which have showy 
flowers and possess great powers of seed-dispersal, would be 
the most abundant and most widely distributed ; and this we 
find to be the case, the Composite possessing all these charac- 
teristics in the highest degree, and being the most generally 
abundant group of plants with conspicuous flowers in all parts 
of the world, 


How the Struggle for Existence Ads among Flowers. 

Let us now consider what will be the action of the struggle 
for existence under the conditions we have seen to exist. 

Everywhere and at all times some species of plants will be 
dominant and aggressive ; while others will be diminishing in 
numbers, reduced to occupy a smaller area, and generally 
having a hard struggle to maintain themselves. Whenever 
a self-fertilising plant is thus reduced in numbers it will be 
in danger of extinction, because, being limited to a small 
area, it will suffer from the effects of too iiniform conditions 
which will produce weakness and infertility. But while this 
change is in progress, any crosses between individuals of 
slightly different constitution will be beneficial, and all varia- 
tions favouring either insect agency on the one hand, or 
wind-dispersal of pollen on the other, will lead to the pro- 
duction of a somewhat stronger and more fertile stock. In- 
creased size or greater brilliancy of the flower, more abundant 
nectar, sweeter odour, or adaptations for more effectual cross- 
fertilisation would all be preserved, and thus would be initiated 
some form of specialisation for insect agency in cross -fertil- 
isation ; and in every different species so circumstanced the 
result would be different, depending as it would on many 
and complex combinations of variation of parts of the flower, 
and of the insect species which most abounded in the district. 

Species thus favourably modified might begin a new era 
of development, and, while spreading over a somewhat wider 
area, give rise to new varieties or species, all adapted in 
various degrees and modes to secure cross -fertilisation by 
insect agency. But in course of ages some change of condi- 
tions might prove adverse. Either the insects required might 
diminish in numbers or be attracted by other competing 
flowers, or a change of climate might give the advantage 
to other more vigorous plants. Then self -fertilisation with 
greater means of dispersal might be more advantageous ; the 
flowers might become smaller and more numerous ; the seeds 
smaller and lighter so as to be more easily dispersed by the 
wind, while some of the special adaptations for insect fertilis- 
ation being useless would, by the absence of selection and by 
the law of economy of growth, be reduced to a rudimentary 


form. With these modifications the species might extend its 
range into new districts, thereby obtaining increased vigour 
by the change of conditions, as appears to have been the case 
with so many of the small flowered self-fertilised plants. Thus 
it might continue to exist for a long scries of ages, till under 
other changes geographical or biological it might again 
suffer from competition or from other adverse circumstances, 
and be at length again confined to a limited area, or reduced 
to very scanty numbers. 

But when this cycle of change had taken place, the species 
would be very different from the original form. The flower 
would have been at one time modified to favour the visits 
of insects and to secure cross -fertilisation by their aid, and 
when the need for this passed away, some portions of these 
structures would remain, though in a reduced or rudi- 
mentary condition. But when insect agency became of 
importance a second time, the new modifications would 
start from a different or more advanced basis, and thus a 
more complex result might be produced. Owing to the 
unequal rates at which the reduction of the various parts 
might occur, some amount of irregularity in the flower might 
arise, and on a second development towards insect cross- 
fertilisation this irregularity, if useful, might be increased by 
variation and selection. 

The rapidity and comparative certainty with which such 
changes as are here supposed do really take place, are well 
shown by the great differences in floral structure, as regards 
the mode of fertilisation, in allied genera and species, and even 
in some cases in varieties of the same species. Thus in the 
Ranunculacete we find the conspicuous part of the flower to be 
the petals in Ranunculus, the sepals in Helleborus, Anemone, 
etc., and the stamens in most species of Thalictrum. In all 
these we have a simple regular flower, but in Aquilegia it is 
made complex by the spurred petals, and in Delphinium and 
Acoriitum it becomes quite irregular. In the more simple class 
self-fertilisation occurs freely, but it is prevented in the more 
complex flowers by the stamens maturing before the pistil 
In the Caprifoliacese wo havo small and regular greenish 
flowers, as in the moschatel (Adoxa) ; more conspicuous regular 
open flowers without honey, as in the elder (Sambucus) ; and 


tubular flowers increasing in length and irregularity, till in 
some, like our common honeysuckle, they are adapted for 
fertilisation by moths only, with abundant honey and 
delicious perfume to attract them. In the Scrophulariaceac 
we find open, almost regular flowers, as Veronica and 
Verbascum, fertilised by flies and bees, but also self -fertilised ; 
Scrophularia adapted in form and colour to be fertilised by 
wasps; and the more complex and irregular flowers of 
Linaria, Rhi nan thus, Melampyrum, Pedicularis, etc., mostly 
adapted to be fertilised by bees. 

In the genera Geranium, Polygonum, Veronica, and several 
others there is a gradation of forms from large and bright 
to small and obscure coloured flowers, and in every case the 
former are adapted for insect fertilisation, often exclusively, 
while in the latter self -fertilisation constantly occurs. In the 
yellow rattle (Rhinanthus Crista-galli) there are two forms 
(which have been named major and minor\ the larger and 
more conspicuous adapted to insect fertilisation only, the 
smaller capable of self-fertilisation ; and two similar forms exist 
in the eyebright (Euphrasia officinalis). In both these cases 
there are special modifications in the length and curvature 
of the style as well as in the size and shape of the corolla ; 
and the two forms are evidently becoming each adapted to 
special conditions, since in some districts the one, in other 
districts the other is most abundant. 1 

These examples show us that the kind of change suggested 
above is actually going on, and has presumably always been 
going on in nature throughout the long geological epochs 
during which the development of flowers has been progressing. 
The two great modes of gaining increased vigour and fertility 
intercrossing and dispersal over wider areas have been 
resorted to again and again, under the pressure of a constant 
struggle for existence and the need for adaptation to ever- 
changing conditions. During all the modifications that ensued, 
useless parts were reduced or suppressed, owing to the absence 
of selection and the principle of economy of growth ; and thus 
at each fresh adaptation some rudiments of old structures were 

1 Muller's Fertilisation of Flowers, pp. 448, 455. Other cases of recent 
degradation and readaptation to insect -fertilisation are given by Professor 
Henslow (see footnote, p. 324). 


re -developed, but not uiifrequently in a different form and for 
a distinct purpose. 

The chief types of flowering plants have existed during the 
millions of ages of the whole tertiary period, and during this 
enormous lapse of time many of them may have been modified 
in the direction of insect fertilisation, and again into that of 
self-fertilisation, not once or twice only, but perhaps scores or 
even hundreds of times ; and at each such modification a 
difference in the environment may have led to a distinct 
line of development. At one epoch the highest specialisation 
of structure in adaptation to a single species or group of insects 
may have saved a plant from extinction ; while, at other times, 
the simplest mode of self-fertilisation, combined with greater 
powers of dispersal and a constitution capable of supporting 
diverse physical conditions, may have led to a similar result. 
With some groups the tendency seems to have been almost 
continuously to greater and greater specialisation, while with 
others a tendency to simplification and degradation has resulted 
in such plants as the grasses arid sedges. 

We are now enabled dimly to perceive how the curious 
anomaly of very simple and very complex methods of securing 
cross-fertilisation both equally effective may have been 
brought about. The simple modes may be the result of a 
comparatively direct modification from the more primitive 
types of flowers, which were occasionally, and, as it were, 
accidentally visited and fertilised by insects ; while the more 
complex modes, existing for the most part in the highly irregular 
flowers, may result from those cases in which adaptation to 
insect-fertilisation, and partial or complete degradation to self- 
fertilisation or to wind -fertilisation, have again and again 
recurred, each time producing some additional complexity, 
arising from the working up of old rudiments for new pur- 
poses, till there have been reached the marvellous flower 
structures of the papilionaceous tribes, of the asclepiads, or of 
the orchids. 

We thus see that the existing diversity of colour and of 
structure in flowers is probably the ultimate result of the 
ever-recurring struggle for existence, combined with the ever- 
changing relations between the vegetable and animal kingdoms 
during countless ages. The constant variability of every part 


and organ, with the enormous powers of increase possessed by 
plants, have enabled them to become again and again readjusted 
to each change of condition as it occurred, resulting in that 
endless variety, that marvellous complexity, and that ex- 
quisite colouring which excite our admiration in the realm of 
flowers, and constitute them the perennial charm and crowning 
glory of nature. 

Flowers the Product of Insect Agency. 

In his Origin of Species, Mr. Darwin first stated that 
flowers had been rendered conspicuous and beautiful in order 
to attract insects, adding : " Hence we may conclude that, if 
insects had not been developed on the earth, our plants would 
not have been decked with beautiful flowers, but would have 
produced only such poor flowers as we see on our fir, oak, nut, 
and ash trees, on grasses, docks, and nettles, which are all 
fertilised through the agency of the wind." The argument in 
favour of this view is now much stronger than when he wrote ; 
for not only have we reason to believe that most of these 
wind-fertilised flowers are degraded forms of flowers which 
have once been insect fertilised, but we have abundant evidence 
that whenever insect agency becomes comparatively ineffective, 
the colours of the flowers become less bright, their size and 
beauty diminish, till they are reduced to such small, greenish, 
inconspicuous flowers as those of the rupture-wort (Herniaria 
glabra), the knotgrass (Polygonum aviculare), or the cleisto- 
gamic flowers of the violet. There is good reason to believe, 
therefore, not only that flowers have been developed in order 
to attract insects to aid in their fertilisation, but that, having 
been once produced, in however great profusion, if the insect 
races were all to become extinct, flowers (in the temperate 
zoiies at all events) would soon dwindle away, and that 
ultimately all floral beauty would vanish from the earth. 

We cannot, therefore, deny the vast change which insects 
have produced upon the earth's surface, and which has been 
thus forcibly and beautifully delineated by Mr. Grant Allen : 
" While man has only tilled a few level plains, a few great river 
valleys, a few peninsular mountain slopes, leaving the vast mass 
of earth untouched by his hand, the insect has spread himself 
over every land in a thousand shapes, and has made the whole 


flowering creation subservient to his daily wants. His butter- 
cup, his dandelion, and his meadow-sweet grow thick in every 
English field. His thyme clothes the hillside; his heather 
purples the bleak gray moorland. High up among the alpine 
heights his gentian spreads its lakes of blue ; amid the snows 
of the Himalayas his rhododendrons gleam with crimson light. 
Even the wayside pond yields him the white crowfoot and the 
arrowhead, while the broad expanses of Brazilian streams are 
beautified by his gorgeous water-lilies. The insect has thus 
turned the whole surface of the earth into a boundless flower- 
garden, which supplies him from year to year with pollen or 
honey, and itself in turn gains perpetuation by the baits that 
it oilers for his allurement." l 

Concluding Remarks on Colour in Nature. 

In the last four chapters I have endeavoured to give a 
general and systematic, though necessarily condensed view of 
the part which is played by colour in the organic world. We 
have seen in what infinitely varied ways the need of conceal- 
ment has led to the modification of animal colours, whether 
among polar snows or sandy deserts, in tropical forests or in 
the abysses of the ocean. We next find these general adapta- 
tions giving way to more specialised types of coloration, 
by which each species has become more and more harmonised 
with its immediate surroundings, till we reach the most 
curiously minute resemblances to natural objects in the leaf 
and stick insects, and those which are so like flowers or moss 
or 'birds' droppings that they deceive the acutest eye. We 
have learnt, further, that these varied forms of protective 
colouring are far more numerous than has been usually sus- 
pected, because, what appear to be very conspicuous colours 
or markings when the species is observed in a museum or in 
a menagerie, are often highly protective when the creature is 
seen under the natural conditions of its existence. From 
these varied classes of facts it seems not improbable that 
fully one-half of the species in the animal kingdom possess 
colours which have been more or less adapted to secure for 
them concealment or protection. 

Passing onward we find the explanation of a distinct type 
1 The Colour Sense, by Grant Allen, p. 95. 


of colour or marking, often superimposed upon protective 
tints, in the importance of easy recognition by many animals 
of their fellows, their parents, or their mates. By this need 
we have been able to account for markings that seem calcu- 
lated to make the animal conspicuous, when the general tints 
and well-known habits of the whole group demonstrate the 
need of concealment. Thus also we are able to explain the 
constant symmetry in the markings of wild animals, as well as 
the numerous cases in which the conspicuous colours are con- 
cealed when at rest and only become visible during rapid motion. 

In striking contrast to ordinary protective coloration we 
have " warning colours," usually very conspicuous and often 
brilliant or gaudy, which serve to indicate that their possess- 
ors are either dangerous or uneatable to the usual enemies 
of their tribe. This kind of coloration is probably more 
prevalent than has been hitherto supposed, because in. the 
case of many tropical animals we are quite unacquainted with 
their special and most dangerous enemies, and are also un- 
able to determine whether they are or are not distasteful to 
those enemies. As a kind of corollary to the "warning 
colours/' we find the extraordinary phenomena of " mimicry," 
in which defenceless species obtain protection by being mis- 
taken for those which, from any cause, possess immunity from 
attack. Although a large number of instances of warning 
colour and of mimicry are now recorded, it is probably still 
an almost unworked field of research, more especially in 
tropical regions and among the inhabitants of the ocean. 

The phenomena of sexual diversities of coloration next 
engaged our attention, and the reasons why Mr. Darwin's theory 
of " sexual selection," as regards colour and ornament, could 
not be accepted were stated at some length, together with 
the theory of animal coloration and ornament we propose 
to substitute for it This theory is held to be in harmony 
with the general facts of animal coloration, while it entirely 
dispenses with the very hypothetical and inadequate agency 
of female choice in producing the detailed colours, patterns, 
and ornaments, which in so many cases distinguish the male 

If my arguments on this point are sound, they will dispose 
also of Mr. Grant Allen's view of the direct action of the 


colour sense on the animal integuments. 1 He argues that the 
colours of insects and birds reproduce generally the colours of 
the flowers they frequent or the fruits they eat, and he 
adduces numerous cases in which flower-haunting insects and 
fruit-eating birds are gaily coloured. This he supposes to be 
due to the colour -taste, developed by the constant presence 
of bright flowers and fruits, being applied to the selection of 
each variation towards brilliancy in their mates ; thus in time 
producing the gorgeous and varied hues they now possess. 
Mr. Allen maintains that "insects are bright where bright 
flowers exist in numbers, and dull where flowers are rare or 
inconspicuous ; " and he urges that " we can hardly explain this 
wide coincidence otherwise than by supposing that a taste 
for colour is produced through the constant search for food 
among entomophilous blossoms, and that this taste has reacted 
upon its possessors through the action of unconscious sexual 

The examples Mr. Allen quotes of bright insects being 
associated with bright flowers seem very forcible, but are 
really deceptive or erroneous ; and quite as many cases could 
be quoted which prove the very opposite. For example, in 
the dense equatorial forests flowers are exceedingly scarce, 
and there is no comparison with the amount of floral colour 
to be met with in our temperate meadows, woods, and hill- 
sides. The forests about Para in the lower Amazon are 
typical in this respect, yet they abound with the most 
gorgeously coloured butterflies, almost all of which frequent 
tho forest depths, keeping near the ground, where there is the 
greatest deficiency of brilliant flowers. In contrast with this 
let us take the Cape of Good Hope the most flowery region 
probably that exists upon the globe, where the country 
is a complete flower-garden of heaths, pelargoniums, mesembry- 
anthemus, exquisite iridaceous and other bulbs, and numerous 
flowering shrubs arid trees ; yet the Cape butterflies are hardly 
equal, either in number or variety, to those of any country 
in South Europe, and are utterly insignificant when compared 
with those of the comparatively flowerless forest-depths of 
the Amazon or of Now Guinea. Neither is there any relation 
between the colours of other insects and their haunts. Few 
1 The Colour Sense, chap, ix. 


are more gorgeous than some of the tiger-beetles and the 
carabi, yet these are all carnivorous ; while many of the most 
brilliant metallic buprestidas and longicorns are always found 
on the bark of fallen trees. So with the humming-birds ; 
their brilliant metallic tints can only be compared with metals 
or gems, and are totally unlike the delicate pinks and purples, 
yellows and reds of the majority of flowers. Again, the 
Australian honey-suckers (Meliphagidae) are genuine flower- 
haunters, and the Australian flora is more brilliant in colour 
display than that of most tropical regions, yet these birds are, 
as a rule, of dull colours, not superior on the average to our 
grain-eating finches. Then, again, we have the grand pheasant 
family, including the gold and the silver pheasants, the gorgeous 
fire-backed and ocellated pheasants, and the resplendent pea- 
cock, all feeding on the ground on grain or seeds or insects, 
yet adorned with the most gorgeous colours. 

There is, therefore, no adequate basis of facts for this theory 
to rest upon, even if there were the slightest reason to believe 
that not only birds, but butterflies and beetles, take any 
delight in colour for its own sake, apart from the food-supply 
of which it indicates the presence. All that has been proved or 
that appears to be probable is, that they are able to perceive 
differences of colour, and to associate each colour with the 
particular flowers or fruits which best satisfy their wants. 
Colour being in its nature diverse, it has been beneficial for 
them to be able to distinguish all its chief varieties, as mani- 
fested more particularly in the vegetable kingdom, and among 
the different species of their own group ; and the fact that 
certain species of insects show some preference for a particular 
colour may bo explained by their having found flowers of 
that colour to yield them a more abundant supply of nectar 
or of pollen. In those cases in which butterflies frequent 
flowers of their own colour, the habit may well have been 
acquired from the protection it affords them. 

It appears to me that, in imputing to insects and birds the 
same love of colour for its own sake and the same aesthetic 
tastes as we ourselves possess, we may be as far from the truth 
as were those writers who held that the bee was a good mathe- 
matician, and that the honeycomb was constructed throughout 
to satisfy its refined mathematical instincts ; whereas it is now 


generally admitted to be the result of the simple principle of 
economy of material applied to a primitive cylindrical cell. 1 

In studying the phenomena of colour in the organic world 
we have been led to realise the wonderful complexity of the 
adaptations which bring each species into harmonious relation 
with all those which surround it, and which thus link together 
the whole of nature in a network of relations of marvellous 
intricacy. Yet all this is but, as it were, the outward show 
and garment of nature, behind which lies the inner structure 
the framework, the vessels, the cells, the circulating fluids, 
and the digestive and reproductive processes, and behind 
these again those mysterious chemical, electrical, and vital 
forces which constitute what we term Life. These forces 
appear to be fundamentally the same for all organisms, as 
is the material of which all are constructed; and we thus 
find behind the outer diversities an inner relationship which 
binds together the myriad forms of life. 

Each species of animal or plant thus forms part of one 
harmonious whole, carrying in all the details of its complex 
structure the record of the long story of organic development ; 
and it was with a truly inspired insight that our great philo- 
sophical poet apostrophised the humble weed 

Flower in the crannied wall, 

I pluck you out of tlie crannies, 

I hold you here, root and all, in my hand, 

Little flower but if I could understand 

What you are, root and all, and all in all, 

I should know what God and man is. 

See Origin of Species, sixth edition, p. 220. 



The facts to be explained The conditions which have determined dis- 
tribution The permanence of oceans Oceanic and continental areas 
Madagascar and New Zealand The teachings of the thousand- 
fathom line The distribution of marsupials The distribution of 
tapirs Powers of dispersal as illustrated by insular organisms Birds 
and insects at sea Insects at great altitudes The dispersal of plants 
Dispersal of seeds by the wind Mineral matter carried by the wind 
Objections to the theory of wind-dispersal answered Explanation 
of north temperate plants in the southern hemisphere No proof of 
glaciation in the tropics Lower temperature not needed to explain 
the facts Concluding remarks. 

THE theory which we may now take as established that all 
the existing forms of life have been derived from other forms 
by a natural process of descent with modification, and that 
this same process has been in action during past geological 
time should enable us to give a rational account not only of 
the peculiarities of form and structure presented by animals 
and plants, but also of their grouping together in certain 
areas, and their general distribution over the earth's surface. 

In the absence of any exact knowledge 'of the facts of 
distribution, a student of the theory of evolution might 
naturally anticipate that all groups of allied organisms would 
be found in the same region, and that, as he travelled farther 
and farther from any given centre, the forms of life would 
differ more and more from those which prevailed at the 
starting-point, till, in the remotest regions to which he could 
penetrate, he would find an entirely new assemblage of 
animals and plants, altogether unlike those with which he was 


familiar. He would also anticipate that diversities of climate 
would always be associated with a corresponding diversity in 
the forms of life. 

Now these anticipations are to a considerable extent justi- 
fied. Eemoteness on the earth's surface is usually an indi- 
cation of diversity in the fauna and flora, while strongly 
contrasted climates are always accompanied by a considerable 
contrast in the forms of life. But this correspondence is by 
no means exact or proportionate, and the converse propositions 
are often quite untrue. Countries which are near to each 
other often differ radically in their animal and vegetable pro- 
ductions ; while similarity of climate, together with moderate 
geographical proximity, are often accompanied by marked 
diversities in the prevailing forms of life. Again, while many 
groups of animals genera, families, and sometimes even 
orders are confined to limited regions, most of the families, 
many genera, and even some species are found in every part 
of the earth. An enumeration of a few of these anomalies will 
better illustrate the nature of the problem we have to solve. 

As examples of extreme diversity, notwithstanding geo- 
graphical proximity, we may adduce Madagascar and Africa, 
whose animal and vegetable productions are far less alike than 
are those of Great Britain and Japan at the remotest ex- 
tremities of the great northern continent ; while an equal, or 
perhaps even a still greater, diversity exists between Australia 
and New Zealand. On the other hand, Northern Africa and 
South Europe, though separated by the Mediterranean Sea, 
have faunas and floras which do not differ from each other 
more than do the various countries of Europe. As a proof 
that similarity of climate and general adaptability have had 
but a small part in determining the forms of life in each 
country, we have the fact of the enormous increase of rabbits 
and pigs in Australia and New Zealand, of horses and cattle 
in South America, and of the common sparrow in North 
America, though in none of these cases are the animals 
natives of the countries in which they thrive so well. 
And lastly, in illustration of the fact that allied forms are 
not always found in adjacent regions, we have the tapirs, 
which are found only on opposite sides of the globe, in 
tropical America and the Malayan Islands ; the camels of 


the Asiatic deserts, whose nearest allies are the llamas 
and alpacas of the Andes; and the marsupials, only found 
in Australia and on the opposite side of the globe, in 
America. Yet, again, although mammalia may be said to 
be universally distributed over the globe, being found abund- 
antly on all the continents and on a great many of the larger 
islands, yet they are entirely wanting in New Zealand, and in 
a considerable number of other islands which are, nevertheless, 
perfectly able to support them when introduced. 

Now most of these difficulties can be solved by means of 
well-known geographical and geological facts. When the pro- 
ductions of remote countries resemble each other, there is 
almost always continuity of land with similarity of climate 
between them. When adjacent countries differ greatly in 
their productions, we find them separated by a sea or strait 
whose great depth is an indication of its antiquity or per- 
manence. When a group of animals inhabits two coun- 
tries or regions separated by wide oceans, it is found that 
in past geological times the same group was much more 
widely distributed, and may have reached the countries it 
inhabits from an intermediate region in which it is now extinct. 
We know, also, that countries now united by land were 
divided by arms of the sea at a not very remote epoch ; while 
there is good reason to believe that others now entirely 
isolated by a broad expanse of sea were formerly united and 
formed a single land area. There is also another important 
factor to be taken account of in considering how animals and 
plants have acquired their present peculiarities of distribution, 
changes of climate. We know that quite recently a glacial 
epoch extended over much of what are now the temperate 
regions of the northern hemisphere, and that consequently 
the organisms which inhabit those parts must be, com- 
paratively speaking, recent immigrants from more southern 
lands. But it is a yet more important fact that, down to 
middle Tertiary times at all events, an equable temperate 
climate, with a luxuriant vegetation, extended to far within 
the arctic circle, over what are now barren wastes, covered 
for ten months of the year with snow and ice. The arctic 
zone has, therefore, been in past times capable of supporting 
almost all the forms of life of our temperate regions ; and wo 


must take account of this condition of things whenever we 
have to speculate on the possible migrations of organisms 
between the old arid new continents. 

The Conditions which have determined Distribution. 

When we endeavour to explain in detail the facts of the 
existing distribution of organic beings, we are confronted by 
several preliminary questions, upon the solution of which will 
depend our treatment of the phenomena presented to us. 
Upon the theory of descent which we have adopted, all the 
different species of a genus, as well as all the genera which 
compose a family or higher group, have descended from some 
common ancestor, and must therefore, at some remote epoch, 
have occupied the same area, from which their descendants 
have spread to the regions they now inhabit. In the numerous 
cases in which the same group now occupies countries separated 
by oceans or seas, by lofty mountain-chains, by wide deserts, 
or by inhospitable climates, we have to consider how the 
migration which must certainly have taken place has been 
effected. It is possible that during some portion of the time 
which has elapsed since the origin of the group the inter- 
posing barriers have not been in existence ; or, on the other 
hand, the particular organisms we are dealing with may have 
the power of overpassing the barriers, and thus reaching their 
present remote dwelling-places. As this is really the funda- 
mental question of distribution on which the solution of all 
its more difficult problems depends, wo have to inquire, in the 
first place, what is the nature of, and what are the limits to, the 
changes of the earth's surface, especially during the Tertiary 
and latter part of the Secondary periods, as it was during those 
periods that most of the existing types of the higher animals 
and plants came into existence ; and, in the next place, what 
are the extreme limits of the powers of dispersal possessed by 
the chief groups of animals and plants. We will first consider 
the question of barriers, more especially those formed by seas 

and oceans. 

The Permanence of Oceans. 

It was formerly a very general belief, even amongst 
geologists, that the great features of the earth's surface, no less 
than the smaller ones, were subject to continual mutations, 


and that during the course of known geological time the 
continents and great oceans had again and again changed 
places with each other. Sir Charles Lyell, in the last edition 
of his Principles of Geology (1872), said: "Continents, there- 
fore, although permanent for whole geological epochs, shift 
their positions entirely in the course of ages ; " and this may 
be said to have been the orthodox opinion down to the very 
recent period when, by means of deep-sea soundings, the nature 
of the ocean bottom was made known. The first person to 
throw doubt on this view appears to have been the veteran 
American geologist, Professor Dana. In 1849, in the Report 
of Wilke's Exploring Expedition, he adduced the argument 
against a former continent in the Pacific during the Tertiary 
period, from the absence of all native quadrupeds. In 1856, 
in articles in the American Journal, he discussed the develop- 
ment of the American continent, and argued for its general 
permanence ; and in his Manual of Geology in 1863 and later 
editions, the same views were more fully enforced and were 
latterly applied to all continents. Darwin, in his Journal of 
Researches, published in 1845, called attention to the fact that 
all the small islands far from land in the Pacific, Indian, and 
Atlantic Oceans are either of coralline or volcanic formation. 
He excepted, however, the Seychelles and St. Paul's rocks ; 
but the former have since been shown to be no exception, as 
they consist entirely of coral rock ; and although Darwin 
himself spent a few hours on St. Paul's rocks on his outward 
voyage in the Beagle, and believed he had found some 
portions of them to be of a " cherty," and others of a 
" felspathic " nature, this also has been shown to be erroneous, 
and the careful examination of the rocks by the Abb6 Bernard 
clearly proves them to be wholly of volcanic origin. 1 We 
have, therefore, at the present time, absolutely no exception 
whatever to the remarkable fact that all the oceanic islands of 
the globe are either of volcanic or coral formation ; and there 
is, further, good reason to believe that those of the latter class 
in every case rest upon a volcanic foundation. 

In his Origin of Species, Darwin further showed that no 
true oceanic island had any native mammals or batrachia 

1 See A. Agassiz, Three Cruises of the Blake (Cambridge, Mass,, 1888), 
voL i. p. 127, footnote. 


when first discovered, this fact constituting the test of the 
class to which an island belongs ; whence he argued that none 
of them had ever been connected with continents, but all had 
originated in mid-ocean. These considerations alone render 
it almost certain that the areas now occupied by the great 
oceans have never, during known geological time, been 
occupied by continents, since it is in the highest degree im- 
probable that every fragment of those continents should have 
completely disappeared, and have buen replaced by volcanic 
islands rising out of profound oceanic abysses; but recent 
research into the depth of the oceans and the nature of the 
deposits now forming on their floors, adds greatly to the 
evidence in this direction, and renders it almost a certainty 
that they represent very ancient if riot primaeval features of 
the earth's surface. A very brief outline of the nature of this 
evidence will be now given. 

The researches of the Clwllcnger expedition into the 
nature of the sea -bottom show, that the whole of the land 
debris brought down by rivers to the ocean (with the ex- 
ception of pumice and other floating matter), is deposited 
comparatively near to the shores, and that the fineness of the 
material is an indication of the distance to which it has been 
carried. Everything in the nature of gravel and sand is laid 
down within a very few miles of land, only the finer muddy 
sediments being carried out for 20 or 50 miles, and 
the very finest of all, under the most favourable conditions, 
rarely extending beyond 150, or at the utmost, 300 miles 
from land into the deep ocean. 1 Beyond these distances, and 
covering the entire ocean floor, are various oozes formed wholly 
from the debris of marine organisms ; while intermingled with 
these are found various volcanic products which have been 
either carried through the air or floated on the surface, and a 
small but perfectly recognisable quantity of meteoric matter. 
Ice-borne rocks are also found abundantly scattered over the 
ocean bottom within a definite distance of the arctic and 
antarctic circles, clearly marking out the limit of floating ice- 
bergs in recent geological times. 

1 Even the extremely fine Mississippi mud is nowhere found beyond a 
hundred miles from the mouths of the river in the Gulf of Mexico (A. Agassiz, 
Three Cruises of the Blake, vol. i. p. 128). 


Now the whole series of marine stratified rocks, from the 
earliest Palaeozoic to the most recent Tertiary beds, consist of 
materials closely corresponding to the land debris now being 
deposited within a narrow belt round the shores of all con- 
tinents; while no rocks have been found which can bo identified 
with the various oozes now forming in the deep abysses of the 
ocean. It follows, therefore, that all the geological formations 
have been formed in comparatively shallow water, and always 
adjacent to the continental land of the period. The great 
thickness of some of the formations is no indication of a deep 
sea, but only of slow subsidence during the time that the 
deposition was in progress. This view is now adopted by 
many of the most experienced geologists, especially by Dr. 
Archibald Geikie, Director of the Geological Survey of Great 
Britain, who, in his lecture on " Geographical Evolution," says : 
" From all this evidence we may legitimately conclude that 
the present land of the globe, though consisting in great 
measure of marine formations, has never lain under the deep 
sea ; but that its site must always have been near land. Even 
its thick marine limestones are the deposits of comparatively 
shallow water." l 

But besides these geological and physical considerations, 
there is a mechanical difficulty in the way of repeated 
change of position of oceans and continents which has not 
yet received the attention it deserves. According to the 
recent careful estimate by Mr. John Murray, the land area 
of the globe is to the water area as *28 to *72. The mean 
height of the land above sea-level is 2250 feet, while the 
mean depth of the ocean is 14,640 feet. Hence the bulk 
of dry land is 23,450,000 cubic miles, and that of the waters 
of the ocean 323,800,000 cubic miles; and it follows that if 
the whole of the solid matter of the earth's surface were 
reduced to one level, it would be everywhere covered by an 
ocean about two miles deep. The accompanying diagram will 
serve to render these figures more intelligible. The length of 
the sections of land and ocean are in the proportion of their 
respective areas, while the mean height of the land and the 
mean depth of the ocean are exhibited on a greatly increased 

1 I have given a full summary of the evidence for the permanence of 
oceanic and continental areas in my Island L\fe, chap, vi. 


vertical scale. If we considered the continents and their 
adjacent oceans separately they would differ a little, but not 
very materially, from this diagram ; in some cases the propor- 
tion of land to ocean would be a little greater, in others a little 

Now, if we try to imagine a process of elevation and 
depression by which the sea and land shall completely change 
places, we shall be met by insuperable difficulties. We must, 
in the first place, assume a general equality between ele- 
vation and subsidence during any given period, because if 
the elevation over any extensive continental area were not 
balanced by some subsidence of approximately equal amount, 

Diagram of proportionate mean height of Land and depth of Oceans. 


Area. -28 of area 

Area. -72 of area of Globe. 

Fio. 32. 

an unsupported hollow would be left under the earth's crust. 
Let us now suppose a continental area to sink, and an adjacent 
oceanic area to rise, it will be seen that the greater part of 
the land will disappear long before the new land has approached 
the surface of the ocean. This difficulty will not be removed 
by supposing a portion of a continent to subside, and the 
immediately adjacent portion of the ocean on the other side 
of the continent to rise, because in almost every case we find 
that within a comparatively short distance from the shores of 
all existing continents, the ocean floor sinks rapidly to a depth 
of from 2000 to 3000 fathoms, and maintains a similar depth, 
generally speaking, over a large portion of the oceanic areas. 
In order, therefore, that any area of continental extent be 
upraised from the great oceans, there must be a subsidence of 
a land area five or six times as great, unless it can be shown 
that an extensive elevation of the ocean floor up to and far 


above the surface could occur without an equivalent depression 
elsewhere. The fact that the waters of the ocean are sufficient 
to cover the whole globe to a depth of two miles, is alone 
sufficient to indicate that the great ocean basins are permanent 
features of the earth's surface, since any process of alterna- 
tion of these with the land areas would have been almost 
certain to result again and again in the total disappearance of 
large portions, if not of all, of the dry land of the globe. But 
the continuity of terrestrial life since the Devonian and Car- 
boniferous periods, and the existence of very similar forms in 
the corresponding deposits of every continent as well as the 
occurrence of sedimentary rocks, indicating the proximity of 
land at the time of their deposit, over a large portion of the 
surface of all the continents, and in every geological period 
assure us that no such disappearance has ever occurred. 

Oceanic and Continental Areas. 

When we speak of the permanence of oceanic and conti- 
nental areas as one of the established facts of modern research, 
we do not mean that existing continents and oceans have 
always maintained the exact areas and outlines that they now 
present, but merely, that while all of them have been under- 
going changes in outline and extent from age to age, they 
have yet maintained substantially the same positions, and 
have never actually changed places with each other. There 
are, moreover, certain physical and biological facts which 
enable us to mark out these areas with some confidence. 

We have seen that there are a large number of islands 
which may be classed as oceanic, because they have never 
formed parts of continents, but have originated in mid-ocean, 
and have derived their forms of life by migration across the 
sea. Their peculiarities are seen to bo very marked in com- 
parison with those islands which there is good reason to 
believe are really fragments of more extensive land areas, and 
are hence termed " continental." These continental islands 
consist in every case of a variety of stratified rocks of various 
ages, thus corresponding closely with the usual structure of 
continents ; although many of the islands are small like 
Jersey or the Shetland Islands, or far from continental 
land like the Falkland Islands or New Zealand. They all 


contain indigenous mammalia or batrachia, and generally a 
much greater variety of birds, reptiles, insects, and plants, 
than do the oceanic islands. From these various character- 
istics we conclude that they have all once formed parts of 
continents, or at all events of much larger land areas, and have 
become isolated, either by subsidence of the intervening land 
or by the effects of long-continued marine denudation. 

Now, if we trace the thousand-fathom line around all our 
existing continents we find that, with only two exceptions, 
every island which can be classed as " continental " falls 
within this line, while all that lie beyond it have the un- 
doubted characteristics of " oceanic " islands. We, therefore, 
conclude that the thousand-fathom lino marks out, approxi- 
mately, the "continental area," that is, the limits within 
which continental development and change throughout known 
geological time have gone on. There may, of course, have 
been some extensions of land beyond this limit, while some 
areas within it may always have been ocean ; but so far as 
we have any direct evidence, this lino may be taken to mark 
out, approximately, the most probable boundary between the 
"continental area," which has always consisted of land and 
shallow sea in varying proportions, and the great oceanic 
basins, within the limits of which volcanic activity has been 
building up numerous islands, but whose profound depths 
have apparently undergone little change. 

Madagascar and New Zealand. 

The two exceptions just referred to are Madagascar and 
New Zealand, and all the evidence goes to show that in these 
cases the land connection with the nearest continental area 
was very remote in time. The extraordinary isolation of the 
productions of Madagascar almost all the most characteristic 
forms of mammalia, birds, and reptiles of Africa being 
absent from it renders it certain that it must have been 
separated from that continent very early in the Tertiary, if 
not as far back as the latter part of the Secondary period ; 
and this extreme antiquity is indicated by a depth of 
considerably more than a thousand fathoms in the Mozam- 
bique Channel, though this deep portion is less than a 
hundred miles wide between the Comoro Islands and the main- 


land. 1 Madagascar is the only island on the globe with a fairly 
rich mammalian fauna which is separated from a continent by 
a depth greater than a thousand fathoms ; and no other island 
presents so many peculiarities in these animals, or has pre- 
served so many lowly organised and archaic forms. The 
exceptional character of its productions agrees exactly with its 
exceptional isolation by means of a very deep arm of the sea. 
New Zealand possesses no known mammals and only a 
single species of batrachian ; but its geological structure is 
perfectly continental. There is also much evidence that it 
does possess one mammal, although no specimens have been 
yet obtained. 2 Its reptiles and birds are highly peculiar and 
more numerous than in any truly oceanic island. Now the 
sea which directly separates New Zealand from Australia is 
more than 2000 fathoms deep, but in a north-west direction 
there is an extensive bank under 1000 fathoms, extending to 
and including Lord Howe's Island, while north of this are 
other banks of the same depth, approaching towards a sub- 
marine extension of Queensland on the one hand, and New 
Caledonia on the other, and altogether suggestive of a land 
union with Australia at some very remote period. Now the 
peculiar relations of the New Zealand fauna and flora with 
those of Australia and of the tropical Pacific Islands to the 
northward indicate such a connection, probably during the 
Cretaceous period ; and here, again, we have the exceptional 
depth of the dividing sea and the form of the ocean bottom 
according well with the altogether exceptional isolation of 
New Zealand, an isolation which has been held by some 
naturalists to be great enough to justify its claim to be one 
of the primary Zoological Regions. 

TJie Teachings of the Thousand-Fathom Line. 

If now we accept the annexed map as showing us approxi- 
mately how far beyond their present limits our continents may 

1 For a full account of the peculiarities of the Madagascar fauna, see my 
Island Life, chap. xix. 

2 See Island Life, p. 446, and the whole of chaps, xxi. xxii. More 
recent soundings have shown that the Map at p. 448, as well as that of the 
Madagascar group at p. 387, are erroneous, the ocean around Norfolk Island 
and in the Straits of Mozambique being more than 1000 fathoms deep. 
The general argument is, however, unaffected. 


have extended during any portion of the Tertiary and Secondary 
periods, we shall obtain a foundation of inestimable value for 
our inquiries into those migrations of animals and plants 
during past ages which have resulted in their present peculi- 
arities of distribution. We see, for instance, that the South 
American and African continents have always been separated 
by nearly as wide an ocean as at present, and that whatever 
similarities there may be in their productions mast be due to 
the similar forms having been derived from a common origin 
in one of the great northern continents. The radical difference 
between the higher forms of life of the two continents accords 
perfectly with their permanent separation. If there had been 
any direct connection between them during Tertiary times, we 
should hardly have found the deep-seated differences between 
the Quadrumana of the two regions no family even being 
common to both ; nor the peculiar Insectivora of the one 
continent, and the equally peculiar Edentata of the other. 
The very numerous families of birds quite peculiar to one or 
other of these continents, many of which, by their structural 
isolation and varied development of generic and specific forms, 
indicate a high antiquity, equally suggest that there has been 
no near approach to a land connection during the same epoch. 
Looking to the two great northern continents, we see indica- 
tions of a possible connection between them both in the North 
Atlantic and the North Pacific oceans ; and when we remember 
that from middle Tertiary times backward so far as we know 
continuously to the earliest Palaeozoic epoch a temperate and 
equable climate, with abundant woody vegetation, prevailed 
up to and within the arctic circle, we see what facilities 
may have been afforded for migration from one continent 
to the other, sometimes between America and Europe, some- 
times between America and Asia. Admitting these highly 
probable connections, no bridging of the Atlantic in more 
southern latitudes (of which there is not a particle of evidence) 
will have been necessary to account for all the intermigration 
that has occurred between the two continents. If, on the 
other hand, we remember how long must have been the route, 
and how diverse must always have been the conditions be- 
tween the more northern and the more southern portions of 
the American and Euro -Asiatic continents, we shall not be 


surprised that many widespread forms in either continent 
have not crossed into the other ; and that while the skunks 
(Mephitis), the pouched rats (Saccomyidae), and the turkeys 
(Meleagris) are confined to America, the pigs and the hedge- 
hogs, the true flycatchers and the pheasants are found only 
in the Euro-Asiatic continent. But, just as there have been 
periods which facilitated intermigration between America and 
the Old World, there have almost certainly been periods, 
perhaps of long duration even geologically, when these con- 
tinents have been separated by seas as wide as, or even wider 
than, those of the present day ; and thus may be explained 
such curious anomalies as the origination of the camel-tribe in 
America, and its entrance into Asia in comparatively recent 
Tertiary times, while the introduction of oxen and bears into 
America from the Euro- Asiatic continent appears to have been 
equally recent. 1 

We shall find on examination that this view of the general 
permanence of the oceanic and continental areas, with constant 
minor fluctuations of land and sea over the whole extent of 
the latter, enables us to understand, and offer a rational 
explanation of, most of the difficult problems of geographical 
distribution ; and further, that our power of doing this is in 
direct proportion to our acquaintance with the distribution of 
fossil forms of life during the Tertiary period. We must, also, 
take due note of many other facts of almost equal importance 
for a due appreciation of the problems presented for solution, 
the most essential being, the various powers of dispersal 
possessed by the different groups of animals arid plants, the 
geological antiquity of the species and genera, and the width 
and depth of the seas which separate the countries they 
inhabit. A few illustrations will now be given of the way in 
which these branches of knowledge enable us to deal with the 
difficulties and anomalies that present themselves. 

The Distribution of Marsupials. 

This singular and lowly organised type of mammals con- 
stitutes almost the sole representative of the class in Australia 

1 For some details of these migrations, see the author's Geographical 
Distribution of Animals, vol. i. p. 140 ; also Heilprin's Geographical and 
Geological Distribution of Animals, 


and New Guinea, while it is entirely unknown in Asia, Africa, 
or Europe. It reappears in America, where several species of 
opossums are found; and it was long thought necessary to postu- 
late a direct southern connection of these distant countries, 
in order to account for this curious fact of distribution. When, 
however, we look to what is known of the geological history 
of the marsupials the difficulty vanishes. In the Upper Eocene 
deposits of Western Europe the remains of several animals 
closely allied to the American opossums have been found ; 
and as, at this period, a very mild climate prevailed far up 
into the arctic regions, there is no difficulty in supposing that 
the ancestors of the group entered America from Europe or 
Northern Asia during early Tertiary times. 

But we must go much further back for the origin of the 
Australian marsupials. All the chief types of the higher 
mammalia were in existence in the Eocene, if riot in the preceding 
Cretaceous period, and as we find none of these in Australia, 
that country must have been finally separated from the Asiatic 
continent during the Secondary or Mesozoic period. Now 
during that period, in the Upper and the Lower Oolite and 
in the still older Trias, the jaw-bones of numerous small 
mammalia have been found, forming eight distinct genera, 
which are believed to have been either marsupials or some 
allied lowly forms. In North America also, in beds of the 
Jurassic and Triassic formations, the remains of an equally great 
variety of these small mammalia have been discovered ; and 
from the examination of more than sixty specimens, belonging 
to at least six distinct genera, Professor Marsh is of opinion 
that they represent a generalised type, from which the more 
specialised marsupials and insectivora were developed. 

From the fact that very similar mammals occur both in 
Europe and America at corresponding periods, and in beds 
which represent a long succession of geological time, and that 
during the whole of this time no fragments of any higher 
forms have been discovered, it seems probable that both the 
northern continents (or the larger portion of their area) were 
then inhabited by no other mammalia than these, with 
perhaps other equally low types. It was, probably, not later 
than the Jurassic age when some of these primitive marsu- 
pials were able to enter Australia, where they have since 


remained almost completely isolated ; and, being free from the 
competition of higher forms, they have developed into the 
great variety of types we now behold there. These occupy the 
place, and have to some extent acquired the form and structure 
of distinct orders of the higher mammals the rodents, the 
insectivora, and the carnivora, while still preserving the 
essential characteristics and lowly organisation of the mar- 
supials. At a much later period probably in late Tertiary 
times the ancestors of the various species of rats and mice 
which now abound in Australia, and which, with the aerial bats, 
constitute its only forms of placental mammals, entered the 
country from some of the adjacent islands. For this purpose 
a land connection was not necessary, as these small creatures 
might easily be conveyed among the branches or in the crevices 
Of trees uprooted by floods and carried down to the sea, and 
then floated to a shore many miles distant. That no actual land 
connection with, or very close approximation to, an Asiatic 
island has occurred in recent times, is sufficiently proved by 
the fact that no squirrel, pig, civet, or other widespread 
mammal of the Eastern hemisphere has been able to reach the 
Australian continent. 

The Distribution of Tapirs. 

These curious animals form one of the puzzles of geographi- 
cal distribution, being now confined to two very remote regions 
of the globe the Malay Peninsula and adjacent islands of 
Sumatra and Borneo, inhabited by one species, and tropical 
America, where there are three or four species, ranging from 
Brazil to Ecuador and Guatemala. If we considered these 
living forms only, we should be obliged to speculate on 
enormous changes of land and sea in order that these tropical 
animals might have passed from one country to the other. But 
geological discoveries have rendered all such hypothetical 
changes unnecessary. During Miocene and Pliocene times 
tapirs abounded over the whole of Europe and Asia, their 
remains having been found in the tertiary deposits of France, 
India, Burmah, and China. In both North and South 
America fossil remains of tapirs occur only in caves and de- 
posits of Post -Pliocene age, showing that they are compara- 
tively recent immigrants into that continent. They perhaps 


entered by the route of Kamchatka and Alaska, where the 
climate, even now so much milder and more equable than on 
the north-east of America, might have been warm enough in 
late Pliocene times to have allowed the migration of these 
animals. In Asia they were driven southwards by the 
competition of numerous higher and more powerful forms, 
but have found a last resting-place in the swampy forests of 
the Malay region. 

Wlmt these Facts Prove. 

Now these two cases, of the marsupials and the tapirs, 
are in the highest degree instructive, because they show 
us that, without any hypothetical bridging of deep oceans, 
and with only such changes of sea and land as are indi- 
cated by the extent of the comparatively shallow seas 
surrounding and connecting the existing continents, we are 
able to account for the anomaly of allied forms occurring 
only in remote and widely separated areas. These examples 
really constitute crucial tests, because, of all classes of animals, 
mammalia are least able to surmount physical barriers. They 
are obviously unable to pass over wide arms of the sea, 
while the necessity for constant supplies of food and water 
renders sandy deserts or snow -clad plains equally impass- 
able. Then, again, the peculiar kinds of food on which 
alone many of them can subsist, and their liability to the 
attacks of other animals, put a further check upon their 
migrations. In these respects almost all other organisms 
have great advantages over mammals. Birds can often fly 
long distances, and can thus cross arms of the sea, deserts, or 
mountain ranges; insects not only fly, but are frequently 
carried great distances by gales of wind, as shown by tho 
numerous cases of their visits to ships hundreds of miles from 
land. Reptiles, though slow of movement, have advantages in 
their greater capacity for enduring hunger or thirst, their power 
of resisting cold or drought in a state of torpidity, and they 
have also some facilities for migration across the sea by means 
of their eggs, which may be conveyed in crevices of timber or 
among masses of floating vegetable matter. And when we 
come to the vegetable kingdom, the means of transport are 
at their maximum, numbers of seeds having special adaptations 

2 A 


for being carried by mammalia or birds, and for floating in the 
water, or through the air, while many are so small and so 
light that there is practically no limit to the distances they 
may be carried by gales and hurricanes. 

We may, therefore, feel quite certain that the means of dis- 
tribution that have enabled the larger mammalia to reach the 
most remote regions from a common starting-point, will be at 
least as efficacious, and usually far more efficacious, with all other 
land animals and plants ; and if in every case the existing 
distribution of this class can be explained on the theory of 
oceanic and continental permanence, with the limited changes 
of sea and land already referred to, no valid objections can be 
taken against this theory founded on anomalies of distribution 
in other orders. Yet nothing is more common than for 
students of this or that group to assert that the theory of 
oceanic permanence is quite inconsistent with the distribution 
of its various species and genera. Because a few Indian 
genera and closely allied species of birds are found in Mada- 
gascar, a land termed "Lemuria" has been supposed to have 
united the two countries during a comparatively recent 
geological epoch ; while the similarity of fossil plants and 
reptiles, from the Permian and Miocene formations of India 
and South Africa, has been adduced as further evidence of this 
connection. But there are also genera of snakes, of insects, 
and of plants, common to Madagascar and South America 
only, which have been held to necessitate a direct land 
connection between these countries. These views evidently 
refute themselves, because any such land connections must 
have led to a far greater similarity in the productions of 
the several countries than actually exists, and would besides 
render altogether inexplicable the absence of all the chief 
types of African and Indian mammalia from Madagascar, and 
its marvellous individuality in every department of the organic 
world. 1 

Powers of Dispersal as illustrated by Insular Organisms. 

Having arrived at the conclusion that our existing oceans 
have remained practically unaltered throughout the Tertiary and 
Secondary periods of geology, and that the distribution of the 

1 For a fall discussion of this question, see Island L\fe t pp. 390-420. 


mammalia is such as might have been brought about by their 
known powers of dispersal, and by such changes of land 'and 
sea as have probably or certainly occurred, we are, of course, 
restricted to similar causes to explain the much wider and 
sometimes more eccentric distribution of other classes of 
animals and of plants. In doing so, we, have to rely partly on 
direct evidence of dispersal, afforded by the land organisms 
that have been observed far out at sea, or which have taken 
refuge on ships, as well as by the periodical visitants to remote 
islands ; but very largely on indirect evidence, afforded by 
the frequent presence of certain groups on remote oceanic 
islands, which some ancestral forms must, therefore, have 
reached by transmission across the ocean from distant lands. 


These vary much in their powers of flight, and their 
capability of traversing wide seas and oceans. Many 
swimming and wading birds can continue long on the wing, 
fly swiftly, and have, besides, the power of resting safely 
on the surface of the water. These would hardly be limited 
by any width of ocean, except for the need of food ; and many 
of them, as the gulls, petrels, and divers, find abundance of 
food on the surface of the sea itself. These groups have a 
wide distribution across the oceans ; while waders especially 
plovers, sandpipers, snipes, and herons are equally cos- 
mopolitan, travelling along the coasts of all the continents, 
and across the narrow seas which separate them. Many of 
these birds seem unaffected by climate, and as the organisms 
on which they feed are equally abundant on arctic, temperate, 
and tropical shores, there is hardly any limit to the range 
even of some of the species. 

Land-birds are much more restricted in their range, owing 
to their usually limited powers of flight, their inability to rest 
on the surface of the sea or to obtain food from it, and their 
greater specialisation, which renders them less able to main- 
tain themselves in the new countries they may occasionally 
reach. Many of them are adapted to live only in woods, or 
in marshes, or in deserts ; they need particular kinds of food 
or a limited range of temperature ; and they are adapted to 
cope only with the special enemies or the particular group of 


competitors among which they have been developed. Such 
birds as these may pass again and again to a new country, but 
are never able to establish themselves in it; and it is this 
organic barrier, as it is termed, rather than any physical 
barrier, which, in many cases, determines the presence of a 
species in one area and its absence from another. We must 
always remember, therefore, that, although the presence of a 
species in a remote oceanic island clearly proves that its 
ancestors must at one time have found their way there, the 
absence of a species does not prove the contrary, since it also 
may have reached the island, but have been unable to main- 
tain itself, owing to the inorganic or organic conditions not 
being suitable to it. This general principle applies to all 
classes of organisms, and there are many striking illustrations 
of it. In the Azores there are eighteen species of land-birds 
which are permanent residents, but there are also several 
others which reach the islands almost every year after great 
storms, but have never been able to establish themselves. In 
Bermuda the facts are still more striking, since there are only 
ten species of resident birds, while no less than twenty other 
species of land -birds and more than a hundred species of 
waders and aquatics are frequent visitors, often in great 
numbers, but are never able to establish themselves. On 
the same principle we account for the fact that, of the many 
continental insects and birds that have been let loose, or 
have escaped from confinement, in this country, hardly 
one has been able to maintain itself, and the same pheno- 
menon is still more striking in the case of plants. Of the 
thousands of hardy plants which grow easily in our gardens, 
very few have ever run wild, and when the experiment 
is purposely tried it invariably fails. Thus A. de Candolle 
informs us that several botanists of Paris, Geneva, and 
especially of Montpellier, have sown the seeds of many 
hundreds of species of exotic hardy plants, in what appeared 
to be the most favourable situations, but that in hardly a 
single case has any one of them become naturalised. 1 Still 
more, then, in plants than in animals the absence of a species 
does not prove that it has never reached the locality, but 
merely that it has not been able to maintain itself in com- 

1 Geoyraphie Botanique, p. 798. 


petition with the native productions. In other cases, as we 
have seen, facts of an exactly opposite nature occur. The rat, 
the pig, and the rabbit, the water-cress, the clover, and many 
other plants, when introduced into New Zealand, flourish 
exceedingly, and even exterminate their native competitors; 
so that in these cases we may feel sure that the species in 
question did not exist in New Zealand simply because 
they had been unable to reach that country by their natural 
means of dispersal. I will now give a few cases, in addition 
to those recorded in my previous works, of birds and insects 
which have been observed far from any land. 

Birds and Insects at Sea. 

Captain D. Fullarton of the ship Timaru recorded in his 
log the occurrence of a great number of small land-birds about 
the ship on 15th March 1886, when in Lat. 48 31' N., Long. 
8 1 6' W. He says : " A great many small land-birds about us ; 
put about sixty into a coop, evidently tired out." And two 
days later, 17th March, "Over fifty of the birds cooped on 
15th died, though fed. Sparrows, finches, water- wagtails, two 
small birds, name unknown, one kind like a linnet, and a largo 
bird like a starling. In all there have been on board over 
seventy birds, besides some that hovered about us for some 
time and then fell into the sea exhausted." Easterly winds 
and severe weather were experienced at the time. 1 The spot 
where this remarkable flight of birds was met with is about 
160 miles due west of Brest, and this is the least distance the 
birds must have been carried. It is interesting to note that 
the position of the ship is nearly in the line from the English 
and French coasts to the Azores, where, after great storms, so 
many bird stragglers arrive annually. These birds were prob- 
ably blown out to sea during their spring migration along the 
south coast of England to Wales and Ireland. During the 
autumnal migration, however, great flocks of birds especially 
starlings, thrushes, and fieldfares have been observed every 
year flying out to sea from the west coast of Ireland, almost 
the whole of which must perish. At the Nash Lighthouse, in 
the Bristol Channel on the coast of Glamorganshire, an enormous 
number of small birds were observed on 3d September, includ- 
1 Nature, 1st April 1886. 


ing nightjars, buntings, white-throats, willow-wrens, cuckoos, 
house -sparrows, robins, wheatears, and blackbirds. These 
had probably crossed from Somersetshire, and had they been 
caught by a storm the larger portion of them must have been 
blown out to sea. 1 

These facts enable us to account sufficiently well for the 
birds of oceanic islands, the number and variety of which are 
seen to be proportionate to their facilities for reaching the 
island and maintaining themselves in it. Thus, though more 
birds yearly reach Bermuda than the Azores, the number of 
residents in the latter islands is much larger, due to the 
greater extent of the islands, their number, and their more 
varied surface. In the Galapagos the land-birds are still more 
numerous, due in part to their larger area and greater proxi- 
mity to the continent, but chiefly to the absence of storms, 
so that the birds which originally reached the islands have 
remained long isolated and have developed into many closely 
allied species adapted to the special conditions. All the 
species of the Galapagos but one are peculiar to the islands, 
while the Azores possess only one peculiar species, and 
Bermuda none a fact which is clearly due to the continual 
immigration of fresh individuals keeping up the purity of 
the breed by intercrossing. In the Sandwich Islands, which 
are extremely isolated, being more than 2000 miles from 
any continent or large island, we have a condition of things 
similar to what prevails in the Galapagos, the land -birds, 
eighteen in number, being all peculiar, and belonging, except 
one, to peculiar genera. These birds have probably all 
descended from three or four original types which reached 
the islands at some remote period, probably by means of 
intervening islets that have since disappeared. In St. Helena 
we have a degree of permanent isolation which has pre- 
vented any land-birds from reaching the island ; for although 
its distance from the continent, 1100 miles, is not so great 
as in the case of the Sandwich Islands, it is situated in an 
ocean almost entirely destitute of small islands, while its 
position within the tropics renders it free from violent storms. 
Neither is there, on the nearest part of the coast of Africa, 
a perpetual stream of migrating birds like that which 
1 Report of the Brit. Assoc. Committee on Migration of Birds during 1886. 


supplies the innumerable stragglers which every year reach 
Bermuda and the Azores. 


Winged insects have been mainly dispersed in the same 
way as birds, by their power of flight, aided by violent or long- 
continued winds. Being so small, and of such low specific 
gravity, they are occasionally carried to still greater distances ; 
and thus no islands, however remote, are altogether without 
them. The eggs of insects, being often deposited in borings 
or in crevices of timber, may have been conveyed long 
distances by floating trees, as may the larvae of those species 
which feed on wood. Several cases have been published of 
insects coming on board ships at great distances from land ; 
and Darwin records having caught a large grasshopper when 
the ship was 370 miles from the coast of Africa, whence the 
insect had probably come. 

In the Entomologists Monthly Magazine for June 1885, Mr. 
MacLachlan has recorded the occurrence of a swarm of moths 
in the Atlantic ocean, from the log of the ship Pleione. 
The vessel was homeward bound from New Zealand, and in 
Lat. 6 47' N., Long. 32 50' W., hundreds of moths appeared 
about the ship, settling in numbers on the spars and rigging. 
The wind for four days previously had been very light from 
north, north-west, or north-east, and sometimes calm. The north- 
east trade wind occasionally extends to the ship's position at 
that time of year. The captain adds that "frequently, in 
that part of the ocean, ho has had moths and butterflies 
come on board." The position is 960 miles south-west of 
the Cape Verde Islands, and about 440 north-east of the 
South American coast. The specimen preserved is Deiopeia 
pulchella, a very common species in dry localities in the 
Eastern tropics, and rarely found in Britain, but, Mr. Mac- 
Lachlan thinks, not found in South America. They must 
have come, therefore, from the Capo Verde Islands, or from 
some parts of the African coast, and must have traversed 
about a thousand miles of ocean with the assistance, no doubt, 
of a strong north-east trade wind for a great part of the distance. 
In the British Museum collection there is a specimen of tho 
same moth caught at sea during the voyage of the Rattlesnake, 


in Lat. 6 N., Long. 22 1 W., being between the former position 
and Sierra Leone, thus rendering it probable that the moths 
came from that part of the African coast, in which case the 
swarm encountered by the Pleione must have travelled more 
than 1200 miles. 

A similar case was recorded by Mr. F. A. Lucas in the 
American periodical Science of 8th April 1887. He states 
that in 1870 he met with numerous moths of many species 
while at sea in the South Atlantic (Lat. 25 S., Long. 24 W.), 
about 1000 miles from the coast of Brazil. As this position 
is just beyond the south-east trades, the insects may have been 
brought from the land by a westerly gale. In the Zoologist 
(1864, p. 8920) is the record of a small longicorn beetle which 
flew on board a ship 500 miles off the west coast of Africa. 
Numerous other cases are recorded of insects at less distances 
from land, and, taken in connection with those already given, 
they are sufficient to show that great numbers must be con- 
tinually carried out to sea, and that occasionally they are able 
to reach enormous distances. But the reproductive powers of 
insects are so great that all we require, in order to stock a 
remote island, is that some few specimens shall reach it even 
once in a century, or once in a thousand years. 

Insects at great Altitudes. 

Equally important is the proof we possess that insects are 
often carried to great altitudes by upward currents of air. 
Humboldt noticed them up to heights of 15,000 and 18,000 
feet in South America, and Mr. Albert Miiller has collected many 
interesting cases of the same character in Europe. 1 A moth 
(Plusia gamma) has been found on the summit of Mont Blanc ; 
small hymenoptera and moths have been seen on the Pyrenees 
at a height of 11,000 feet, while numerous flies and beetles, 
some of considerable size, have been caught on the glaciers 
and snow-fields of various parts of the Alps. Upward 
currents of air, whirlwinds and tornadoes, occur in all parts 
of the world, and large numbers of insects are thus carried 
up into the higher regions of the atmosphere, where they 
are liable to be caught by strong winds, and thus conveyed 
enormous distances over seas or continents. With such 
1 Trans. Ent. Soc., 1871, p. 184, 


powerful means of dispersal the distribution of insects over 
the entire globe, and their presence in the most remote 
oceanic islands, offer no difficulties. 

The Dispersal of Plants. 

The dispersal of seeds is effected in a greater variety of 
ways than are available in the case of any animals. Some 
fruits or seed-vessels, and some seeds, will float for many 
weeks, and after immersion in salt water for that period 
the seeds will often germinate. Extreme cases are the double 
cocoa-nut of the Seychelles, which has been found on the coast 
of Sumatra, about 3000 miles distant; the fruits of the 
Sapindus saponaria (soap-berry), which has been brought to 
Bermuda by the Gulf Stream from the West Indies, and has 
grown after a journey in the sea of about 1500 miles ; and the 
West Indian bean, Entada scandens, which reached the Azores 
from the West Indies, a distance of full 3000 miles, and after- 
wards germinated at Kew. By these means we can account 
for the similarity in the shore flora of the Malay Archipelago 
and most of the islands of the Pacific ; and from an examination 
of the fruits and seeds, collected among drift during the voyage 
of the Challenger, Mr. Hemsley has compiled a list of 121 
species which are probably widely dispersed by this means. 

A still larger number of species owe their dispersal to birds 
in several distinct ways. An immense number of fruits in all 
parts of the world are devoured by birds, and have been 
attractively coloured (as we have seen), in order to be so 
devoured, because the seeds pass through the birds' bodies and 
germinate where they fall. We have seen how frequently 
birds are forced by gales of wind across a wide expanse of 
ocean, and thus seeds must be occasionally carried. It is a 
very suggestive fact, that all the trees and shrubs in the Azores 
bear berries or small fruits which are eaten by birds ; while all 
those which bear larger fruits, or are eaten chiefly by mammals 
such as oaks, beeches, hazels, crabs, etc. are entirely 
wanting. Game-birds and waders often have portions of mud 
attached to their feet, and Mr. Darwin has proved by experi- 
ment that such mud frequently contains seeds. One partridge 
had such a quantity of mud attached to its foot as to contain 
seeds from which eighty-two plants germinated ; this proves that 


a very small portion of mud may serve to convey seeds, and 
such an occurrence repeated even at long intervals may greatly 
aid in stocking remote islands with vegetation. Many seeds 
also adhere to the feathers of birds, and thus, again, may be 
conveyed as far as birds are ever carried. Dr. Guppy found 
a small hard seed in the gizzard of a Cape Petrel, taken about 
550 miles east of Tristan da Cunha. 

Dispersal of Seeds by the Wind. 

In the preceding cases we have been able to obtain direct 
evidence of transportal; but although we know that many seeds 
are specially adapted to be dispersed by the wind, we cannot 
obtain direct proof that they are so carried for hundreds or 
thousands of miles across the sea, owing to the difficulty of 
detecting single objects which are so small and inconspicuous. 
It is probable, however, that the wind as an agent of dispersal 
is really more effective than any of those wo have hitherto 
considered, because a very large number of plants have seeds 
which are very small and light, and are often of such a form 
as to facilitate aerial carriage for enormous distances. It is 
evident that such seeds are especially liable to bo transported 
by violent winds, because they become ripe in autumn at the 
time when storms are most prevalent, while they either lie 
upon the surface of the ground, or are disposed in dry capsules 
on the plant ready to be blown away. If inorganic particles 
comparable in weight, size, or form with such seeds are 
carried for great distances, we may be sure that seeds will also 
be occasionally carried in the same way. It will, therefore, 
be necessary to give a few examples of wind-carriage of small 

On 27th July 1875 a remarkable shower of small pieces of 
hay occurred at Monkstown, near Dublin. They appeared 
floating slowly down from a great height, as if falling from a 
dark cloud which hung overhead. The pieces picked up were 
wet, and varied from single blades of grass to tufts weighing 
one or two ounces. A similar shower occurred a few days 
earlier in Denbighshire, and was observed to travel in a 
direction contrary to that of the wind in the lower atmosphere. 1 
There is no evidence of the distance from which the hay was 
1 Nature (1875), vol. xii. pp. 279, 298. 


brought, but as it had been carried to a great height, it was 
in a position to be conveyed to almost any distance by a 
violent wind, had such occurred at the time. 

Mineral Matter carried by the Wind. 

The numerous cases of sand and volcanic dust being carried 
enormous distances through the atmosphere sufficiently prove 
the importance of wind as a carrier of solid matter, but un- 
fortunately the matter collected has not been hitherto examined 
with a view to determine the maximum size and weight of the 
particles. A few facts, however, have been kindly furnished 
me by Professor Judd, F.RS. Some dust which fell at 
Genoa on 15th October 1885, and was believed to have been 
brought from the African desert, consisted of quartz, horn- 
blende, and other minerals, and contained particles having a 
diameter of ^J-^- inch, each weighing -junj-^nnr grain. This 
dust had probably travelled over 600 miles. In the dust from 
Krakatoa, which fell at Batavia, about 100 miles distant, 
during the great eruption, there are many solid particles even 
larger than those mentioned above. Some of this dust was given 
me by Professor Judd, and I found in it several ovoid particles 
of a much larger size, being -fa inch long, and -fa wide and 
deep. The dust from the same eruption, which fell on board 
the ship Arabella, 970 miles from the volcano, also contained 
solid particles T fo inch diameter. Mr. John Murray of the 
Challenger Expedition writes to me that he finds in the deep 
sea deposits 500 and even 700 miles west of the coast of 
Africa, rounded particles of quartz, having a diameter of 
g-jhj- inch, and similar particles are found at equally great 
distances from the south-west coasts of Australia ; and he 
considers these to be atmospheric dust carried to that 
distance by the wind. Taking the sp. gr. of quartz at 
2*6, these particles would weigh about ^fanr grain each. 
These interesting facts can, however, by no means bo taken 
as indicating the extreme limits of the power of wind in 
carrying solid particles. During the Krakatoa eruption 
no gale of special violence occurred, and the region is 
one of comparative calms. The grains of quartz found by 
Mr. Murray more nearly indicate the limit, but the very 
small portions of matter brought up by the dredge, as com- 




pared with the enormous areas of sea-bottom, over which the 
atmospheric dust must have been scattered, render it in the 
highest degree improbable that the maximum limit either of 
size of particles, or of distance from land has been reached. 

Let us, however, assume that the quartz grains, found by 
Mr. Murray in the deep-sea ooze 700 miles from land, give us 
the extreme limit of the power of the atmosphere as a carrier 
of solid particles, and let us compare with these the weights 
of some seeds. From a small collection of the seeds of thirty 



No of Seed* 
in one Gram 




Draba verna 


in. in. in. 

Oval, flat. 


Hypericnm perforatum 
Astilbe rivularis . 



T>*0 Xy^ff 

Elongate, flat, tailed, wavy. 


Saxifraga coriophylla 


&x 7 V 

Surface rough, adhere to the 

dry capsules. 


(Enothera rosea . 





Hyporicum hirsutum 
Mimulus luteus . 



A x iiu 

Cylindrical, rough. 
Oval, minute. 


Penthorum sedoides 



Flattened, very minute. 


Sagina procumbens 



Sub-triangular, flat. 


Orchis maculata 


Margined, flat, very minute. 


Gentiana purpurea 



Wavy, rough, with this cori- 

aceous margins. 


Silene alpina 
Adenophora communis 


A X 5-V 

Flat, with fringed margins. 
Very thin, wavy, light. 

Quartz grains 



Deep sea . . 700 miles. 




Genoa . . . GOO mile. 

species of herbaceous plants sent mo from Kew, those in the 
above table were selected, and small portions of eight of 
them carefully weighed in a chemical balance. 1 By counting 
these portions I was able to estimate the number of seeds 
weighing one grain. Tho three very minute species, whoso 
numbers are marked with an asterisk (*), were estimated by 
the comparison of their sizes with those of the smaller weighed 

If now we compare the seeds with the quartz grains, we 

1 I am indebted to Professor E. Meldola of the Finsbury Technical Inotitute, 
and Rev. T. D. Titmaa of Charterhouse for furnishing me with the weights 


find that several are from twice to three times the weight 
of the grains found by Mr. Murray, and others five times, 
eight times, and fifteen times as heavy ; but they are pro- 
portionately very much larger, and, being usually irregular in 
shape or compressed, they expose a very much larger surface to 
the air. The surface is often rough, and several have dilated 
margins or tailed appendages, increasing friction and rendering 
the uniform rate of failing through still air immensely less 
than in the case of the smooth, rounded, solid quartz grains. 
With these advantages it is a moderate estimate that seeds 
ton times the weight of the quartz grains could be carried 
quite as far through the air by a violent gale and under the 
most favourable conditions. These limits will include five 
of the seeds here given, as well as hundreds of others which do 
not exceed thorn in weight ; and to these we may add some 
larger seeds which have other favourable characteristics, as is 
the case with numbers 11-13, which, though very much larger 
than the rest, are so formed as in all probability to be still more 
easily carried great distances by a gale of wind. It appears, 
therefore, to be absolutely certain that every autumnal gale 
capable of conveying solid mineral particles to great distances, 
must also carry numbers of small seeds at least as far ; and if 
this is so, the wind alone will form one of the most effective 
agents in the dispersal of plants. 

Hitherto this mode of conveyance, as applying to the 
transmission of seeds for great distances across the ocean, has 
been rejected by botanists, for two reasons. In the first place, 
there is said to be no direct evidence of such conveyance j and, 
secondly, the peculiar plants of remote oceanic islands do not 
appear to have seeds specially adapted for aerial transmission. 
I will consider briefly each of these objections. 

Objection to the Theory of Wind-Dispersal. 

To obtain direct evidence of the transmission of such 
minute and perishable objects, which do not exist in great 
quantities, and are probably carried to the greatest distances 
but rarely and as single specimens, is extremely difficult. A 
bird or insect can be seen if it comes on board ship, but who 
would ever detect the seeds of Mimulus or Orchis even if a 
score of them fell on a ship's deck ? Yet if but one such seed 


per century were carried to an oceanic island, that island 
might become rapidly overrun by the plant, if the conditions 
were favourable to its growth and reproduction. It is further 
objected that search has been made for such seeds, and they 
have not been found. Professor Kerner of Innsbruck examined 
the snow on the surface of glaciers, and assiduously collected 
all the seeds he could find, and these were all of plants which 
grew in the adjacent mountains or in the same district. In 
like manner, the plants growing on moraines were found to 
be those of the adjacent mountains, plateaux, or lowlands. 
Hence he concluded that the prevalent opinion that seeds 
may be carried through the air for very great distances " is 
not supported by fact." 1 The opinion is certainly not 
supported by Kerner 's facts, but neither is it opposed by 
them. It is obvious that the seeds that would be carried by 
the wind to moraines or to the surface of glaciers would be, first 
and in the greatest abundance, those of the immediately 
surrounding district; then, very much more rarely, those 
from more remote mountains ; and lastly, in extreme rarity, 
those from distant countries or altogether distinct mountain 
ranges. Let us suppose the first to be so abundant that a 
single seed could be found by industrious search on each 
square yard of the surface of the glacier ; the second so scarce 
that only one could possibly be found in a hundred yards 
square ; while to find one of the third class it would be 
necessary exhaustively to examine a square mile of surface. 
Should we expect that one ever to be found, and should the fact 
that it could not be found be taken as a proof that it was not 
there ? Besides, a glacier is altogether in a bad position to 
receive such remote wanderers, since it is generally surrounded 
by lofty mountains, often range behind range, which would 
intercept the few air-borne seeds that might have been carried 
from a distant land. The conditions in an oceanic island, on the 
other hand, are the most favourable, since the land, especially 
if high, will intercept objects carried by the wind, and will 
thus cause more of the solid matter to fall on it than on an 
equal area of ocean. We know that winds at sea often blow 
violently for days together, and the rate of motion is indicated 
by the fact that 72 miles an hour was the average velocity 
1 See Nature, vol. vi. p. 164, for a summary of Kerner 's paper. 


of the wind observed during twelve hours at the Ben Nevis 
observatory, while the velocity sometimes rises to 120 miles 
an hour. A twelve hours' gale might, therefore, carry 
light seeds a thousand miles as easily and certainly as it 
could carry quartz -grains of much greater specific gravity, 
rotundity, and smoothness, 500 or even 100 miles; and it is 
difficult even to imagine a sufficient reason why they should 
not be so carried perhaps very rarely and under exceptionally 
favourable conditions, but this is all that is required. 

As regards the second objection, it has been observed that 
orchideae, which have often exceedingly small and light seeds, 
are remarkably absent from oceanic islands. This, however, 
may bo very largely due to their extreme specialisation and 
dependence on insect agency for their fertilisation ; while the 
fact that they do occur in such very remote islands as the 
Azores, Tahiti, and the Sandwich Islands, proves that they 
must have once reached these localities either by the agency 
of birds or by transmission through the air ; and the facts I 
have given above render the latter mode at least as probable 
as the former. Sir Joseph Hooker remarks on the composite 
plant of Kerguelen Island (Cotula plumosa) being found also on 
Lord Auckland and MacQuarrio Islands, and yet having no 
pappus, while other species of the genus possess it. This is 
certainly remarkable, and proves that the plant must have, or 
once have had, some other means of dispersal across wide 
oceans. 1 One of the most widely dispersed species in the 
whole world (Sonchus oleraceus) possesses pappus, as do four 
out of five of the species which are common to Europe and 
New Zealand, all of which have a very wide distribution. 
The same author remarks on the limited area occupied by 
most species of Composite, notwithstanding their facilities for 
dispersal by means of their feathered seeds ; but it has been 

1 It seems quite possible that the absence of pappus in this case is a recent 
adaptation, and that it has been brought about by causes similar to those 
which have reduced or aborted the wings of insects in oceanic islands. For 
when a plant has once reached one of the storm-swept islands of the southern 
ocean, the pappus will be injurious for the same reason that the wings of 
insects are injurious, since it will lead to the seeds being blown out to sea and 
destroyed. The seeds which are heaviest and have least pappus will have the 
best chance of falling on the ground and remaining there to germinate, and 
this process of selection might rapidly lead to the entire disappearance of the 


already shown that limitations of area are almost always due 
to the competition of allied forms, facilities for dispersal being 
only one of many factors in determining the wide range of 
species. It is, however, a specially important factor in the 
case of the inhabitants of remote oceanic islands, since, whether 
they are peculiar species or not, they or their remote ancestors 
must at some time or other have reached their present posi- 
tion by natural means. 

I have already shown elsewhere, that the flora of the 
Azores strikingly supports the view of the species having been 
introduced by aerial transmission only, that is, by the agency 
of birds and the wind, because all plants that could not possibly 
have been carried by these means are absent. 1 In the same 
way we may account for the extreme rarity of Leguminosse in 
all oceanic islands. Mr. Hemsley, in his Report on Insular 
Floras, says that they "are wanting in a large number of 
oceanic islands where there is no true littoral flora," as St. 
Helena, Juan Fernandez, and all the islands of the South 
Atlantic and South Indian Oceans. Even in the tropical 
islands, such as Mauritius and Bourbon, there are no endemic 
species, and very few in the Galapagos and the remoter Pacific 
Islands. All these facts are quite in accordance with the absence 
of facilities for transmission through the air, either by birds 
or the wind, owing to the comparatively large size and weight 
of the seeds ; and an additional proof is thus afforded of the 
extreme rarity of the successful floating of seeds for great 
distances across the ocean. 2 

Explanation of North Temperate Plants in the Southern Hemisphere. 

If we now admit that many seeds which are either minute 
in size, of thin texture or wavy form, or so fringed or 
margined as to afford a good hold to the air, are capable of 
being carried for many hundreds of miles by exceptionally 

1 See Island Life, p. 251. 

2 Mr. Hemsley suggests that it is not so much the difficulty of transmission 
by floating, as the bad conditions the seeds are usually exposed to when they 
reach land. Many, even if they germinate, are destroyed by the waves, as 
Burchell noticed at St. Helena ; while even a Hat and sheltered shore would 
be an unsuitable position for many inland plants. Air-borne seeds, on the 
other hand, may be carried far inland, and so scattered that some of them 
are likely to reach suitable stations. 


violent and long-continued gales of wind, we shall not only be 
better able to account for the floras of some of the remotest 
oceanic islands, but shall also find in the fact a sufficient ex- 
planation of the wide diffusion of many genera, and even species, 
of arctic and north temperate plants in the southern hemisphere 
or on the summits of tropical mountains. Nearly fifty of the 
flowering plants of Tierra-del-Fuego are found also in North 
America or Europe, but in no intermediate country ; while fifty- 
eight species are common to New Zealand and Northern Europe; 
thirty ^eight to Australia, Northern Europe, and Asia ; and no 
less ihan seventy-seven common to New Zealand, Australia, 
and South America. 1 On lofty mountains far removed from each 
other, identical or closely allied plants often occur. Thus the 
fine Primula imperialis of a single mountain peak in Java has 
been found (or a closely allied species) in the Himalayas ; 
and many other plants of the high mountains of Java, Ceylon, 
and North India are either identical or closely allied forms. So, 
in Africa, some species, found on the summits of the Cameroons 
and Fernando Po in West Africa, are closely allied to species 
in the Abyssinian highlands and in Temperate Europe ; while 
other Abyssinian and Cameroons species have recently been 
found on the mountains of Madagascar. Some peculiar Aus- 
tralian forms have been found represented on the summit of 
Kini Balu in Borneo. Again, on the summit of the Organ 
mountains in Brazil there are species allied to those of the 
Andes, but not found in the intervening lowlands. 

No Proof of It emit Lower Temperature in the Tropics. 

Now all these facts, and numerous others of like character, 
were supposed by Mr. Darwin to be due to a lowering of 
temperature during glacial epochs, which allowed these tem- 
perate forms to migrate across the intervening tropical low- 
lands. But any such change within the epoch of existing species 
is almost inconceivable. In the first place, it would necessitate 
the extinction of much of the tropical flora (and with it of the 
insect life), because without such extinction alpine herbaceous 
plants could certainly never spread over tropical forest low- 

1 For fuller particulars, see Sir J. Hooker's Introduction to Floras of N&o 
Zealand and Australia, and a summary in my Island Life, chaps, xxii. 




lands ; and, in the next place, there is not a particle of direct 
evidence that any such lowering of temperature in inter- 
tropical lowlands ever took place. The only alleged evidence 
of the kind is that adduced by the late Professor Agassiz and 
Mr. Hartt ; but I am informed by my friend, Mr. J. C. Branner 
(now State Geologist of Arkansas, U.S.), who succeeded Mr. 
Hartt, and spent several years completing the geological 
survey of Brazil, that the supposed moraines and glaciated 
granite rocks near Eio Janeiro and elsewhere, as well as the 
so-called boulder- clay of the same region, are entirely ex- 
plicable as the results of sub-aerial denudation and weathering, 
and that there is no proof whatever of glaciatiori in any 
part of Brazil. 

Lower Temperature not needed to Explain the Facts. 
But any such vast physical change as that suggested by 
Darwin, involving as it does such tremendous issues as re- 
gards its effects on the tropical fauna and flora of the whole 
world, is really quite uncalled for, because the facts to be 
explained are of the same essential nature as those presented 
by remote oceanic islands, between which and the nearest con- 
tinents no temperate land connection is postulated. In pro- 
portion to their limited area and extreme isolation, the Azores, 
St. Helena, the Galapagos, and the Sandwich Islands, each 
possess a fairly rich the last a very rich indigenous flora ; 
and the means which sufficed to stock them with a great 
variety of plants would probably suffice to transmit others 
from mountain-top to mountain-top in various parts of the 
globe. In the case of the Azores, we have large numbers of 
species identical with those of Europe, and others closely allied, 
forming an exactly parallel case to the species found on the 
various mountain summits which have been referred to. The 
distances from Madagascar to the South African mountains 
and to Kilimandjaro, and from the latter to Abyssinia, are no 
greater than from Spain to the Azores, while there are other 
equatorial mountains forming stepping-stones at about an 
equal distance to the Cameroons. Between Java and the 
Himalayas we have the lofty mountains of Sumatra and of 
North-western Burma, forming steps at about the same distance 
apart ; while between Kini Balu and the Australian Alps we 


have the unexplored snow mountains of New Guinea, the 
Bellenden Ker mountains in Queensland, and the New England 
and Blue Mountains of New South Wales. Between Brazil 
and Bolivia the distances are no greater ; while the unbroken 
range of mountains from Arctic America to Tierra-del-Fuego 
offers the greatest facilities for transmission, the partial gap 
between the lofty peak of Chiriqui and the high Andes of New 
Grenada being far less than from Spain to the Azores. Thus, 
whatever means have sufficed for stocking oceanic islands must 
have been to some extent effective in transmitting northern 
forms from mountain to mountain, across the equator, to the 
southern hemisphere ; while for this latter form of dispersal 
there are special facilities, in the abundance of fresh and un- 
occupied surfaces always occurring in mountain regions, owing 
to avalanches, torrents, mountain-slides, and rock-falls, thus 
affording stations on which airborne seeds may germinate 
and find a temporary home till driven out by the inroads of 
the indigenous vegetation. These temporary stations may be 
at much lower altitudes than the original habitat of the species, 
if other conditions are favourable. Alpine plants often descend 
into the valleys on glacial moraines, while some arctic species 
grow equally well on mountain summits and on the seashore. 
The distances above referred to between the loftier mountains 
may thus be greatly reduced by the occurrence of suitable 
conditions at lower altitudes, and the facilities for trans- 
mission by means of aerial currents proportionally increased. 1 

Fads Explained ly the Wind-Carriage of Seeds. 

But if we altogether reject aerial transmission of seeds for 
great distances, except by the agency of birds, it will be 
difficult, if not impossible, to account for the presence of so 
many identical species of plants on remote mountain summits, 
or for that " continuous current of vegetation " described by 
Sir Joseph Hooker as having apparently long existed from 
the northern to the southern hemisphere. It may be admitted 
that we can, possibly, account for the greater portion of the 
floras of remote oceanic islands by the agency of birds alone ; 
because, when blown out to sea land-birds must reach some island 

1 For a fuller discussion of this subject, see my Island Life, chap, xxiii. 


or perish, and all which come within sight of an island will 
struggle to reach it as their only refuge. But, with mountain 
summits the case is altogether different, because, being sur- 
rounded by land instead of by sea, no bird would need to fly, 
or to be carried by the wind, for several hundred miles at a 
stretch to another mountain summit, but would find a refuge 
in the surrounding uplands, ridges, valleys, or plains. As a 
rule the birds that frequent lofty mountain tops are peculiar 
species, allied to those of the surrounding district ; and there 
is no indication whatever of the passage of birds from one 
remote mountain to another in any way comparable with 
the flights of birds which are known to reach the Azores 
annually, or even with the few regular migrants from 
Australia to New Zealand. It is almost impossible to con- 
ceive that the seeds of the Himalayan primula should have 
been thus carried to Java ; but, by means of gales of wind, 
ancj intermediate stations from fifty to a few hundred miles 
apart, where the seeds might vegetate for a year or two and 
produce fresh seed to be again carried on in the same 
manner, the transmission might, after many failures, be at 
last effected. 

A very important consideration is the vastly larger scale 
on which wind-carriage of seeds must act, as compared with 
bird-carriage. It can only be a few birds which carry seeds 
attached to their feathers or feet. A very small proportion of 
these would carry the seeds of Alpine plants ; while an almost 
infinitesimal fraction of these latter would convey the few 
seeds attached to them safely to an oceanic island or remote 
mountain. But winds, in the form of whirlwinds or tornadoes, 
gales or hurricanes, are perpetually at work over large areas 
of land and sea. Insects and light particles of matter are 
often carried up to the tops of high mountains ; and, from the 
very nature and origin of winds, they usually consist of 
ascending or descending currents, the former capable of 
suspending such small and light objects as are many seeds 
long enough for them to be carried enormous distances. For 
each single seed carried away by external attachment to the 
feet or feathers of a bird, countless millions are probably 
carried away by violent winds ; and the chance of conveyance 
to a great distance and in a definite direction must be many 


times greater by the latter mode than by the former. 1 We 
have seen that inorganic particles of much greater specific 
gravity than seeds, and nearly as heavy as the smallest kinds, 
are carried to great distances through the air, and we can 
therefore hardly doubt that some seeds are carried as far. 
The direct agency of the wind, as a supplement to bird- 
transport, will help to explain the presence in oceanic islands 
of plants growing in dry or rocky places whose small seeds 
are not likely to become attached to birds ; while it seems to 
be the only effective agency possible in the dispersal of those 
species of alpine or sub-alpine plants found on the summits 
of distant mountains, or still more widely separated in the 
temperate zones of the northern and southern hemispheres. 

Concluding Remarks. 

On the general principles that have been now laid down, it 
will be found that all the chief facts of the geographical dis- 
tribution of animals and plants can be sufficiently understood. 
There will, of course, be many cases of difficulty and some 
seeming anomalies, but these can usually be seen to depend on 
our ignorance of some of the essential factors of the problem. 
Either wo do not know the distribution of the group in recent 
geological times, or we are still ignorant of the special methods 
by which the organisms are able to cross the sea. The latter 
difficulty applies especially to the lizard tribe, which are found 

1 A very remarkable case of wind conveyance of seeds on a large scale is 
described in a letter from Mr. Thomas Hanbury to his brother, the late 
Daniel Hanbury, which has been kindly communicated to me by Mr. Hemsley 
of Kew. The letter is dated "Shanghai, 1st May 1856," and the passage 
referred to is as follows : 

" For the past three days we have had very warm weather for this time of 
year, in fact almost as warm as the middle of summer. Last evening the 
wind suddenly changed round to the north and blow all night with consider- 
able violence, making a great change in the atmosphere. 

' ' This morning, myriads of small white particles are floating about in the 
air ; there is not a single cloud and no mist, yet the sun is quite obscured by 
this substance, and it looks like a white fog in England. I enclose thee a 
sample, thinking it may interest. Tt is evidently a vegetable production ; I 
think, apparently, some kind of seed." 

Mr. Hemsley adds, that this substance proves to be the plumose seeds of 
a poplar or willow. In order to produce the effects described quite obscuring 
the sun like a white fog, the seeds must have filled the air to a very great 
height ; and they must have been brought from some district where there were 
extensive tracts covered with the tree which produced them. 


in almost all the tropical oceanic islands ; but the particular 
mode in which they are able to traverse a wide expanse of 
ocean, which is a perfect barrier to batrachia and almost so to 
snakes, has not yet been discovered. Lizards are found in all 
the larger Pacific Islands as far as Tahiti, while snakes do not 
extend beyond the Fiji Islands ; and the latter are also absent 
from Mauritius and Bourbon, where lizards of seven or eight 
species abound. Naturalists resident in the Pacific Islands 
would make a valuable contribution to our science by study- 
ing the life-history of the native lizards, and endeavouring to 
ascertain the special facilities they possess for crossing over 
wide spaces of ocean. 



What wo may expect The number of known species of extinct animals 
Causes of the imperfection of the geological record Geological 
evidences of evolution Shells Crocodiles The rhinoceros tribe 
The pedigree of the horse tribe Development of deer's horns Brain 
development Local 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 distribution of 
insects Geological succession of vertebrata Concluding remarks. 

TlIE theory of evolution in the organic world necessarily im- 
plies that the forms of animals and plants have, broadly 
speaking, progressed from a more generalised to a more 
specialised structure, and from simpler to more complex 
forms. We know, however, that this progression has been 
by no means regular, but has been accompanied by repeated 
degradation and degeneration ; while extinction on an 
enormous scale has again and again stopped all progress in 
certain directions, and has often compelled a fresh start 
in development from some comparatively low and imperfect 

The enormous extension of geological research in recent 
times has made us acquainted with a vast number of extinct 
organisms, so vast that in some important groups such as 
the mollusca the fossil are more numerous than the living 
species; whilo in the mammalia they are not much less 
numerous, the preponderance of living species being chiefly in 
the smaller and in the arboreal forms which have not been so 
well preserved as the members of the larger groups. With 
such a wealth of material to illustrate the successive stages 


through which animals have passed, it will naturally be ex- 
pected that we should find important evidence of evolution. 
We should hope to learn the steps by which some isolated 
forms have been connected with their nearest allies, and in 
many cases to have the gaps filled up which now separate 
genus from genus, or species from species. In some cases these 
expectations are fulfilled, but in many other cases we seek 
in vain for evidence of the kind we desire ; and this absence 
of evidence with such an apparent wealth of material is held 
by many persons to throw doubt on the theory of evolution 
itself. They urge, with much appearance of reason, that all 
the arguments we have hitherto adduced fall short of demon- 
stration, and that the crucial test consists in being able to 
show, in a great number of cases, those connecting links which 
we say must have existed. Many of the gaps that still remain 
are so vast that it seems incredible to these writers that they 
could over have been filled up by a close succession of species, 
since these must have spread over so many ages, and have 
existed in such numbers, that it seems impossible to account 
for their total absence from deposits in which great numbers 
of species belonging to other groups are preserved and have 
been discovered. In order to appreciate the force, or weakness, 
of these objections, we must inquire into the character and 
completeness of that record of the past life of the earth which 
geology has unfolded, and ascertain the nature and amount 
of the evidence which, under actual conditions, we may expect 
to find. 

The Number of known Species of Extinct Animals, 

When we state that the known fossil mollusca are consider- 
ably more numerous than those which now live on the earth, 
it appears at first sight that our knowledge is very complete, 
but this is far from being the case. The species have been 
continually changing throughout geological time, and at each 
period have probably been as numerous as they are now. If 
we divide the fossiliferous strata into twelve great divisions 
the Pliocene, Miocene, Eocene, Cretaceous, Oolite, Lias, 
Trias, Permian, Carboniferous, Devonian, Silurian, and Cam- 
brian, we find not only that each has a very distinct and 
characteristic molluscan fauna, but that the different sub- 


divisions often present a widely different series of species ; so 
that although a certain number of species are common to 
two or more of the great divisions, the totality of the species 
that have lived upon the earth must be very much more than 
twelve times perhaps even thirty or forty times the 
number now living. In like manner, although the species of 
fossil mammals now recognised by more or less fragmentary 
fossil remains may not be much less numerous than the 
living species, yet the duration of existence of these was 
comparatively so short that they were almost completely 
changed, perhaps six or seven times, during the Tertiary 
period ; and this is certainly only a fragment of the geological 
time during which mammalia existed on the globe. 

There is also reason to believe that the higher animals 
were much more abundant in species during past geological 
epochs than now, owing to the greater equability of the climate 
which rendered even the arctic regions as habitable as the 
temperate zones are in our time. 

The same equable climate would probably cause a more 
uniform distribution of moisture, and render what are now 
desert regions capable of supporting abundance of animal life. 
This is indicated by the number and variety of the species of 
large animals that have been found fossil in very limited areas 
which they evidently inhabited at one period. M. Albert 
Gaudry found, in the deposits of a mountain stream at 
Pikermi in Greece, an abundance of large mammalia such as 
are nowhere to be found living together at the present time. 
Among them were two species of Mastodon, two different 
rhinoceroses, a gigantic wild boar, a camel and a giraffe 
larger than those now living, several monkeys, carnivora 
ranging from martens and civets to lions and hyamas of the 
largest size, numerous antelopes of at least five distinct genera, 
and besides these many forms altogether extinct. Such were 
the great herds of Hipparion, an ancestral form of horse ; the 
Helladotherium, a huge animal bigger than the giraffe ; the 
Ancylotherium, one of the Edentata ; the huge Dinotherium ; 
the Aceratherium, allied to the rhinoceros ; and the monstrous 
Chalicotherium, allied to the swine and ruminants, but as large 
as a rhinoceros ; and to prey upon these, the great Mac- 
hairodus or sabre-toothed tiger. And all these remains were 


found in a spaco 300 paces long by 60 paces broad, many of 
the species existing in enormous quantities. 

The Pikermi fossils belong to the Upper Miocene forma- 
tion, but an equally rich deposit of Upper Eocene age has 
been discovered in South- Western France at Quercy, where M. 
Filhol has determined the presence of no less than forty-two 
species of beasts of prey alone. Equally remarkable are the 
various discoveries of mammalian fossils in North America, 
especially in the old lake bottoms now forming what are 
called the " bad lands " of Dakota and Nebraska, belonging to 
the Miocene period. Here are found an enormous assemblage 
of remains, often perfect skeletons, of herbivora and carnivora, 
as varied and interesting as those from the localities already 
referred to in Europe ; but altogether distinct, and far ex- 
ceeding, in number and variety of species of the larger animals, 
the whole existing fauna of North America. Very similar 
phenomena occur in South America and in Australia, leading 
us to the conclusion that the earth at the present time is 
impoverished as regards the larger animals, and that at each 
successive period of Tertiary time, at all events, it contained 
a far greater number of species than now inhabit it. The 
very richness and abundance of the remains which we find 
in limited areas, serve to convince us how imperfect and 
fragmentary must be our knowledge of the earth's fauna at 
any one past epoch ; since we cannot believe that all, or 
nearly all, of the animals which inhabited any district were 
entombed in a single lake, or overwhelmed by the floods of a 
single river. 

But the spots where such rich deposits occur are ex- 
ceedingly few and far between when compared with the vast 
areas of continental land, and wo have every reason to believe 
that in past ages, as now, numbers of curious species were 
rare or local, the commoner and more abundant species giving 
a very imperfect idea of the existing series of animal forms. 
Yet more important, as showing the imperfection of our 
knowledge, is the enormous lapse of time between the several 
formations in which we find organic remains in any abundance, 
so vast that in many cases we find ourselves almost in a new 
world, all the species and most of the genera of the higher 
animals having undergone a complete change. 


Causes of the Imperfection of the Geological Record. 

These facts are quite in accordance with the conclusions of 
geologists as to the necessary imperfection of the geological 
record, since it requires the concurrence of a number of 
favourable conditions to preserve any adequate representation 
of the life of a given epoch. In the first place, the animals to 
be preserved must not die a natural death by disease, or old 
age, or by being the prey of other animals, but must be 
destroyed by some accident which shall lead to their being 
embedded in the soil. They must be either carried away by 
floods, sink into bogs or quicksands, or be enveloped in the 
mud or ashes of a volcanic eruption ; and when thus embedded 
they must remain undisturbed amid all the future changes of 
the earth's surface. 

But the chances against this are enormous, because de- 
nudation is always going on, and the rocks we now find at 
the earth's surface are only a small fragment of those which 
were originally laid down. The alternations of marine and 
freshwater deposits, and the frequent unconformability of 
strata with those which overlie them, tell us plainly of 
repeated elevations and depressions of the surface, and of 
denudation on an enormous scale. Almost every mountain 
range, with its peaks, ridges, and valleys, is but the remnant 
of some vast plateau eaten away by sub-aerial agencies ; every 
range of sea-cliffs tell us of long slopes of land destroyed by 
the waves ; while almost all the older rocks which now form 
the surface of the earth have been once covered with newer 
deposits which have long since disappeared. Nowhere are 
the evidences of this denudation more apparent than in North 
and South America, where granitic or metamorphic rocks cover 
an area hardly less than that of all Europe. The same rocks 
are largely developed in Central Africa and Eastern Asia ; 
while, besides those portions that appear exposed on the 
surface, areas of unknown extent are buried under strata 
which rest on them uncomformably, and could not, there- 
fore, constitute the original capping under which the whole of 
these rocks must once have been deeply buried ; because 
granite can only be formed, and metamorphism can only go 
on, deep down in the crust of the earth. What an over- 


whelming idea does this give us of the destruction of whole 
piles of rock, miles in thickness and covering areas comparable 
with those of continents ; and how great must have heen the 
loss of the innumerable fossil forms which those rocks con- 
tained ! In view of such destruction wo are forced to conclude 
that our paloeontological collections, rich though they may 
appear, are really but small and random samples, giving no 
adequate idea of the mighty series of organism which have 
lived upon the earth. 1 

Admitting, however, the extreme imperfection of the geo- 
logical record as a whole, it may bo urged that certain limited 
portions of it are fairly complete as, for example, the various 
Miocene deposits of India, Europe, and North America, 
and that in these we ought to find many examples of species 
and genera linked together by intermediate forms. It may be 
replied that in several cases this really occurs ; and the reason 
why it does not occur more often is, that the theory of 
evolution requires that distinct genera should be linked 
together, not by a direct passage, but by the descent of both 
from a common ancestor, which may have lived in some much 
earlier age the record of which is either wanting or very in- 
complete. An illustration given by Mr. Darwin will make this 
more clear to those who have not studied the subject. The 
fan tail and pouter pigeons are two very distinct and unlike 
breeds, which we yet know to have bee.n both derived from the 
common wild rock-pigeon. Now, if we had every variety of 
living pigeon before us, or even all those which have lived 
during the present century, we should find no intermediate 
types between these two none combining in any degree the 
characters of the pouter with that of the fantail. Neither 
should we ever find such an intermediate form, even had there 
been preserved a specimen of every breed of pigeon since 
the ancestral rock -pigeon was first tamed by man a 
period of probably several thousand years. We thus see 
that a complete passage from one very distinct species to 
another could not be expected even had we a complete record 
of the life of any one period. What we require is a complete 

1 The reader who desires to understand this subject more fully, should 
study chap. x. of the Origin of Species t and chap. xiv. of Sir Charles Lyell's 
Principles of Geology. 


record of all the species that have existed since the two forms 
began to diverge from their common ancestor, and this the 
known imperfection of the record renders it almost impossible 
that wo should over attain. All that we have a right to 
expect is, that, as we multiply the fossil forms in any group, 
the gaps that at first existed in that group shall become less 
wide and less numerous; and also that, in some cases, a tolerably 
direct series shall be found, by which the more specialised 
forms of the present day shall be connected with more 
generalised ancestral types. We might also expect that when 
a country is now characterised by special groups of animals, 
the fossil forms that immediately preceded them shall, for the 
most part, belong to the same groups ; and further, that, com- 
paring the more ancient with the more modern types, we 
should find indications of progression, the earlier forms being, 
on the whole, lower in organisation, and less specialised in 
structure than the later. Now evidence of evolution of these 
varied kinds is what we do find, and almost every fresh discovery 
adds to their number and cogency. In order, therefore, to show 
that the testimony given by geology is entirely in favour of 
the theory of descent with modification, some of the more 
striking of the facts will now be given. 

Geological Evidences of Evolution. 

In an article in Nature (vol. xiv. p. 275), Professor Judd 
calls attention to some recent discoveries in the Hungarian 
plains, of fossil lacustrine shells, and their careful study by Dr. 
Neumayr and M. Paul of the Austrian Geological Survey. 
The beds in which they occur have accumulated to the thick- 
ness of 2000 feet, containing throughout abundance of fossils, 
and divisible into eight zones, each of which exhibits a well- 
marked and characteristic fauna. Professor Judd then de- 
scribes the bearing of these discoveries as follows 

" The group of shells which affords the most interesting 
evidence of the origin of new forms through descent with modi- 
fication is that of the genus Vivipara or Paludina, which occurs 
in prodigious abundance throughout the whole series of fresh- 
water strata. We shall not, of course, attempt in this place 
to enter into any details concerning the forty distinct forms of 
this genus (Dr. Neumayr very properly hesitates to call them all 


species), which are named and described in this monograph, and 
between which, as the authors show, so many connecting links, 
clearly illustrating the derivation of the newer from the older 
types, havo been detected. On the minds of those who care- 
fully examine the admirably engraved figures given in the 
plates accompanying this valuable memoir, or still better, the 
very large series of specimens from among which the subjects of 
these figures are selected, and which are now in the museum 
of the Reichsanstalt of Vienna, but little doubt will, we 
suspect, remain that the authors have fully made out their 
case, and have demonstrated that, beyond all controversy, the 
series with highly complicated ornamentation were variously 
derived by descent the lines of which are in most cases 
perfectly clear and obvious from the simple and unorna- 
mented Vivipara achatinoides of the Congerien-Schichten (the 
lower division of the series of strata). It is interesting to 
notice that a large portion of these unquestionably derived 
forms depart so widely from the type of the genus Vivipara, 
that they have been separated on so high an authority as that 
of Sandberger, as a new genus, under the name of Tulotoma. 
And hence we are led to the conclusion that a vast number 
of forms, certainly exhibiting specific distinctions, and accord- 
ing to some naturalists, differences even entitled to be regarded 
of generic value, have all a common ancestry." 

It is, as Professor Judd remarks, owing to the exceptionally 
favourable circumstances of a long -continued and unbroken 
series of deposits being formed under physical conditions 
either identical or very slowly changing, that we owe so com- 
plete a record of the process of organic change. Usually, 
some disturbing elements, such as a sudden change of physical 
conditions, or the immigration of new sets of forms from other 
areas and the consequent retreat or partial extinction of the 
older fauna, interferes with the continuity of organic development, 
and produces those puzzling discordances so generally met 
with in geological formations of marine origin. While a case 
of the kind now described affords evidence of the origin of 
species complete and conclusive, though on a necessarily very 
limited scale, the very rarity of the conditions which are essential 
to such completeness servos to explain why it is that in most 
cases the direct evidence of evolution is not to be obtained. 


Another illustration of the filling up of gaps between 
existing groups is afforded by Professor Huxley's researches 
on fossil crocodiles. The gap between the existing crocodiles 
and the lizards is very wide, but as we go back in geological 
time we meet with fossil forms which are to some extent 
intermediate and form a connected series. The three living 
genera Crocodilus, Alligator, and Gavialis are found in the 
Eocene formation, and allied forms of another genus, Holops, 
in the Chalk. From the Chalk backward to the Lias another 
group of genera occurs, having anatomical characteristics 
intermediate between the living crocodiles and the most 
ancient forms. These, forming two genera Belodon and 
Stagonolepis, are found in a still older formation, the Trias. 
They have characters resembling some lizards, especially the 
remarkable Hatteria of New Zealand, and have also some 
resemblances to the Dinosaurians reptiles which in some 
respects approach birds. Considering how comparatively few 
are the remains of this group of animals, the evidence which it 
affords of progressive development is remarkably clear. 1 

Among the higher animals the rhinoceros, the horse, and 
the deer afford good evidence of advance in organisation and 
of the filling up of the gaps which separate the living forms 
from their nearest allies. The earliest ancestral forms of the 
rhinoceroses occur in the Middle Eocene of the United States, 
and were to some extent intermediate between the rhinoceros 
and tapir families, having like the latter four toes to the front 
feet, and three to those behind. These are followed in the 
Upper Eocene by the genus Amynodon, in which the skull 
assumes more distinctly the rhinocerotic type. Following this 
in the Lower Miocene we have the Aceratherium, like the last 
in its feet, but still more decidedly a rhinoceros in its general 
structure. From this there are two diverging lines one in 
the Old World, the other in the New. In the former, to which 
the Aceratherium is supposed to have migrated in early 
Miocene times, when a mild climate and luxuriant vegetation 
prevailed far within the arctic circle, it gave rise to the 
Ceratorhinus and the various horned rhinoceroses of late 
Tertiary times and of those now living. In America a 

1 On " Stagonolepis Robertson! and on the Evolution of the Crocodilia," in 
Q. J. of Geological Society, 1875 ; aud abstract in Nature, vol. xii. p. 38. 


number of large hornless rhinoceroses were developed 
they are found in the Upper Miocene, Pliocene, and Post- 
Pliocene formations and then became extinct. The true 
rhinoceroses have three toes on all the feet. 1 

The Pedigree of the Horse Tribe. 

Yet more remarkable is the evidence afforded by the 
ancestral forms of the horse tribe which have been discovered 
in the American tcrtiaries. The family Equidre, comprising 
the living horse, asses, and zebras, differ widely from all other 
mammals in the peculiar structure of the feet, all of which 
terminate in a single large toe forming the hoof. They have 
forty teeth, the molars being formed of hard and soft material 
in crescentic folds, so as to be a powerful agent in grinding 
up hard grasses and other vegetable food. The former peculi- 
arities depend upon modifications of the skeleton, which have 
been thus described by Professor Huxley : 

" Let us turn in the first place to the fore-limb. In most 
quadrupeds, as in ourselves, the fore-arm contains distinct 
bones, called the radius and the ulna. The corresponding 
region in the horse seems at first to possess but one bone. 
Careful observation, however, enables us to distinguish in this 
bone a part which clearly answers to the upper end of the 
ulna. This is closely united with the chief mass of the bone 
which represents the radius, and runs out into a slender shaft, 
which may be traced for some distance downwards upon the 
back of the radius, and then in most cases thins out and 
vanishes. It takes still more trouble to make sure of what is 
nevertheless the fact, that a small part of the lower end of the 
bone of a horse's fore-arm, which is only distinct in a very 
young foal, is really the lower extremity of the ulna. 

" What is commonly called the knee of a horse is its wrist. 
The ' cannon bone ' answers to the middle bone of the five 
metacarpal bones which support the palm of the hand in our- 
selves. The pastern, coronary, and coffin bones of veterin- 
arians answer to the joints of our middle fingers, while the 
hoof is simply a greatly enlarged and thickened nail. But if 

1 From a paper by Messrs. Scott and Osborne, "On the Origin and 
Development of the Rhinoceros Group," read before the British Association 
In 1883. 


what lies below the horse's l knee ' thus corresponds to the 
middle finger in ourselves, what has become of the four other 
fingers or digits ? We find in the places of the second and 
fourth digits only two slender splintlike bones, about two- 
thirds as long as the cannon bone, which gradually taper to 
their lower ends and bear no finger joints, or, as they are 
termed, phalanges. Sometimes, small bony or gristly nodules 
are to be found at the bases of these two metacarpal splints, 
and it is probable that these represent rudiments of the first 
and fifth toes. Thus, the part of tho horse's skeleton which 
corresponds with that of the human hand, contains one over- 
grown middle digit, and at least two imperfect lateral digits ; 
and these answer, respectively, to the third, the second, and 
tho fourth fingers in man. 

" Corresponding modifications arc found in the hind limb. 
In ourselves, and in most quadrupeds, the leg contains two 
distinct bones, a large bone, the tibia, and a smaller and more 
slender bone, the fibula. But, in tho horse, the fibula seems, 
at first, to be reduced to its upper end ; a short slender bone 
united with the tibia, and ending in a point below, occupying 
its place. Examination of the lower end of a young foal's 
shin-bond, however, shows a distinct portion of osseous matter 
which is the lower end of the fibula ; so that the, apparently 
single, lower end of tho shin-bone is really made up of the 
coalesced ends of tho tibia and fibula, just as the, apparently 
single, lower end of the forearm bone is composed of the coal- 
esced radius and ulna. 

" The heel of the horse is the part commonly known as 
tho hock. The hinder cannon bono answers to the middle 
metatarsal bone of the human foot, the pastern, coronary, 
and coffin bones, to the middle toe bones ; the hind hoof to 
the nail ; as in tho forefoot. And, as in the forefoot, there 
are merely two splints to represent the second and the fourth 
toes. Sometimes a rudiment of a fifth too appears to bo 

" Tho teeth of a horse arc not less peculiar than its limbs. 
The living engine, like all others, must bo well stoked if it is 
to do its work ; and the horse, if it is to make good its wear 
and tear, and to exert tho enormous amount of force required 
for its propulsion, must be well and rapidly fed. To this end, 

2 <; 


good cutting instruments and powerful and lasting crushers 
are needful. Accordingly, the twelve cutting teeth of a horse 
are close-set and concentrated in the forepart of its mouth, 
like so many adzes or chisels. The grinders or molars are 
large, and have an extremely complicated structure, being 
composed of a number of different substances of unequal hard- 
ness. The consequence of this is that they wear away at 
different rates ; and, hence, the of each grinder is 
always as uneven as that of a good millstone." 1 

We thus see that the Equidte differ very widely in structure 
from most other mammals. Assuming the truth of the theory 
of evolution, we should expect to find traces among extinct 
animals of the steps by which this great modification has 
been effected ; and we do really find traces of these steps, 
imperfectly among European fossils, but far more completely 
among those of America. 

It is a singular fact that, although no horse inhabited 
America when discovered by Europeans, yet abundance of 
remains of extinct horses have been found both in North and 
South America in Post-Tertiary and Upper Pliocene deposits ; 
and from these an almost continuous series of modified forms 
can be traced in the Tertiary formation, till we reach, at 
the very base of the series, a primitive form so unlike our 
perfected animal, that, had we not the intermediate links, few 
persons would believe that the one was the ancestor of the 
other. The tracing out of this marvellous history we owe 
chiefly to Professor Marsh of Yale College, who has himself 
discovered no less than thirty species of fossil Equida) ; and 
we will allow him to tell the story of the development of the 
horse from a humble progenitor in his own words. 

" The oldest representative of the horse at present known 
is the diminutive Eohippus from the Lower Eocene. Several 
species have been found, all about the size of a fox. Like 
most of the early mammals, these ungulates had forty-four 
teeth, the molars with short crowns and quite distinct in form 
from the premolars. The ulna and fibula were entire and 
distinct, and there were four well-developed toes and a rudi- 
ment of another on the forefeet, arid three toes behind. In 
the structure of the feet and teeth, the Eohippus unmistak- 
1 American Addresses, pp. 73-76, 


ably indicates that the direct ancestral line to the modern 
horse has already separated from the other perissodactyles, or 
odd-toed ungulates. 

" In the next higher division of the Eocene another genus, 
Orohippus, makes its appearance, replacing Eohippus, and 
showing a greater, though still distant, resemblance to the 
equine type. The rudimentary first digit of the forefoot has 
disappeared, and the last premolar has gone over to the molar 
series. Orohippus was but little larger than Eohippus, and 
in most other respects very similar. Several species have 
been found, but none occur later than the Upper Eocene. 

" Near the base of the Miocene, we find a third closely allied 
genus, Mesohippus, which is about as large as a sheep, and one 
stage nearer the horse. There are only three toes and a 
rudimentary splint on the forefeet, and three toes behind. 
Two of tho premolar teeth are quite like the molars. The 
ulna is no longer distinct or the fibula entire, and other 
characters show clearly that the transition is advancing. 

" In the Upper Miocene Mesohippus is not found, but in its 
place a fourth form, Miohippus, continues the line. This 
genus is near the Anchitherium of Europe, but presents 
several important differences. The three toes in each foot 
are more nearly of a size, and a rudiment of the fifth meta- 
carpal bone is retained. All the known species of this genus 
are larger than those of Mesohippus, and none of them pass 
above the Miocene formation. 

" The genus Protohippus of the Lower Pliocene is yet more 
equine, and some of its species equalled the ass in size. There 
are still three toes on each foot, but only the middle one, 
corresponding to the single toe of the horse, comes to the 
ground. This genus resembles most nearly the Hipparion of 

" In the Pliocene we have tho last stage of the series before 
reaching the horse, in the genus Pliohippus, which has lost 
the small hooflots, and in other respects is very equine. 
Only in the Upper Pliocene does the true Equus appear and 
complete the genealogy of the horse, which in the Post-Tertiary 
roamed over the whole of North and South America, and soon 
after became extinct. This occurred long before the dis- 
covery of the continent by Europeans, and no satisfactory 



Kr 'Si' 




Fro toh ippus 





FIG. 83 Geological development of the horse tribe (Eohlppus since discovered). 


reason for the extinction has yet been given. Besides the 
characters I have mentioned, there are many others in the 
skeleton, skull, teeth, and brain of the forty or more inter- 
mediate species, which show that the transition from the 
Eocene Eohippus to the modern Equus has taken place in the 
order indicated r l (see Eig. 33). 

Well may Professor Huxley say that this is demonstrative 
evidence of evolution ; the doctrine resting upon exactly as 
secure a foundation as did the Copernican theory of the 
motions of the heavenly bodies at the time of its promulga- 
tion. Both have the same basis the coincidence of the 
observed facts with the theoretical requirements. 

Development of Deer's Horns. 

Another clear and unmistakable proof of evolution is 
afforded by one of the highest and latest developed tribes of 
mammals the true deer. These differ from all other ruminants 
in possessing solid deciduous horns which are always more or 
less branched. They first appear in the Middle Miocene 
formation, and continue down to our time ; and their develop- 
ment has been carefully traced by Professor Boyd Dawkins, 
who thus summarises his results : 

"In the middle stage of the Miocene the cervine antler 
consists merely of a simple forked crown (as in Cervus dicro- 
ceros), which increases in size in the Upper Miocene, although 
it still remains small and erect, like that of the roe. In Cervus 
Matheroni it measures 1T4 inches, and throws off not more 
than four tines, all small. The deer living in Auvergne in 
the succeeding or Pliocene age, present us with another stage 
in the history of antler development. There, for the first 
time, we see antlers of the Axis and Kusa type, larger and 
longer, and more branching than any antlers were before, and 
possessing three or more well-developed tines. Deer of this 
type abounded in Pliocene Europe. They belong to the 
Oriental division of the Cervida?, and their presence in Europe 
confirms the evidence of the flora, brought forward by the 
Comte do Saporta, that the Pliocene climate was warm. 
They have probably disappeared from Europe in consequence 

1 Lecture on the Introduction and Succession of Vertebrate Life in America, 
Nature, vol. xvi, p. 471. 


of the lowering of the temperature in the Pleistocene age, 
while their descendants have found a congenial home in the 
warmer regions of Eastern Asia. 

" In the latest stage of the Pliocene the Upper Pliocene of 
the Val d'Arno the Cervus dicranios of Nesti presents us with 
antlers much smaller than those of the Irish elk, but very 
complicated in their branching. This animal survived into 
the succeeding age, and is found in the pre-glacial forest 
bed of Norfolk, being described by Dr. Falconer under the 
name of Sedgwick's deer. The Irish elk, moose, stag, reindeer, 
and fallow deer appear in Europe in the Pleistocene age, all 
with highly complicated antlers in the adult, and the first 
possessing the largest antlers yet known. Of these the Irish 
elk disappeared in the Prehistoric age, after having lived in 
countless herds in Ireland, while the rest have lived on into 
our own times in Euro-Asia, and, with the exception of the 
last, also in North America. 

"From this survey it is obvious that the cervine antlers 
have increased in size and complexity from the Mid-Miocene 
to the Pleistocene age, and that their successive changes are 
analogous to those which are observed in the development of 
antlers in the living deer, which begin with a simple point, 
and increase in number of tines till their limit of growth bo 
reached. In other words, the development of antlers indicated 
at successive and widely-separated pages of the geological 
record is the same as that observed in the history of a single 
living species. It is also obvious that the progressive 
diminution of size and complexity in the antlers, from the 
present time back into the early Tertiary age, shows that we 
are approaching the zero of antler development in the Mid- 
Miocene. No trace of any antler-bearing ruminant has been 
met with in the lower Miocenes, either of Europe or the 
United States." 1 

Progressive Brain-Development. 

The three illustrations now given sufficiently prove that, 
whenever the geological record approaches to completeness, 
we have evidence of the progressive change of species in 
definite directions, and from less developed to more de- 

1 Nature, vol. xxv, p. 84. 


veloped types exactly such a change as we may expect to 
find if the evolution theory be the true one. Many other 
illustrations of a similar change could l>o given, but the 
animal groups in which they occur being less familiar, the 
details would be less interesting, and perhaps hardly intel- 
ligible. There is, however, one very remarkable proof of 
development that must be briefly noticed that afforded by 
the steady increase in the size of the brain. This may be 
best stated in the words of Professor Marsh : 

"The real progress of mammalian life in America, from 
the beginning of the Tertiary to the present, is well illus- 
trated by the brain-growth, in which we have the key to 
many other changes. The earliest known Tertiary mammals 
all had very small brains, and in some forms this organ was 
proportionally less than in certain reptiles. There was a 
gradual increase in the size of the brain during this period, 
and it is interesting to find that this growth was mainly 
confined to the cerebral hemispheres, or higher portion of the 
brain. In most groups of mammals the brain has gradually 
become more convoluted, and thus increased in quality as 
well as quantity. In some also the cerebellum and olfactory 
lobes, the lower parts of the brain, have even diminished in 
size. In the long struggle for existence during Tertiary time 
the big brains won, then as now ; and the increasing power 
thus gained rendered useless many structures inherited from 
primitive ancestors, but no longer adapted to new conditions." 

This remarkable proof of development in the organ of 
the mental faculties, forms a fitting climax to the evidence 
already adduced of the progressive evolution of the general 
structure of the body, as illustrated by the bony skeleton. 
We now pass on to another class of facts equally suggestive 
of evolution. 

The Local Relations of Fossil and Living Animals. 

If all existing animals have been produced from ancestral 
forms mostly extinct under the law of variation and natural 
selection, we may expect to find in most cases a close rela- 
tion between the living forms of each country and those which 
inhabited it in the immediately preceding epoch. But if 
species have originated in some quite different way, either by 


any kind of special creation, or by sudden advances of organisa- 
tion in the offspring of preceding types, such close relationship 
would not be found ; and facts of this kind become, therefore, 
to some extent a test of evolution under natural selection or 
some other law of gradual change. Of course the relationship 
will not appear when extensive migration has occurred, by 
which the inhabitants of one region have been able to take 
possession of another region, and destroy or drive out its 
original inhabitants, as has sometimes happened. But such 
cases are comparatively rare, except where great changes of 
climate are known to have occurred ; and we usually do find 
a remarkable continuity between the existing fauna and flora 
of a country and those of the immediately preceding age. A 
few of the more remarkable of these cases will now be briefly 

The mammalian fauna of Australia consists, as is well 
known, wholly of the lowest forms the Marsupials and Mono- 
tremata except only a few species of mice. This is accounted 
for by the complete isolation of the country from the Asiatic 
continent during the whole period of the development of the 
higher animals. At some earlier epoch the ancestral mar- 
supials, which abounded both in Europe and North America 
in the middle of the Secondary period, entered the country, 
and have since remained there, free from the competition of 
higher forms, and have undergone a special development in 
accordance with the peculiar conditions of a limited area. 
While in the large continents higher forms of mammalia have 
been developed, which have almost or wholly exterminated the 
less perfect marsupials, in Australia these latter have become 
modified into such varied forms as the leaping kangaroos, tho 
burrowing wombats, the arboreal phalangers, the insectivorous 
bandicoots, and the carnivorous Dasyuridae or native cats, 
culminating in the Thylacinus or " tiger- wolf " of Tasmania 
animals as unlike each other as our sheep, rabbits, squirrels, 
and dogs, but all retaining the characteristic features of the 
marsupial type. 

Now in the caves and late Tertiary or Post-Tertiary deposits 
of Australia the remains of many extinct mammalia have been 
found, but all are marsupials. There are many kangaroos, 
some larger than any living species, and others more allied to 


the tree-kangaroos of New Guinea ; a large wombat as large 
as a tapir ; the Diprotodon, a thick-limbed kangaroo the size of 
a rhinoceros or small elephant ; and a quite different animal, 
the Nototherium, nearly as large. The carnivorous Thyla- 
cinus of Tasmania is also found fossil ; and a huge phalanger, 
Thylacoleo, the size of a lion, believed by Professor Owen 
and by Professor Oscar Schmidt to have been equally carni- 
vorous and destructive. 1 Besides these, there are many other 
species more resembling the living forms both in size and 
structure, of which they may be, in some cases, the direct 
ancestors. Two species of extinct Echidna, belonging to the 
very low Monotremata, have also been found in New South 

Next to Australia, South America possesses the most re- 
markable assemblage of peculiar mammals, in its numerous 
Edentata the sloths, ant-eaters, and armadillos ; its rodents, 
such as the cavies and chinchillas ; its marsupial opossums, and 
its quadrumana of the family Cebidae. Kemains of extinct 
species of all these have been found in the caves of Brazil, of 
Post-Pliocene age ; while in the earlier Pliocene deposits of the 
pampas many distinct genera of these groups have been found, 
some of gigantic size and extraordinary form. There are 
armadillos of many types, some being as large as elephants ; 
gigantic sloths of the genera Megatherium, Megalonyx, Mylodon, 
Lestodon, and many others ; rodents belonging to the American 
families Cavidne and Chinchillida3 ; and ungulates allied to the 
llama ; besides many other extinct forms of intermediate types 
or of uncertain affinities. 2 The extinct Moas of New Zealand 
huge wingless birds allied to the living Apteryx illustrate 
the same general law. 

The examples now quoted, besides illustrating and enforcing 
the general fact of evolution, throw some light on the usual 
character of the modification and progression of animal forms. 
In the cases where the geological record is tolerably complete, 
we find a continuous development of some kind either in 
complexity of ornamentation, as in the fossil Paludinas of the 
Hungarian lake-basins ; in size and in the specialisation of the 

1 See The Mammalia in their Relation to Primeval Times, p. 102. 

2 For a brief enumeration and description of these fossils, see the author's 
Geographical Distribution of Animals, vol. i. p. 146. 


feot and teeth, as in the American fossil horses; or in the in- 
creased development of the branching horns, as in the true 
deer. In each of these cases specialisation and adaptation to 
the conditions of the environment appear to have reached their 
limits, arid any change of these conditions, especially if it be 
at all rapid or accompanied by the competition of less developed 
but more adaptable forms, is liable to cause the extinction of 
the most highly developed groups. Such we know was the 
case with the horse tribe in America, which totally disappeared 
in that continent at an epoch so recent that we cannot be 
sure that the disappearance was not witnessed, perhaps caused, 
by man ; while even in the Eastern hemisphere it is the 
smaller species the asses and the zebras that have persisted, 
while the larger arid more highly developed true horses have 
almost, if not quite, disappeared in a state of nature. So wo 
find, both in Australia and South America, that in a quite 
recent period many of the largest and most specialised forms 
have become extinct, while only the smaller types have sur- 
vived to our day ; and a similar fact is to be observed in many 
of the earlier geological epochs, a group progressing and reach- 
ing a maximum of size or complexity and then dying out, 
or leaving at most but few and pigmy representatives. 

Cause of Extinction of Larye Animals. 

Now there are several reasons for the repeated extinction 
of large rather than of small animals. In the first place, 
animals of great bulk require a proportionate supply of food, 
and any adverse change of conditions would affect them more 
seriously than it would smaller animals. In the next place, 
the extreme specialisation of many of these large animals 
would render it less easy for them to be modified in any new 
direction suited to changed conditions. Still more important, 
perhaps, is the fact that very large animals always increase 
slowly as compared with small ones the elephant producing 
a single young one every three years, while a rabbit may have 
a litter of seven or eight young two or three times a year. 
Now the probability of favourable variations will be in direct 
proportion to the population of the species, and as the smaller 
animals are not only many hundred times more numerous than 
the largest, but also increase perhaps a hundred times ae 


rapidly, they are able to become quickly modified by variation 
arid natural selection in harmony with changed conditions, 
while the large and bulky species, being unable to vary quickly 
enough, are obliged to succumb in the struggle for exist- 
ence. As Professor Marsh well observes : "In every vigorous 
primitive type which was destined to survive many geological 
changes, there seems to have been a tendency to throw off' 
lateral branches, which became highly specialised and soon 
died out, because they were unable to adapt themselves to new 
conditions." And he goes on to show how the whole narrow 
path of the persistent Suilline type, throughout the entire 
series of the American tertiaries, is strewed with the remains of 
such ambitious offshoots, many of them attaining the size of 
a rhinoceros ; " while the typical pig, with an obstinacy never 
lost, has held on in spite of catastrophes and evolution, and still 
lives in America to-day." 

Indications of General Progression in Plants and Animals. 

One of the most powerful arguments formerly adduced 
against evolution was, that geology afforded no evidence of 
the gradual development of organic forms, but that whole 
tribes and classes appeared suddenly at definite epochs, and 
often in great variety and exhibiting a very perfect organisa- 
tion. The mammalia, for example, were long thought to have 
first appeared in Tertiary times, where they are represented in 
some of the earlier deposits by all the great divisions of the 
class fully developed carnivora, rodents, insectivora, mar- 
supials, and even the perissodactyle and artiodactyle divisions of 
the ungulata as clearly defined as at the present day. The 
discovery in 1818 of a single lower jaw in the Stonesfield 
Slate of Oxfordshire hardly threw doubt on the generalisation, 
since either its mammalian character was denied, or the 
geological position of the strata, in which it was found, was 
held to have been erroneously determined. But since then, at 
intervals of many years, other remains of mammalia have been 
discovered in the Secondary strata, ranging from the Upper 
Oolite to the Upper Trias both in Europe and the United 
States, and one even (Tritylodon) in the Trias of South Africa. 
All these are either marsupials, or of some still lower type of 
mammalia ; but they consist of many distinct forms classed in 


about twenty genera. Nevertheless, a great gap still exists 
between these mammals and those of the Tertiary strata, since 
no mammal of any kind has been found in any part of the 
Cretaceous formation, although in several of its subdivisions 
abundance of land plants, freshwater shells, and air-breathing 
reptiles have been discovered. So with fishes. In the last 
century none had been obtained lower than the Carboniferous 
formation ; thirty years later they were found to be very 
abundant in the Devonian rocks, and later still they were 
discovered in the Upper Ludlow and Lower Ludlow beds of 
the Silurian formation. 

We thus see that such sudden appearances are deceptive, 
and are, in fact, only what we ought to expect from the known 
imperfection of the geological record. The conditions favour- 
able to the fossilisation of any group of animals occur com- 
paratively rarely, and only in very limited areas ; while the 
conditions essential for their permanent preservation in the 
rocks, amid all the destruction caused by denudation or meta- 
morphism, are still more exceptional. And when they are 
thus preserved to our day, the particular part of the rocks in 
which they lie hidden may not be on the surface but buried 
down deep under other strata, and may thus, except in the 
case of mineral -bearing deposits, be altogether out of our 
reach. Then, again, how large a proportion of the earth 
consists of wild and uncivilised regions in which no exploration 
of the rocks has been yet made, so that whether we shall find 
the fossilised remains of any particular group of animals 
which lived during a limited period of the earth's history, and 
in a limited area, depends upon at least a fivefold combination 
of chances. Now, if we take each of these chances separately 
as only ten to one against us (and some are certainly more 
than this), then the actual chance against our finding the 
fossil remains, say of any one order of mammalia, or of land 
plants, at any particular geological horizon, will be about a 
hundred thousand to one. 

It may be said, if the chances are so great, how is it that 
we find such immense numbers of fossil species exceeding in 
number, in some groups, all those that are now living ? But 
this is exactly what we should expect, because the number of 
species of organisms that have ever lived upon the earth, since 


the earliest geological times, will probably be many hundred 
times greater than those now existing of which wo have any 
knowledge; and hence the enormous gaps and chasms in the 
geological record of extinct forms is not to be wondered at. 
Yet, notwithstanding these chasms in our knowledge, if 
evolution is true, there ought to have been, on the whole, 
progression in all the chief types of life. The higher arid more 
specialised forms should have come into existence later than the 
lower and more generalised forms ; and however fragmentary 
the portions wo possess of the whole tree of life upon the 
earth, they ought to show us broadly that such a progressive 
evolution has taken place. We have seen that in some special 
groups, already referred to, such a progression is clearly 
visible, and we will now cast a hasty glance over the entire 
series of fossil forms, in order to see if a similar progression is 
manifested by them as a whole. 

The Progressive Development of Plants. 

Ever since fossil plants have been collected and studied, the 
broad fact has been apparent that the early plants those of 
the Coal formation were mainly cryptogamous, while in the 
Tertiary deposits the higher flowering plants prevailed. In the 
intermediate secondary epoch the gymnosperms cycads and 
coniferae formed a prominent part of the vegetation, and as 
these have usually been held to be a kind of transition form 
between the flowerloss and flowering plants, the geological 
succession has always, broadly speaking, been in accordance 
with the theory of evolution. Beyond this, however, the facts 
wore very puzzling. The highest cryptogams ferns, lycopods, 
and equisetacea* appeared suddenly, and in immense profusion 
in the Coal formation, at which period they attained a develop- 
ment they have never since surpassed or even equalled ; while 
the highest plants the dicotyledonous and monocotyledonous 
angiosperms which now form the bulk of the vegetation of 
the world, and exhibit the most wonderful modifications of 
form and structure, were almost unknown till the Tertiary 
period, when they suddenly appeared in full development, and, 
for the most part, under the same generic forms as now exist. 

During the latter half of the present century, however, 
groat additions have been made to our knowledge of fossil 


plants ; and although there arc still indications of vast gaps in 
our knowledge, due, no doubt, to the very exceptional conditions 
required for the preservation of plant remains, we now possess 
evidence of a more continuous development of the various 
types of vegetation. According to Mr. Lester F. Ward, 
between 8000 and 9000 species of fossil plants have been 
described or indicated ; and, owing to the careful study 
of the nervation of leaves, a large number of these are 
referable to their proper orders or genera, and therefore give 
us some notion which, though very imperfect, is probably 
accurate in its main outlines of the progressive development 
of vegetation on the earth. 1 The following is a summary of 
the facts as given by Mr. Ward : 

The lowest forms of vegetable life the cellular plants 
have been found in Lower Silurian deposits in the form of three 
species of marine alga3 ; and in the whole Silurian formation 
fifty species have been recognised. We cannot for a moment 
suppose, however, that this indicates the first appearance of 
vegetable life upon the earth, for in these same Lower 
Silurian beds the more highly organised vascular cryptogams 
appear in the form of rhizocarps plants allied to Marsilea 
and Azolla, and a very little higher, ferns, lycopods, and even 
conifers appear. We have indications, however, of a still 
more ancient vegetation, in the carbonaceous shales and thick 
beds of graphite far down in the Middle Laurentian, since 
there is no other known agency than the vegetable cell 
by means of which carbon can be extracted from the atmo- 

1 Sketch of Palaeobotany in Fifth Annual Report of U.S. Geological Survey, 
1883-84, pp. 363-452, with diagrams. Sir J. William Davvson, speaking of 
the value of leaves for the determination of fossil plants, says : " In my own 
experience I have often found determinations of the leaves of trees confirmed 
by the discovery of their fruits or of the structure of their stems. Thus, in 
the rich cretaceous plant-beds of the Dunvegaii series, we have beech-nuts 
associated in the same bed with leaves referred to feigns. In the Laraiuie 
beds I determined many years ago nuts of the Trapn or water chestnut, and 
subsequently Lesquereux found in beds in the United States leaves which he 
referred to the same genus. Later, I found in collections made on the Red Deer 
River of Canada my fruits and Lesquereux's leaves on the same slab. The 
presence of trees of the genera Carya and Juglans in the same formation was 
inferred from their leaves, and specimens have since been obtained of silicified 
wood with the microscopic structure of the modem butternut. Still we are 
willing to admit that determinations from leaves alone are liable to doubt," 
The (feological History of Plants, p. 196. 


sphere and fixed in the solid state. These great beds of 
graphite, therefore, imply the existence of abundance of 
vegetable life at the very commencement of the era of which 
we have any geological record. 1 

Ferns, as already stated, begin in the Middle Silurian forma- 
tion with the Eoptoris Morrieri. In tho Devonian, we have 79 
species, in the Carboniferous 6 2 7, and in the Permian 186 species; 
after which fossil ferns diminish greatly, though they are 
found in every formation ; and the fact that fully 3000 living 
species are known, while the richest portion of the Tertiary in 
fossil plants the Miocene has only produced 87 species, will 
serve to indicate the extreme imperfection of the geological 

The Equisetacea; (horsetails) which also first appear in 
tho Silurian and reach their maximum development in the 
Coal formation, are, in all succeeding formations, far less 
numerous than ferns, and only thirty living species are known. 
Lycopodiacese, though still more abundant in the Coal form- 
ation, are very rarely found in any succeeding deposit, though 
the living species are tolerably numerous, about 500 having 
been described. As we cannot suppose them to have really 
diminished and then increased again in this extraordinary 
manner, we have another indication of the exceptional nature 
of plant preservation and the extreme and erratic character of 
the imperfection of the record. 

Passing now to the next higher division of plants the 
gymriosperms we find Coniferae appearing in the Upper 
Silurian, becoming tolerably abundant in the Devonian, and 
reaching a maximum in the Carboniferous, from which form- 
ation more than 300 species are known, equal to the 
number recorded as now living. They occur in all succeeding 
formations, being abundant in tho Oolite, and excessively so 
in the Miocene, from which 250 species have been described. 
The allied family of gyrnnosperms, the Cycadacerc, first appear 
in the Carboniferous era, but very scantily ; are most abundant 
in the Oolite, from which formation 116 species are known, 
and then steadily diminish to the Tertiary, although there are 
seventy-five living species. 

We now come to the true flowering plants, and we first 

1 Sir J. William Dawsoii's Oeoloyical History of Plants, p. 18. 


meet with monocotyledons in the Carboniferous and Permian 
formations. The character of these fossils was long disputed, 
but is now believed to be well established ; and the sub- 
class continues to be present in small numbers in all succeeding 
deposits, becoming rather plentiful in the Upper Cretaceous, 
and very abundant in the Eocene and Miocene. In the latter 
formation 272 species have been discovered; but the 11G 
species in the Eocene form a larger proportion of the total 
vegetation of the period. 

True dicotyledons appear very much later, in the Cretaceous 
period, and only in its upper division, if we except a single 
species from the Urgonian beds of Greenland. The remark- 
able thing is that we here find the sub-class fully developed 
and in great luxuriance of types, all the three divisions 
Apetalae, Polypetalre, and GamopetaljB being represented, with 
a total of no less than 770 species. Among them are such 
familiar forms as the poplar, the birch, the beech, the sycamore, 
and the oak ; as well as the fig, the true laurel, the sassafras, 
the persimmon, the maple, the walnut, the magnolia, and even 
the apple and the plum tribes. Passing on to the Tertiary 
period the numbers increase, till they reach their maximum 
in the Miocene, where more than 2000 species of dicoty- 
ledons have been discovered. Among these the proportionate 
number of the higher gamopetalae has slightly increased, but 
is considerably less than at the present day. 

Possible Cause of sudden late Appearance of Exogens. 

The sudden appearance of fully developed exogenous 
flowering plants in the Cretaceous period is very analogous to 
the equally sudden appearance of all the chief types of 
placental mammalia in the Eocene ; and in both cases wo 
must feel sure that this suddenness is only apparent, due to 
unknown conditions which have prevented their preservation 
(or their discovery) in earlier formations. The case of the 
dicotyledonous plants is in some respects the most extra- 
ordinary, because in the earlier Mesozoic formations we appear 
to have a fair representation of the flora of the period, 
including such varied forms as ferns, equisetums, cycads, 
conifers, and monocotyledons. The only hint at an explana 
tion of this anomaly has been given by Mr. Ball, who supposes 


that all these groups inhabited the lowlands, where there was 
not only excessive heat arid moisture, but also a super- 
abundance of carbonic acid in the atmosphere conditions 
under which these groups had been developed, but which 
were prejudicial to the dicotyledons. These latter are 
supposed to have originated on the high table-lands and 
mountain ranges, in a rarer and drier atmosphere in which 
the quantity of carbonic acid gas was much less ; and any 
deposits formed in lake beds at high altitudes arid at such a 
remote epoch have been destroyed by denudation, and hence 
wo have no record of their existence. 1 

During a few weeks spent recently in the Rocky Mountains, 
I was struck by the great scarcity of monocotyledons and 
ferns in comparison with dicotyledons a scarcity due 
apparently to the dry ness and rarity of the atmosphere 
favouring the higher groups. If we compare Coulter's Rocky 
Mountain Botany with Gray's Hotnvy of the Northern (East] 
United States, we have two areas which differ chiefly in the 
points of altitude and atmospheric moisture. Unfortunately, 
in neither of these works are the species consecutively 
numbered ; but by taking the pages occupied by the two 
divisions of dicotyledons on the one hand, monocotyledons 
and ferns on the other, wo can obtain a good approximation. 
In this way we find that in the flora of the North-Eastern 
States the monocotyledons and ferns ore to the dicotyledons in 
the proportion of 45 to 100 ; in the Rocky Mountains they 
are in the proportion of only 34 to 100 ; while if we take an 
exclusively Alpine flora, as given by Mr. Ball, there are not 
one-fifth as many monocotyledons as dicotyledons. These 
facts show that even at the present day elevated plateaux 
and mountains are more favourable to dicotyledons than to 
monocotyledons, and we may, therefore, well suppose that the 
former originated within such elevated areas, and were for 
long ages confined to them. It is interesting to note that their 
richest early remains have been found in the central regions 
of the North American continent, where they now, proportion- 
ally, most abound, and where the conditions of altitude and a 
dry atmosphere were probably present at a very early period. 

1 " On the Origin of the Flora of the European Alps," Proc. of Roy. Geog. 
Society, vol. i. (1879), pp. 564-588. 

2 D 




The diagram (Fig. 34), slightly modified from one given 


by Mr. Ward, will illustrate our present knowledge of the 
development of the vegetable kingdom in geological time. 


The shaded vertical Lands exhibit the proportions of the fossil 
forms actually discovered, while the outline extensions are 
intended to show what we may fairly presume to have been 
the approximate periods of origin, and progressive increase of 
the number of species, of the chief divisions of the vegetable 
kingdom. These seem to accord fairly well with their respec- 
tive grades of development, and thus offer no obstacle to the 
acceptance of the belief in their progressive evolution. 

Geological Distribution of Insects. 

The marvellous development of insects into such an endless 
variety of forms, their extreme specialisation, and their adapta- 
tion to almost every possible condition of life, would almost 
necessarily imply an extreme antiquity. Owing, however, to 
their small size, their lightness, and their usually aerial habits, 
no class of animals has been so scantily preserved in the 
rocks ; and it is only recently that the whole of the scattered 
material relating to fossil insects and their allies have been 
brought together by Mr. Samuel H. Scudder of Boston, and 
we have thus learned their bearing on the theory of evolution. 1 

The most striking fact which presents itself on a glance at 
the distribution of fossil insects, is the completeness of the 
representation of all the chief types far back in the Secondary 
period, at which time many of the existing families appear to 
have been perfectly differentiated. Thus in the Lias wo find 
dragonflies "apparently as highly specialised as to-day, no 
less than four tribes being present/* Of beetles we have 
undoubted Curculionidse from the Lias and Trias ; Chrysome- 
lidoe in the same deposits ; Cerambycida} in the Oolites ; 
Scaraboeidoe in the Lias ; Buprestida3 in the Trias ; Elateridae, 
Trogositidie, and Nitidulidie in the Lias ; Staphylinidje in the 
English Purbecks; while Hydrophilida), Gyrinidje, and Carabidra 
occur in the Lias. All these forms are well represented, but 
there are many other families doubtfully identified in equally 
ancient rocks. Diptera of the families Empida?, Asilidae, 
and Tipulidffl have been found as far back as the Lias. 
Of Lepidoptera, Sphingidao and Tineido? have been found 

1 Systematic Review of our Present Knowledge of Fossil Insects, including 
Myriapods and Arachnids (Lull, of U. S. Oeol. Survey, No. 81, Washington, 


in the Oolite ; while ants, representing the highly specialised 
Hymenoptera, have occurred in the Piirbeck and Lias. 

This remarkable identity of the families of very ancient 
with those of existing insects is quite comparable with the 
apparently sudden appearance of existing genera of trees in 
the Cretaceous epoch. In both cases we feel certain that we 
must go very much farther back in order to find the ancestral 
forms from which they were developed, and that at any 
moment some fresh discovery may revolutionise our ideas as 
to the antiquity of certain groups. Such a discovery was 
made while Mr. Scudder's work was passing through the press. 
Up to that date all the existing orders of true insects appeared 
to have originated in the Trias, the alleged moth and beetle of 
the Coal formation having been incorrectly determined. But 
now, undoubted remains of beetles have been found in the Coal 
measures of Silesia, thus supporting the interpretation of the 
borings in carboniferous trees as having been made by insects 
of this order, and carrying back this highly specialised form of 
insect life well into Palaeozoic times. Such a discovery renders 
all speculation as to the origin of true insects premature, 
because we may feel sure that all the other orders of insects, 
except perhaps hymenoptera and lepidoptera, were contempo- 
raneous with the highly specialised beetles. 

The less highly organised terrestrial arthropoda the 
Arachnida and Myriapoda are, as might be expected, much 
more ancient. A fossil spider has been found in the Carboni- 
ferous, and scorpions in the Upper Silurian rocks of Scotland, 
Sweden, and the United States. Myriapoda have been found 
abundantly in the Carboniferous and Devonian formations ; 
but all are of extinct orders, exhibiting a more generalised 
structure than living forms. 

Much more extraordinary, however, is the presence in the 
Palaeozoic formations of ancestral forms of true insects, termed 
by Mr. Scudder Palaeodictyoptera. They consist of general- 
ised cockroaches and walking-stick insects (Orthopteroidea) ; 
ancient mayflies and allied forms, of which there are six 
families and more than thirty genera (Neuropteroidca) ; three 
genera of Hemipteroidea resembling various Homoptera and 
Hemiptora, mostly from the Carboniferous formation, a few 
from the Devonian, and one ancestral cockroach (Palaeoblattina) 


from the Middle Silurian sandstone of France. If this 
occurrence of a true hexapod insect from the Middle Silurian 
be really established, taken in connection with the well- 
defined Coleoptera from the Carboniferous, the origin of the 
entire group of terrestrial arthropoda is necessarily thrown 
back into the Cambrian epoch, if not earlier. And this cannot 
bo considered improbable in view of the highly differentiated 
land plants ferns, equisetums, and lycopods in the Middle or 
Lower Silurian, and even a conifer (Cordaites Robbii) in the 
Upper Silurian ; while the beds of graphite in the Laurentian 
were probably formed from terrestrial vegetation. 

On the whole, then, we may affirm that, although the 
geological record of the insect life of the earth is exceptionally 
imperfect, it yet decidedly supports the evolution hypothesis. 
The most specialised order, Lepidoptera, is the most recent, 
only dating back to the Oolite ; the Ilymenoptera, Diptera, 
and Hornoptera go as far as the Lias ; while the Orthoptera 
and Neuroptera extend to the Trias. The recent discovery of 
Coleoptera in the Carboniferous shows, however, that the 
preceding limits are not absolute, and will probably soon be 
overpassed. Only the more generalised ancestral forms of 
winged insects have been traced back to Silurian time, and 
along with them the less highly organised scorpions ; facts 
which serve to show us the extreme imperfection of our 
knowledge, and indicate possibilities of a world of terrestrial 
life in tho remotest Palaeozoic times. 

Geological Succession of fartebrata. 

The lowest forms of vertebrates are the fishes, and these appear 
first in the geological record in the Upper Silurian formation. 
The most ancient known fish is a Pteraspis, one of the buck- 
lered ganoids or plated fishes by no means a very low type 
allied to the sturgeon (Accipenser) and alligator - gar 
(Lepidosteus), but, as a group, now nearly extinct. Almost 
equally ancient are the sharks, which under various forms 
still abound in our seas. We cannot suppose these to bo nearly 
the earliest fishes, especially as the two lowest orders, now 
represented by the Amphioxus or lancelet and tho lampreys, 
have not yet been found fossil. Tho ganoids were greatly 
developed in the Devonian era, and continued till the 


Cretaceous, when they gave way to the true osseous fishes, 
which had first appeared in the Jurassic period, and have con- 
tinued to increase till the present day. This much later 
appearance of the higher osseous fishes is quite in accordance 
with evolution, although some of the very lowest forms, the 
lancelet and the lampreys, together with the archaic ceratodus, 
have survived to our time. 

The Amphibia, represented by the extinct labyrinthodons, 
appear first in the Carboniferous rocks, and these peculiar forms 
became extinct early in the Secondary period. The labyrin- 
thodons were, however, highly specialised, and do not at all 
indicate the origin of the class, which may bo as ancient as the 
lower forms of fishes. Hardly any recognisable remains of our 
existing groups the frogs, toads, and salamanders are found 
before the Tertiary period, a fact which indicates the extreme 
imperfection of the record as regards this class of animals. 

True reptiles have not been found till we reach the Per- 
mian where Prohatteria and Proterosaurus occur, the former 
closely allied to the lizard-like Sphenodon of New Zealand, 
the latter having its nearest allies in the same group of 
reptiles Rhyncocephala, other forms of which occur in the 
Trias. In this last-named formation the earliest crocodiles 
Phytosaurus (Belodon) and Stagonolepis occur, as well as the 
earliest tortoises Chelytherium, Proganochelys, and Psepho- 
derma. 1 Fossil serpents have been first found in the Cre- 
taceous formation, but the conditions for the preservation of 
these forms have evidently been unfavourable, and the record 
is correspondingly incomplete. The marine Plesiosauri and 
Ichthyosauri, the flying Pterodactyles, the terrestrial Iguan- 
odon of Europe, and the huge Atlantosaurus of Colorado 
the largest land animal that has ever lived upon the earth 2 
all belong to special developments of the reptilian type which 
flourished during the Secondary epoch, and then became 

1 For the facts as to the early appearance of the above named groups oi 
reptiles I am indebted to Mr. R. Lydekker of the Geological Department of 
the Natural History Museum. 

2 According to Professor Marsh this creature was 50 or 60 feet long, and 
when erect, at least 30 feet in height. It fed upon the foliage of the 
mountain forests of the Cretaceous epoch, the remains of which are preserved 
with it. 


Birds are among the rarest of fossils, due, no doubt, to their 
aerial habits removing them from the ordinary dangers of 
flood, bog, or ice which overwhelm mammals and reptiles, and 
also to their small specific gravity which keeps them floating 
on the surface of water till devoured. Their remains were long 
confined to Tertiary deposits, where many living genera arid 
a few extinct forms have been found. The only birds yet 
known from the older rocks are the toothed birds (Odontor- 
riithes) of the Cretaceous beds of the United States, belong- 
ing to two distinct families and many genera ; a penguin-like 
form (Enaliornis) from the Upper Greensand of Cambridge ; 
and the well-known long- tailed Archscopteryx from the Upper 
Oolite of Bavaria. The record is thus imperfect and fragment- 
ary in the extreme ; but it yet shows us, in the few birds dis- 
covered in the older rocks, more primitive and generalised 
types, while the Tertiary birds had already become specialised 
like those living, and had lost both the teeth and the long 
vertebral tail, which indicate reptilian affinities in the earlier 

Mammalia have been found, as already stated, as far back 
as the Trias formation, in Europe in the United States and 
in South Africa, all being very small, and belonging either 
to the Marsupial order, or to some still lower and more 
generalised type, out of which both Marsupials and Insectivora 
were developed. Other allied forms have been found in the 
Lower and Upper Oolite both of Europe and the United States. 
But there is then a great gap in the whole Cretaceous 
formation, from which no mammal has been obtained, although 
both in the Wealden and the Upper Chalk in Europe, and in 
the Upper Cretaceous deposits of the United States an 
abundant and well-preserved terrestrial flora has been dis- 
covered. Why no mammals have left their remains here it is 
impossible to say. We can only suppose that the limited 
areas in which land plants have been so abundantly preserved, 
did not present the conditions which are needed for the fossil- 
isation and preservation of mammalian remains. 

When we come to the Tertiary formation, we find mammals 
in abundance ; but a wonderful change has taken place. The 
obscure early types have disappeared, and we discover in their 
place a whole series of forms belonging to existing orders, 




and even sometimes to existing families. Thus, in the Eocene 
we have remains of the opossum family ; bats apparently 
belonging to living genera; rodents allied to the South 
American cavies and to dormice and squirrels ; hoofed animals 
belonging to the odd -toed and even -toed groups; and an- 
cestral forms of cats, civets, dogs, with a number of more 
generalised forms of carnivora. Besides these there are 
whales, lemurs, and many strange ancestral forms of pro- 
boscidea. 1 

The great diversity of forms and structures at so remote 
an epoch would require for their development an amount of 
time, which, judging by the changes that have occurred in 
other groups, would carry us back far into the Mesozoic 
period. In order to understand why we have no record of 
these changes in any part of the world, we must fall back 
upon some such supposition as we made in the case of the 
dicotyledonous plants. Perhaps, indeed, the two cases are 
really connected, and the upland regions of the primeval world, 
which saw the development of our higher vegetation, may 
have also afforded the theatre for the gradual development 
of the varied mammalian types which surprise us by their 
sudden appearance in Tertiary times. 

Notwithstanding these irregularities arid gaps in the record, 
the accompanying table, summarising our actual knowledge of 
the geological distribution of the five classes of vortebrata, 













Fishes . 
Birds . 
Mammalia . 

1 For fuller details, see the author's Geographical Distribution of Animals, 
and Heilprin's Geographical and Geological Distribution of A nimals. 


exhibits a steady progression from lower to higher types, 
excepting only the deficiency in the bird record which is 
easily explained. The comparative perfection of type in 
which each of these classes first appears, renders it certain that 
the origin of each and all of them must be sought much 
farther back than any records which have yet been discovered. 
The researches of palaeontologists and embryologists indicate 
a reptilian origin for biids and mammals, while reptiles and 
amphibia arose, perhaps independently, from fishes. 

Concluding Remarks. 

The brief re vie w we have now taken of the more suggestive 
facts presented by the geological succession of organic forms, 
is sufficient to show that most, if not all, of the supposed 
difficulties which it presents in the way of evolution, are due 
either to imperfections in the geological record itself, or to our 
still very incomplete knowledge of what is really recorded in 
the earth's crust. We learn, however, that just as discovery 
progresses, gaps are filled up and difficulties disappear ; while, 
in the case of many individual groups, we have already 
obtained all the evidence of progressive development that can 
reasonably be expected. We conclude, therefore, that the 
geological difficulty has now disappeared ; and that this noble 
science, when properly understood, affords clear and weighty 
evidence of evolution. 



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 tho 
origin of spines Variation and selection overpower the effects of use 
and disuseSupposed 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. 

HAVING now set forth and illustrated at some length the 
most important of the applications of the development 
hypothesis in tho explanation of the broader and more 
generally interesting phenomena presented by tho organic 
world, we propose to discuss some of the more fundamental 
problems and difficulties which have recently been adduced 
by eminent naturalists. It is the more necessary to do this, 
because there is now a tendency to minimise tho action of 
natural selection in tho production of organic forms, and to 
set up in its place certain fundamental principles of variation 
or laws of growth, which it is urged are tho real originators 
of the several lines of development, and of most of the variety 
of form and structure in the vegetable and animal kingdoms. 
These views have, moreover, been seized upon by popular 
writers to throw doubt and discredit on the whole theory of 


evolution, and especially on Darwin's presentation of that 
theory, to the bewilderment of the general public, who are 
quite unable to decide how far the new views, even if well 
established, tend to subvert the Darwinian theory, or whether 
they are really more than subsidiary parts of it, and quite 
powerless without it to produce any effect whatever. 

The writers whose special views we now propose to 
consider are: (1) Mr. Herbert Spencer, on modification of 
structures arising from modification of functions, as set forth 
in his Factors of Organic Evolution. (2) Dr. E. D. Cope, who 
advocates similar views in detail, in his work entitled The 
Origin of the Fittest, and may be considered the head of a 
school of American naturalists who minimise the agency of 
natural selection. (3) Dr. Karl Semper, who has especially 
studied the direct influence of the environment in the whole 
animal kingdom, and has set forth his views in a volume on 
The Natural Conditions of Existence as they Affect Animal Life. 
(4) Mr. Patrick Geddcs, who urges that fundamental laws of 
growth, and the antagonism of vegetative and reproductive 
forces, account for much that has been imputed to natural 

We will now endeavour to ascertain what are the more 
important facts and arguments adduced by each of the above 
writers, and how far they oner a substitute for the action of 
natural selection ; having done which, a brief account will be 
given of the views of Dr. Aug. Wcismann, whose theory of 
heredity will, if established, strike at the very root of the 
arguments of the first three of the writers above referred to. 

Mr. Herbert Spencers Factors of Organic Evolution. 

Mr. Spencer, while fully recognising the importance and 
wide range of the principle of natural selection, thinks that 
sufficient weight has not been given to the effects of use 
and disuse as a factor in evolution, or to the direct action 
of the environment in determining or modifying organic 
structures. As examples of the former class of actions, he 
adduces the decreased size of the jaws in the civilised races 
of mankind, the inheritance of nervous disease produced by 
overwork, the great and inherited development of the udders 
in cows and goats, and the shortened legs, jaws, and snout in 


improved races of pigs the two latter examples being quoted 
from Mr. Darwin, and other cases of like nature. As 
examples of the latter, Mr. Darwin is again quoted as 
admitting that there are many cases in which the action of 
similar conditions appears to have produced corresponding 
changes in different species ; arid we have a very elaborate 
discussion of the direct action of the medium in modifying the 
protoplasm of simple organisms, so as to bring about the 
difference between the outer surface and the inner part that 
characterises the cells or other units of which they are formed. 
Now, although this essay did little more than bring together 
facts which had been already adduced by Mr. Darwin or by 
Mr. Spencer himself, and lay stress upon their importance, its 
publication in a popular review was immediately seized upon 
as "an avowed and definite declaration against some of the 
leading ideas on which the Mechanical Philosophy depends," 
and as being "fatal to the adequacy of the Mechanical 
Philosophy as any explanation of organic evolution," 1 an 
expression of opinion which would be repudiated by every 
Darwinian. For, even admitting the interpretation which Mr. 
Spencer puts on the facts he adduces, they are all included in 
the causes which Darwin himself recognised as having acted 
in bringing about the infinitude of forms in the organic world. 
In the concluding chapter of the Origin of fipecies he says : 
" I have now recapitulated the facts and considerations which 
have thoroughly convinced me that species have been modified 
during a long course of descent. This has been effected 
chiefly through the natural selection of numerous successive, 
slight, favourable variations ; aided in an important manner 
by the inherited effects of the use and disuse of parts ; and 
in an unimportant manner that is, in relation to adaptive 
structures whether past or present, by the direct action of 
external conditions, and by variations which seem to us, in 
our ignorance, to arise spontaneously." This passage, sum- 
marising Darwin's whole inquiry, and explaining his final 
point of view, shows how very inaccurate may be the popular 
notion, as expressed by the Duke of Argyll, of any supposed 
additions to the causes of change of species as recognised 
by Darwin. 

1 Sec the Duke of Argyll's letter in Nature, vol. xxxiv. p. 336. 


But, as we shall see presently, there is now much reason 
to believe that the supposed inheritance of acquired modifica- 
tions that is, of the effects of use and disuse, or of the direct 
influence of the environment is not a fact ; and if so, the very 
foundation is taken away from the whole class of objections 
on which so much stress is now laid. It therefore becomes 
important to inquire whether the facts adduced by Darwin, 
Spencer, and others, do really necessitate such inheritance, or 
whether any other interpretation of them is possible. I 
believe there is such an interpretation ; and we will first 
consider the cases of disuse on which Mr. Spencer lays most 

The cases Mr. Spencer adduces as demonstrating the effects 
of disuse in diminishing the size and strength of organs are, 
the diminished size of the jaws in the races of civilised men, 
and the diminution of the muscles used in closing the jaws in 
the case of pet -clogs fed for generation? on soft food. He 
argues that the minute reduction in any one generation could 
not possibly have been useful, and, therefore, not the subject 
of natural selection ; and against the theory of correlation of 
the diminished jaw with increased brain in man, he urges that 
there are cases of largo brain development, accompanied by 
jaws above the average size. Against the theory of economy 
of nutrition in the case of the pet-dogs, he places the abundant 
food of these animals which would render such economy need- 

But neither he nor Mr. Darwin has considered the effects 
of the withdrawal of the action of natural selection in keep- 
ing up the parts in question to their full dimensions, which, 
of itself, seems to me quite adequate to produce the results 
observed. Recurring to the evidence, adduced in Chapter III, 
of the constant variation occurring in all parts of the organism, 
while selection is constantly acting on these variations in 
eliminating all that fall below the best working standard, and 
preserving only those that are fully up to it; and, remembering 
further, that, of the whole number of the increase produced 
annually, only a small percentage of the best adapted can be 
preserved, we shall see that every useful organ will be kept 
up nearly to its higher limit of si/e and efficiency. Now Mr. 
Galton has proved experimentally that, when any part has 


thus been increased (or diminished) by selection, there is in 
the offspring a strong tendency to revert to a mean or average 
size, which tends to check further increase. And this mean 
appears to be, not the mean of the actual existing individuals 
but a lower mean, or that from which they had been recently 
raised by selection. 1 He calls this the law of "Degression 
towards Mediocrity," and it has been proved by experiments 
with vegetables and by observations on mankind. This regres- 
sion, in every generation, takes place even when both parents 
have been selected for their high development of the organ in 
question ; but when there is no such selection, and crosses are 
allowed among individuals of every grade of development, the 
deterioration will be very rapid ; and after a time not only 
will the average size of the part be greatly reduced, but the 
instances of full development will become very rare. Thus 
what Weismann terms " panmixia," or free intercrossing, will 
co-operate with Galton's law of "regression towards mediocrity," 
and the result will be that, whenever selection ceases to act on 
any part or organ which has heretofore been kept up to a 
maximum of size and efficiency, the organ in question will 
rapidly decrease till it reaches a mean value considerably 
below the mean of the progeny that has usually been produced 
each year, and very greatly below the mean of that portion 
which has survived annually ; and this will take place by the 
general law of heredity, and quite irrespective of any use or 
disuse of the part in question. Now, no observations have been 
adduced by Mr. Spencer or others, showing that the average 
amount of change supposed to be due to disuse is greater than 
that due to the law of regression towards mediocrity ; while 
even if it were somewhat greater, we can see many possible 
contributory causes to its production. In the case of civilised 
man's diminished jaw, there may well be some correlation 
between the jaw and the brain, seeing that increased mental 
activity would lead to the withdrawal of blood and of nervous 
energy from adjacent parts, and might thus lead to diminished 
growth of those parts in the individual. And in the case of 
pet-dogs, the selection of small or short-headed individuals 
would imply the unconscious selection of those with less 
massive temporal muscles, and thus lead to the concomitant 
1 Journal of the Anthropological Institute, vol. xv. pp. 246-260. 


reduction of those muscles. The amount of reduction observed 
by Darwin in the wing-bones of domestic ducks and poultry, 
and in the hind legs of tame rabbits, is very small, and is 
certainly no greater than the above causes will well account 
for; while so many of the external characters of all our 
domestic animals have been subject to long-continued artificial 
selection, and we are so ignorant of the possible correlations 
of different parts, that the phenomena presented by them 
seem sufficiently explained without recurrence to the assump- 
tion that any changes in the individual, due to disuse, are 
inherited by the offspring. 

Supposed Effects of Disuse among Wild Animals. 

It may be urged, however, that among wild animals we have 
many undoubted results of disuse much more pronounced than 
those among domestic kinds, results which cannot be explained 
by the causes already adduced. Such are the reduced size of 
the wings of many birds on oceanic islands ; the abortion of 
the eyes in many cave animals, and in some which live under- 
ground ; and the loss of the hind limbs in whales and in some 
lizards. These cases differ greatly in the amount of the re- 
duction of parts which has taken place, and may be due to 
different causes. It is remarkable that in some of the birds of 
oceanic islands the reduction is little if at all greater than in 
domestic birds, as in the water-hen of Tristan d'Acunha. Now 
if the reduction of wing were due to the hereditary effects of 
disuse, we should expect a very much greater effect in a bird 
inhabiting an oceanic island than in a domestic bird, where the 
disuse has been in action for an indefinitely shorter period. 
In the case of many other birds, however as some of the New 
Zealand rails and the extinct dodo of Mauritius the wings 
have been reduced to a much more rudimentary condition, 
though it is still obvious that they were once organs of flight ; 
and in these cases we certainly require some other causes than 
those which have reduced the wings of our domestic fowls. 
One such cause may have been of the same nature as that 
which has been so efficient in reducing the wings of the insects 
of oceanic islands the destruction of those which, during the 
occasional use of their wings, were carried out to sea. This 
form of natural selection may well have acted in the ease of 


birds whose powers of flight were already somewhat reduced, 
and to whom, there being no enemies to escape from, their use 
was only a source of danger. We may thus, perhaps, account 
for the fact that many of these birds retain small but useless 
wings with which they never fly ; for, the wings having been 
reduced to this functionless condition, no power could reduce 
them further except correlation of growth or economy of 
nutrition, causes which only rarely come into play. 

The complete loss of eyes in some cave animals may, 
perhaps, be explained in a somewhat similar way. When- 
ever, owing to the total darkness, they became useless, they 
might also become injurious, on account of their delicacy of 
organisation and liability to accidents and disease ; in which 
case natural selection would begin to act to reduce, and finally 
abort them ; and this explains why, in some cases, the rudi- 
mentary eye remains, although completely covered by a pro- 
tective outer skin. Whales, like moas and cassowaries, carry 
us back to a remote past, of whoso conditions we know too 
little for safe speculation. We are quite ignorant of the ances- 
tral forms of either of these groups, and are therefore without 
the materials needful for determining the steps by which the 
change took place, or the causes which brought it about. l 

On a review of the various examples that have been given 
by Mr. Darwin and others of organs that have been reduced 
or aborted, there seems too much diversity in the results for 
all to be due to so direct and uniform a cause as the individual 
effects of disuse accumulated by heredity. For if that were 
the only or chief efficient cause, and a cause capable of pro- 
ducing a decided effect during the comparatively short period 

1 The idea of the non-heredity of acquired variations was suggested by 
the summary of Professor Weismann's views, in Nature, referred to later on. 
But since this chapter was written I have, through the kindness of Mr. E. B. 
Poulton, seen some of the proofs of the forthcoming translation of Weismann's 
K.ssays on Heredity, in which he sets forth an explanation very similar 
to that here given. On the difficult question of the almost entire disap- 
pearance of organs, as in the limbs of snakes and of some lizards, he adduces 
"a certain form of correlation, which Roux calls 'the struggle of the parts in 
the organism,' " as playing an important part. Atrophy following disuse is 
nearly always attended by the corresponding increase of other organs : blind 
animals possess more developed organs of touch, hearing, and smell ; the Joss 
of power in the wings is accompanied by increased strength of the legs, etc. 
Now as these latter character*, being useful, will be selected, it is easy to 
understand that a congenital increase of these will be accompanied by a cor 


of the existence of animals in a state of domestication, we 
should expect to find that, in wild species, all unused parts or 
organs had been reduced to the smallest rudiments, or had 
wholly disappeared. Instead of this we find various grades 
of reduction, indicating the probable result of several distinct 
causes, sometimes acting separately, sometimes in combination, 
such as those we have already pointed out. 

And if we find no positive evidence of disuse, acting by its 
direct effect on the individual, being transmitted to the offspring, 
still less can we find such evidence in the case of the use of 
organs. For here the very fact of ?w, in a wild state, implies 
utility, and utility is the constant subject for the action of 
natural selection ; while among domestic animals those parts 
which are exceptionally used are so used in the service of man, 
and have thus become the subjects of artificial selection. 
Thus " the great and inherited development of the udders in 
cows and goats," quoted by Spencer from Darwin, really affords 
no proof of inheritance of the increase due to use, because, 
from tho earliest period of the domestication of these animals, 
abundant milk-production has been highly esteemed, and has 
thus been the subject of selection ; while there are no cases 
among wild animals that may not be better explained by 
variation and natural selection. 

Difficulty as to Co-adaptation of Parts ly Variation and Selection. 
Mr. Spencer again brings forward this difficulty, as he 
lid in his Principles of Biology twenty -five years ago, and 
urges that all the adjustments of bones, muscles, blood-vessels, 
and nerves which would be required during, for example, the 
iovelopment of the neck and fore-limbs of the giraffe, could 

espondiug congenital diminution of the unused organ ; and in cases where 
;he means of nutrition are deficient, every diminution of these useless parts 
rill be a gain to the whole organism, and thus their complete disappearance 
trill, in some cases, be brought al>out directly by natural selection. This 
sorresponds with what we know of these rudimentary organs. 

Jt must, however, be pointed out that the non-heredity of acquired char- 
icters was maintained by Mr. Francis Galton more than twelve years ago, on 
heoretical considerations almost identical with those urged by Professor Weis- 
nann ; while the insufficiency of the evidence for their hereditary trans- 
nisnion was shown, by similar arguments to those used above and in the work 
>f Professor Weismann already referred to (see "A Theory of Heredity," iu 
Anthrop. InstiL, vol. v. pp. 343-345). 
2 K 


not have been effected by " simultaneous fortunate spontaneous 
variations/ 7 But this difficulty is fully disposed of by the 
facts of simultaneous variation adduced in our third chapter, 
and has also been specially considered in Chapter VI, p. 127. 
The best answer to this objection may, perhaps, be found in 
the fact that the very thing said to be impossible by variation 
arid natural selection has been again and again effected by 
variation and artificial selection. During the process of forma- 
tion of such breeds as the greyhound or the bull-dog, of the 
race-horse and cart-horse, of the fan tail pigeon or the otter- 
sheep, many co-ordinate adjustments have been produced ; and 
no difficulty has occurred, whether the change has been effected 
by a single variation as in the last case named or by slow 
steps, as in all the others. It seems to be forgotten that most 
animals have such a surplus of vitality and strength for all the 
ordinary occasions of life that any slight superiority in one 
part can be at once utilised ; while the moment any want of 
balance occurs, variations in the insufficiently developed parts 
will be selected to bring back the harmony of the whole 
organisation. The fact that, in all domestic animals, variations 
do occur, rendering them swifter or stronger, larger or smaller, 
stouter or slenderer, and that such variations can be separately 
selected and accumulated for man's purposes, is sufficient to 
render it certain that similar or even greater changes may be 
effected by natural selection, which, as Darwin well remarks, 
"acts on every internal organ, on every shade of constitu- 
tutional difference, on the whole machinery of life." The 
difficulty as to co-adaptation of parts by variation and natural 
selection appears to me, therefore, to be a wholly imaginary 
difficulty which has no place whatever in the operations of 

Direct Action of the Environment. 

Mr. Spencer's last objection to the wide scope given by 
Darwinians to the agency of natural selection is, that organisms 
are acted upon by the environment, which produces in them 
definite changes, and that these changes in the individual are 
transmitted by inheritance, and thus become increased in 
successive generations. That such changes are produced in 
the individual there is ample evidence, but that they are in- 


herited independently of any form of selection or of reversion 
is exceedingly doubtful, and Darwin nowhere expresses him- 
self as satisfied with the evidence. The two very strongest 
cases he mentions are the twenty-nine species of American 
trees which all differed in a corresponding way from their 
nearest European allies ; and the American maize which 
became changed after three generations in Europe. But in 
the case of the trees the differences alleged may be partly due 
to correlation with constitutional peculiarities dependent on 
climate, especially as regards the deeper tint of the fading leaves 
and the smaller size of the buds and seeds in America than in 
Europe ; while the less deeply toothed or serrated leaves in the 
American species are, in our present complete ignorance of the 
causes and uses of serration, quite as likely to be due to some 
form of adaptation as to any direct action of the climate. 
Again, we are not told how many of the allied species do not 
vary in this particular manner, and this is certainly an im- 
portant factor in any conclusion we may form on the question. 

In the case of the maize it appears that one of the more 
remarkable and highly selected American varieties was culti- 
vated in Germany, and in three years nearly all resem- 
blance to the original parent was lost ; and in the sixth year 
it closely resembled a common European variety, but was of 
somewhat more vigorous growth. In this case no selection 
appears to have been practised, and the effects may have been 
due to that " reversion to mediocrity " which invariably occurs, 
and is more especially marked in the case of varieties which 
have been rapidly produced by artificial selection. It may be 
considered as a partial reversion to the wild or unimproved 
stock ; and the same thing would probably have occurred, 
though perhaps less rapidly, in America itself. As this is 
stated by Darwin to be the most remarkable case known to 
him " of the direct and prompt action of climate on a plant," 
wo must conclude that such direct effects have not been proved 
to be accumulated by inheritance, independently of reversion 
or selection. 

The remaining part of Mr. Spencer's essay is devoted 
to a consideration of the hypothetical action of the environ- 
ment on the lower organisms which consist of simple cells or 
formless masses of protoplasm; and he shows with great 


elaboration that the outer and inner parts of these are 
necessarily subject to different conditions ; and that the outer 
actions of air or water lead to the formation of integuments, 
and sometimes to other definite modifications of the surface, 
whence arise permanent differences of structure. Although 
in these cases also it is very difficult to determine how much 
is due to direct modification by external agencies transmitted 
and accumulated by inheritance, and how much to spontaneous 
variations accumulated by natural selection, the probabilities 
in favour of the former mode of action are here greater, 
because there is no differentiation of nutritive and reproductive 
cells in these simple organisms ; and it can be readily seen 
that any change produced in the latter will almost certainly 
affect the next generation. 1 We are thus carried back almost 
to the origin of life, and can only vaguely speculate on what 
took place under conditions of which we know so little. 

The American School of Evolutionists. 

The tentative views of Mr. Spencer which we have just dis- 
cussed, are carried much further, and attempts have been made 
to work them out in great detail, by many American naturalists, 
whose best representative is Dr. E. D. Cope of Philadelphia. 2 
This school endeavours to explain all the chief modifications 
of form in the animal kingdom by fundamental laws of growth 
and the inherited effects of use and effort, returning, in fact, to 
the teachings of Lamarck as being at least equally important 
with those of Darwin. 

The following extract will serve to show the high position 
claimed by this school as original discoverers, and as having 
made important additions to the theory of evolution : 

"Wallace and Darwin have propounded as the cause of 
modification in descent their law of natural selection. This 
law has been epitomised by Spencer as the ' survival of the 
fittest/ This neat expression no doubt covers the case, but it 
leaves the origin of the fittest entirely untouched. Darwin 
assumes a ' tendency to variation ' in nature, and it is plainly 

1 This explanation is derived from Weismann's Theory of the Continuity 
of the Germ-Plasm as summarised in Nature. 

2 See a collection of his essays under the title, The Origin of the Fittest : 
Essays on Evolution. D. Appleton and Co. New York. 1887. 


necessary to do this, in order that materials for the exercise 
of a selection should exist. Darwin and Wallace's law is then 
only restrictive, directive, conservative, or destructive of some- 
thing already created. I propose, then, to seek for the origin- 
ative laws by which these subjects are furnished ; in other 
words, for the causes of the origin of the fittest." l 

Mr. Cope lays great stress on the existence of a special 
developmental force termed "bathmism" or growth- force, 
which acts by means of retardation and acceleration "without 
any reference to fitness at all ;" that "instead of being controlled 
by fitness it is the controller of fitness." He argues that " all 
the characteristics of generalised groups from genera up (ex- 
cepting, perhaps, families) have been evolved under the law of 
acceleration and retardation," combined with some intervention 
of natural selection ; and that specific characters, or species, 
have been evolved by natural selection with some assistance 
from the higher law. He, therefore, makes species arid genera 
two absolutely distinct things, the latter not developed out of 
the former ; generic characters and specific characters are, in 
his opinion, fundamentally different, and have had different 
origins, and whole groups of species have been simultaneously 
modified, so as to belong to another genus ; whence he thinks 
ifc " highly probable that the same specific form has existed 
through a succession of genera, and perhaps in different epochs 
of geologic time." 

Useful characters, he concludes, have been produced by the 
special location of growth-force by use ; useless ones have been 
produced by location of growth-force without the influence of 
use. Another element which determines the direction of 
growth-force, and which precedes use, is effort; and "it is 
thought that effort becomes incorporated into the metaphysical 
acquisitions of the parent, and is inherited with other meta- 
physical qualities by the young, which, during the period of 
growth, is much more susceptible to modifying influences, and 
is likely to exhibit structural change in consequence." 2 

From these few examples of their teachings, it is clear that 

1 Origin of the Fittest, p. 174. 

2 Ibid. p. 29. It may be here noted that Darwin found these theories 
unintelligible. In a letter to Professor E. T. Morse in 1877, he writes: 
"Thoro is one point which I regret you did not make clear in your Ad- 


these American evolutionists have departed very widely from 
the views of Mr. Darwin, and in place of the well-established 
causes and admitted laws to which he appeals have introduced 
theoretical conceptions which have not yet been tested by 
experiments or facts, as well as metaphysical conceptions 
which are incapable of proof. And when they come to 
illustrate these views by an appeal to palaeontology or 
morphology, we find that a far simpler and more complete 
explanation of the facts is afforded by the established principles 
of variation and natural selection. The confidence with which 
these new ideas are enunciated, and the repeated assertion 
that without them Darwinism is powerless to explain the 
origin of organic forms, renders it necessary to bestow a little 
more time on the explanations they give us of well-known 
phenomena with which, they assert, other theories are incom- 
petent to grapple. 

As examples of use producing structural change, Mr. Cope 
adduces the hooked and toothed beaks of the falcons and the 
butcher-birds, and he argues that the fact of these birds belong- 
ing to widely different groups proves that similarity of use has 
produced a similar structural result. But no attempt is made 
to show any direct causal connection between the use of a bill 
to cut or tear flesh and the development of a tooth on the 
mandible. Such use might conceivably strengthen the bill 
or increase its size, but not cause a special tooth-like outgrowth 
which was not present in the ancestral thrush-like forms of 
the butcher-bird. On the other hand, it is clear that any 
variations of the bill tending towards a hook or tooth would give 
the possessor some advantage in seizing and tearing its prey, 
and would thus be preserved and increased by natural selection. 
Again, Mr. Cope urges the effects of a supposed " law of polar 
or centrifugal growth " to counteract a tendency to un- 
symmetrical growth, where one side of the body is used more 
than the other. But the undoubted hurtfulness of want of 
symmetry in many important actions or functions would 
rapidly eliminate any such tendency. When, however, it has 

dress, namely, what is the meaning and importance of Professors Cope and 
Hyatt's views on acceleration and retardation ? I have endeavoured, and 
given up in despair, the attempt to grasp their meaning" (Life and Letters, 
vol. iii. p, 233). 


become useful, as in the case of the single enlarged claw of 
many Crustacea, it has been preserved by natural selection. 

Origin of the Fed of the Ungulates. 

Perhaps the most original and suggestive of Mr. Cope's 
applications of the theory of use and effort in modifying 
structure are, his chapters "On the Origin of the Foot-Structure 
of the Ungulates ; " and that " On the Effect of Impacts and 
Strains on the Feet of Mammalia ; " and they will serve also 
to show the comparative merits of this theory and that of 
natural selection in explaining a difficult case of modification, 
especially as it is an explanation claimed as new and 
original when first enunciated in 1881. Let us, then, see 
how he deals with the problem. 

The remarkable progressive change of a four or five-toed 
ancestor into the one-toed horse, and the equally remarkable 
division -of the whole group of ungulate animals into the odd- 
toed and oven-toed divisions, Mr. Cope attempts to explain 
by the effects of impact and use among animals which 
frequented hard or swampy ground respectively. On hard 
ground, it is urged, the long middle toe would be most 
used and subjected to the greatest strains, and would 
therefore acquire both strength and development. It would 
then be still more exclusively used, and the extra nourish- 
ment required by it would be drawn from the adjacent less- 
used toes, which would accordingly diminish in size, till, after 
a long series of changes, the records of which are so well 
preserved in the American tertiary rocks, the true one-toed 
horse was developed. In soft or swampy ground, on the other 
hand, the tendency would be to spread out the foot so that 
there were two toes on each side. The two middle toes 
would thus be most used and most subject to strains, and 
would, therefore, increase at the expense of the lateral toes. 
There would be, no doubt, an advantage in these two func- 
tional toes being of equal sixe, so as to prevent twisting of the 
foot while walking ; and variations tending to bring this about 
would be advantageous, and would therefore be preserved. 
Thus, by a parallel series of changes in another direction, 
adapted to a distinct set of conditions, we should arrive at the 
symmetrical divided hoofs of our deer and cattle. The fact- 


that sheep and goats are specially mountain and rock-loving 
animals may be explained by their being a later modification, 
since the divided hoof once formed is evidently well adapted 
to secure a firm footing on rugged and precipitous ground, 
although it could hardly have been first developed in such 
localities. Mr. Cope thus concludes : " Certain it is that the 
length of the bones in the feet of the ungulate orders has a 
direct relation to the dryness of the ground they inhabit, and 
the possibility of speed which their habit permits them or 
necessarily imposes on them." 1 

If there is any truth in the explanation hero briefly 
summarised, it must entirely depend on the fact of individual 
modifications thus produced being hereditary, and we yet 
await the proof of this. In the meantime it is clear that the 
very same results could have been brought about by variation 
and natural selection. For the toes, like all other organs, 
vary in size and proportions, and in their degree of union or 
separation ; and if in one group of animals it was beneficial to 
have the middle toe larger and longer, and in another set to 
have the two middle toes of the same size, nothing can be 
more certain than that these particular modifications would 
be continuously preserved, and the very results we see ulti- 
mately produced. 

The oft-repeated objections that the cause of variations is 
unknown, that there must be something to determine variations 
in the right direction ; that " natural selection includes no 
actively progressive principle, but must wait for the develop- 
ment of variation, and then, after securing the survival of the 
best, wait again for the best to project its own variations for 
selection," we have already sufficiently answered by showing 
that variation in abundant or typical species is always 
present in ample amount ; that it exists in all parts arid 
organs ; that these vary, for the most part, independently, so 
that any required combination of variations can be secured ; 
and finally, that all variation is necessarily either in excess or 
defect of the mean condition, arid that, consequently, the right 
or favourable variations are so frequently present that the 
unerring power of natural selection never wants materials to 
work upon. 

1 Origin qf the Fittest, p. 374. 


Supposed Action of Animal Intelligence. 

The following passage briefly summarises Mr. Cope's 
position : " Intelligence is a conservative principle, arid 
will always direct effort and use into lines which will be 
beneficial to its possessor. Here we have the source of the 
fittest, i.e. addition of parts by increase and location of 
growth-force, directed by the influence of various kinds of 
compulsion in the lower, and intelligent option among higher 
animals. Thus intelligent choice, taking advantage of the 
successive evolution of physical conditions, may be regarded 
as the originator of the fittest, while natural selection is the 
tribunal to which all results of accelerated growth are sub- 
mitted. This preserves or destroys them, and determines the 
new points of departure on which accelerated growth shall 
build." 1 

This notion of "intelligence" the intelligence of the 
animal itself determining its own variation, is so evidently a 
very partial theory, inapplicable to the whole vegetable king- 
dom, and almost so to all the lower forms of animals, amongst 
which, nevertheless, there is the very same adaptation and 
co-ordination of parts and functions as among the highest, that 
it is strange to see it put forward with such confidence as 
necessary for the completion of Darwin's theory. Tf " the 
various kinds of compulsion " by which are apparently meant 
the laws of variation, growth, and reproduction, the struggle 
for existence, and the actions necessary to preserve life under 
the conditions of the animal's environment are sufficient to 
have developed the varied forms of the lower animals and of 
plants, we can see no reason why the same " compulsion " 
should not have carried on the development of the higher 
animals also. The action of this " intelligent option " is alto- 
gether unproved ; while the acknowledgment that natural 
selection is the tribunal which either preserves or destroys the 
variations submitted to it, seems quite inconsistent with the 
statement that intelligent choice is the "orginator of the 
fittest," since whatever is really " the fittest " can never be 
destroyed by natural selection, which is but another name for 
the survival of the fittest If " the fittest " is always definitely 
1 Origin qf the Fittest, p. 40. 



produced by some other power, then natural selection is not 
wanted. If, on the other hand, both fit and unfit are produced, 
and natural selection decides between them, that is pure Dar- 
winism, and Mr. Cope's theories have added nothing to it. 

Fio. 35. Transformation of Artemia salina to A. Milhanscnii ; 1, tail-lobe of A. salina, 
and its transition through 2, 3, 4, 5, to f>, into that of A. Milhausenii; 7, 
post-abdomen of A. salina; 8, post-abdomen of a form bred in brackish 
water; 0, gill of A. Milhausenii ; 10, gill of A. salina. (From Schmanke- 

Semper on the Direct Influence of the Environment. 

Another eminent naturalist, Professor Karl Semper of 
Wurzburg, also adopts the view of the direct transforming 
power of the- environment, and has brought together an 



immense body of interesting facts showing the influence of 
food, of light, of temperature, of still water and moving water, 
of the atmosphere and its currents, of gravitation, and of other 
organisms, in modifying the forms and other characteristics of 
animals. 1 He believes that these various influences produce 
a direct and important effect, and that this effect is accumu- 
lated by inheritance ; yet he acknowledges that we have no 
direct evidence of this, and there is hardly a single case 
adduced in the book which is not equally well explained by 
adaptation, brought about by the survival of beneficial varia- 
tions. Perhaps the most remarkable case he has brought 
forward is that of the transformation of species of crustaceans 
by a change in the saltness of the water (see Fig. 35). Artemia 
salina lives in brackish water, while A. Milhausenii inhabits 
water which is much salter. They differ greatly in the form of 
the tail-lobes, and in the presence or absence of spines upon the 
tail, and had always been considered perfectly distinct species. 
Yet either was transformed into the other in a few generations, 
during which the saltness of the water was gradually altered. 
Yet more, A. salina was gradually accustomed to fresher 
water, and in the course of a few generations, when the water 
had become perfectly fresh, the species 
was changed into Branchipus stag- 
nalis, which had always been con- 
sidered to belong to a different genus 
on account of differences in the form 
of the antennae and of the posterior 
segments of the body (see Fig. 36). 
This certainly appears to be a proof 
of change of conditions producing 
a change of form independently of 
selection, and of that change of form, 
while remaining under the same con- 
ditions, being inherited. Yet there 
is this peculiarity in the case, that 
there is a chemical change in the water, and that this water 
permeates the whole body, and must be absorbed by the 
tissues, and thus affect the ova and oven the reproductive 

1 The Natural Conditions of Existence as they Affect Animal Life. 
London, 1883. 

FKJ. 30. 

Jiranelu pus stugnalis. 
b. Artomia salma. 


elements, and in this way may profoundly modify the whole 
organisation. Why and how the external effects are limited 
to special details of the structure we do not know ; hut it does 
not seem as if any far-reaching conclusions as to the cumula- 
tive effect of external conditions on the higher terrestrial 
animals and plants, can be drawn from such an exceptional 
phenomenon. It seems rather analogous to those effects of 
external influences on the very lowest organisms in which the 
vegetative and reproductive organs are hardly differentiated, 
in which case such effects are doubtless inherited. 1 

Professor Geddes's Theory of Variation in Plants. 

Iji a paper read before the Edinburgh Botanical Society in 
1886 Mr. Patrick Geddes laid down the outlines of a funda- 
mental theory of plant variation, which he has further ex- 
tended in the article " Variation and Selection " in the 
Encyclopedia Britannica, and in a paper read before the Linnsean 
Society but not yet published. 

A theory of variation should deal alike witk the origin of 
specific distinctions and with those vaster differences which 
characterise the larger groups, and he thinks it should answer 
such questions as How an axis comes to bo arrested to form 
a flower ? how the various forms of inflorescence were evolved ? 
how did perigynous or epigynous flowers arise from hypogynous 
flowers ? and many others equally fundamental. Natural selec- 
tion acting upon numerous accidental variations will not, he 
urges, account for such general facts as these, which must 
depend on some constant law of variation. This law he 
believes to be the well-known antagonism of vegetative and 
reproductive growth acting throughout the whole course of 
plant development ; and he uses it to explain many of the 
most characteristic features of the structure of flowers and 

1 In Dr. Weismaim's essay on " Heredity," already referred to, he considers 
it not improbable that changes in organisms produced by climatic influences 
may be inherited, because, as these changes do not affect the external parts 
of an organism only, but often, as in the case of warmth or moisture per- 
meate the whole structure, they may possibly modify the germ -plasm 
itself, and thus induce variations in the next generation. In this way, ho 
thinks, may possibly be explained the climatic varieties of certain butterflies, 
and some other changes which seem to be effected by change of climate in a 
few generations. 


Commencing with the origin of the flower, which all botanists 
agree in regarding as a shortened branch, he explains this 
shortening as an inevitable physiological fact, since the cost of 
the development of the reproductive elements is so great as 
necessarily to check vegetative growth. In the same manner 
the shortening of the inflorescence from raceme to spike or 
umbel, and thence to the capitulum or dense flower-head 
of the composite plants is brought about. This shortening, 
carried still further, produces the flattened leaf-like receptacle 
of Dorstenia, and further still the deeply hollowed fruity 
receptacle of the fig. 

The flower itself undergoes a parallel modification due to a 
similar cause. It is formed by a series of modified leaves 
arranged round a shortened axis. In its earlier stages the 
number of these modified leaves is indefinite, as in many 
Ranunculacese ; and the axis itself is not greatly shortened, as 
in Myosurus. The first advance is to a definite number of 
parts and a permanently shortened axis, in the arrangement 
termed hypogynous, in which all the whorls are quite distinct 
from each other. In the next stage there is a further shorten- 
ing of the central axis, leaving the outer portion as a ring on 
which the petals are inserted, producing the arrangement 
termed perigynous. A still further advance is made by the 
contraction of the axis, so as to leave the central part form- 
ing the ovary quite below the flower, which is then termed 

These several modifications are said to be parallel and 
definite, and to be determined by the continuous checking of 
vegetation by reproduction along what is an absolute groove 
of progressive change. This being the case, the importance of 
natural selection is greatly diminished. Instead of selecting 
and accumulating spontaneous indefinite variations, its function 
is to retard them after the stage of maximum utility has been 
independently reached. The same simple conception is said 
to unlock innumerable problems of vegetable morphology, large 
and small alike. It explains the inevitable development of 
gyjnnosperm into angiosperm by the checked vegetative growth 
of the ovule-bearing leaf or carpel ; while such minor adapta- 
tions as the splitting fruit of the geranium or the cupped stigma 
of the pansy, can be no longer looked upon as achievements 


of natural selection, but must be regarded as naturally trace- 
able to the vegetative checking of their respective types of 
leaf organ. Again, a detailed examination of spiny plants 
practically excludes the hypothesis of mammalian selection 
altogether, and shows spines to arise as an expression of the 
diminishing vegetativencss in fact, the ebbing vitality of a 
species. 1 

Objections to the Theory. 

The theory here sketched out is enticing, and at first sight 
seems calculated to throw much light on the history of plant 
development ; but on further consideration, it seems wanting 
in deiiniteness, while it is beset with difficulties at every step. 
Take first the shortening of the raceme into the umbel and the 
capitulum, said to be caused by arrest of vegetative growth, 
due to the antagonism of reproduction. If this were the 
whole explanation of the phenomenon, we should expect the 
quantity of seed to increase as this vegetative growth dimin- 
ished, since the seed is the product of the reproductive energy 
of the plant, and its quantity the best measure of that energy. 
But is this the case ? The ranunculus has comparatively few 
seeds, and the flowers are not numerous ; while in the same 
order the larkspur and the columbine have far more seeds as 
well as more flowers, but there is no shortening of the raceme 
or diminution of the foliage, although the flowers are large and 
complex. So, the extremely shortened and compressed flower- 
heads of the composite produce comparatively few seeds 
one only to each flower ; while the foxglove, with its long 
spike of showy flowers, produces an enormous number. 

Again, if the shortening of the central axis in the successive 
stages of hypogynous, perigynous, and epigynous flowers were an 
indication of preponderant reproduction and diminished vegeta- 
tion, we should find everywhere some clear indications of this 
fact. The plants with hypogynous flowers should, as a rule, 
have less seed and more vigorous and abundant foliage than 
those at the other extreme with epigynous flowers. But the 

1 This brief indication of Professor Geddes's views is taken from the 
article " Variation and Selection " in the Encyclopaedia Britannica, and a paper 
" On the Nature and Causes of Variation in Plants " in Trans, and Proc. of the 
Edinburgh Botanical Society, 1886 ; and is, for the most part, expressed in 
his own words. 


hypogynous poppies, pinks, and St. John's worts have abund- 
ance of seed and rather scanty foliage ; while the epigynous 
dogwoods and honeysuckles have few seeds and abundant 
foliage. If, instead of the number of the seeds, we take the 
size of the fruit as an indication of reproductive energy, we find 
this at a maximum in the gourd family, yet their rapid and 
luxuriant growth shows no diminution of vegetative power. 
So that the statement that plant modifications proceed " along 
an absolute groove of progressive change " is contradicted by 
innumerable facts indicating advance and regression, improve- 
ment or degradation, according as the ever-changing environ- 
ment renders one form more advantageous than the other. 
As one instance I may mention the Anonaceae or custard-apple 
tribe, which are certainly an advance from the Kanunculaceae ; 
yet in the genus Polyalthea the fruit consists of a number of 
separate carpels, each borne on a long stalk, as if reverting to 
the primitive stalked carpellary leaves. 

On the Origin of Spines. 

But perhaps the most extraordinary application of the 
theory is that which considers spines to be an indication of the 
" ebbing vitality of a species," and which excludes " mammalian 
selection altogether." If this were true, spines should occur 
mainly in feeble, rare, and dying-out species, instead of which 
we have the hawthorn, one of our most vigorous shrubs or trees, 
with abundant vitality and an extensive range over the whole 
Palsearctic region, showing that it is really a dominant species. 
In North America the numerous thorny species of Crataegus 
are equally vigorous, as are the false acacia (Robinia) and 
the honey-locust (Gleditschia). Neither have the numerous 
species of very spiny Acacias been noticed to be rarer or less 
vigorous than the unarmed kinds. 

On the other point that spines are not due to mammalian 
selection we are able to adduce what must be considered direct 
and conclusive evidence. For if spines, admittedly produced by 
aborted branches, petioles, or peduncles, are due solely or mainly 
to diminished vegetativeness or ebbing vitality, they ought to 
occur in all countries alike, or at all events in all whose similar 
conditions tend to check vegetation ; whereas, if they are, 
solely or mainly, developed as a protection against the attacks 


of herbivorous mammals, they ought to be most abundant 
where these are plentiful, and rare or absent where indigenous 
mammalia are wanting. Oceanic islands, as compared with 
continents, would thus furnish a crucial test of the two theories ; 
and Mr. Hemsley of Kew, who has specially studied insular 
floras, has given me some valuable information on this point. 
He says : " There are no spiny or prickly plants in the in- 
digenous element of the St. Helena flora. The relatively rich 
flora of the Sandwich Isles is not absolutely without a prickly 
plant, but almost so. All the endemic genera are unarmed, 
and the endemic species of almost every other genus. Even such 
genera as Zanthoxylon, Acacia, Xylosoma, Lycium, and Solanum, 
of which there are many armed species in other countries, 
are only represented by unarmed species. The two endemic 
Kubi have the prickles reduced to the setaceous condition, and 
the two palms are unarmed. 

" The flora of the Galapagos includes a number of prickly 
plants, among them several cacti (these have not been inves- 
tigated and may be American species), but I do not think one 
of the known endemic species of any family is prickly or 

" Spiny and prickly plants are also rare in New Zealand, 
but there are the formidably armed species of wild Spaniard 
( Aciphylla), one species of Rubus, the pungent-leaved Epacridese 
and a few others." 

Mr. J. G. Baker of Kew, who has specially studied the 
flora of Mauritius and the adjacent islands, also writes mo on 
this point. He says : " Taking Mauritius alone, I do not 
call to mind a single species that is a spinoso endemic tree or 
shrub. If you take the whole group of islands (Mauritius, 
Bourbon, Seychelles, and Rodriguez), there will be about a 
dozen species, but then nine of these are palms. Leaving 
out palms, the trees and shrubs of that part of the world are 
exceptionally non-spinose." 

These are certainly remarkable facts, and quite inexplicable 
on the theory of spines being caused solely by checked vege- 
tative growth, due to weakness of constitution or to an arid soil 
and climate. For the Galapagos and many parts of the Sand- 
wich Islands are very arid, as is a considerable part of the 
North Island of New Zealand. Yet in our own moist climate 


and with our very limited number of trees and shrubs we 
have about eighteen spiny or prickly species, more, apparently, 
than in the whole endemic floras of the Mauritius, Sandwich 
Islands, and Galapagos, though these are all especially rich 
in shrubby arid arboreal species. In New Zealand the prickly 
TCubus is a leafless trailing plant, and its prickles are probably 
a protection against the large snails of the country, several of 
which have shells from two to three and a half inches long. 1 
The " wild Spaniards " are very spiny herbaceous Umbelliferse, 
and may have gained their spines to preserve them from being 
trodden down or eaten by the Moas, which, for countless ages, 
took the place of mammals in New Zealand. The exact use 
or meaning of the spines in palms is more doubtful, though 
they are, no doubt, protective against some animals ; but it is 
certainly an extraordinary fact that in the entire flora of the 
Mauritius, so largely consisting of trees and shrubs, not a 
single endemic species should be thorny or spiny. 

If now we consider that every continental flora produces 
a considerable proportion of spiny and thorny species, and that 
these rise to a maximum in South Africa, where herbivorous 
mammalia were (before the settlement of the country), perhaps, 
more abundant and varied than in any other part of the 
world ; while another district, remarkable for well-armed 
vegetation, is Chile, where the camel-like vicugnas, llamas, and 
alpacas, and an abundance of large rodents wage perpetual 
war against shrubby vegetation, we shall see the full signifi^ 
cance of the almost total absence of thorny and spiny plants in 
the chief oceanic islands ; and so far from " excluding the 
hypothesis of mammalian selection altogether," we shall find 
in this hypothesis the only satisfactory explanation of the 

From the brief consideration of Professor Geddes's theory 
now given, we conclude that, although the antagonism between 
vegetative and reproductive growth is a real agency, and must 
be taken account of in our endeavour to explain many of the 
fundamental facts in the structure and form of plants, yet it 
is so overpowered and directed at every step by the natural 
selection of favourable variations, that the results of its 

1 Placostylis l>ovinus f 3 inches long ; Paryphanta BusLyi, 3 in. diam. ; 
P. Hochstetteri, !>J in. diani. 

2 F 


exclusive and unmodified action are nowhere to be found in 
nature. It may be allowed to rank as one of those " laws of 
growth," of which so many have now been indicated, and 
which were always recognised by Darwin as underlying all 
variation ; but unless we bear in mind that its action must 
always be subordinated to natural selection, and that it is 
continually checked, or diverted, or even reversed by the 
necessity of adaptation to the environment, we shall bo liable 
to fall into such glaring errors as the imputing to " ebbing 
vitality " alone such a widespread phenomenon as the occur- 
rence of spines and thorns, while ignoring altogether the 
influence of the organic environment in their production. 1 

The sketch now given of the chief attempts that have been 
made to prove that either the direct action of the environment 
or certain fundamental laws of variation are independent causes 
of modification of species, shows us that their authors have, 
in every case, failed to establish their contention. Any direct 
action of the environment, or any characters acquired by use 
or disuse, can have no effect whatever upon the race unless 
they are inherited ; and that they are inherited in any case, 

1 The general arguments and objections here set forth will apply with equal 
force to Professor G. Henslow's theory of the origin of the various forms 
and structures of flowers as due to "the responsive actions of the protoplasm 
in consequence of the irritations set up by the weights, pressures, thrusts, 
tensions, etc., of the insect visitors" (The Origin of Floral Structures through 
Insect and other Agencies, p. 340). On the assumption that acquired char- 
acters are inherited, such irritations may have had something to do with 
the initiation of variations and with the production of certain details of 
structure, but they are clearly incompetent to have brought about the 
more important structural and functional modifications of flowers. Such 
are, the various adjustments of length and position of the stamens to bring 
the pollen to the insect and from the insect to the stigma ; the various 
motions of stamens and styles at the right time and the right direction ; 
the physiological adjustments bringing about fertility or sterility in hetero- 
styled plants ; the traps, springs, and complex movements of various parts 
of orchids ; and innumerable other remarkable phenomena. 

For the explanation of these we have no resource but variation and selec- 
tion, to the effects of which, acting alternately with regression or degradation 
as above explained (p. 828) must be imputed the development of the count- 
less floral structures we now behold. Even the primitive flowers, whose 
initiation may, perhaps, have been caused, or rendered possible, by the 
irritation set up by insects' visits, must, from their very origin, have been 
modified, in accordance with the supreme law of utility, by means of varia- 
tion and survival of the fittest. 


except when they directly affect the reproductive cells, has 
not been proved. On the other hand, as we shall presently 
show, there is much reason for believing that such acquired 
characters are in their nature non-heritable. 

Variation and Selection Overpower the Effects of Use and Disuse. 

But there is another objection to this theory arising from 
the very nature of the effects produced. In each generation 
the effects of use or disuse, or of effort, will certainly be very 
small, while of this small effect it is riot maintained that the 
whole will be always inherited by the next generation. How 
small the effect is we have no means of determining, except 
in the case of disuse, which Mr. Darwin investigated carefully. 
He found that in twelve fancy breeds of pigeons, which are 
often kept in aviaries, or if free fly but little, the sternum 
had been reduced by about one-seventh or one-eighth of its 
entire length, and that of the scapula about one-ninth. In 
domestic ducks the weight of the wing-bones in proportion to 
that of the whole skeleton had decreased about one-tenth. 
In domestic rabbits the bones of the legs were found to have 
increased in weight in due proportion to the increased weight 
of the body, but those of the hind legs were rather less in 
proportion to those of the fore legs than in the wild animal, 
a difference which may be imputed to their being less used 
in rapid motion. The pigeons, therefore, afford the greatest 
amount of reduction by disuse one-seventh of the length of 
the sternum. But the pigeon has certainly been domesticated 
four or five thousand years ; and if the reduction of the wings 
by disuse has only been going on for the last thousand years, 
the amount of reduction in each generation would be absolutely 
imperceptible, and quite within the limits of the reduction 
due to the absence of selection, as already explained. But, as 
we have seen in Chapter III, the fortuitous variation of every 
part or organ usually amounts to one-tenth, and often to one- 
sixth of the average dimensions that is, the fortuitous varia- 
tion in one generation among a limited number of the in- 
dividuals of a species is as great as the cumulative effects of 
disuse in a thousand generations ! If we assume that the 
effects of use or of effort in the individual are equal to the 
effects of disuse, or even ten or a hundred times greater, they 


will even then not equal, in each generation, the amount of 
the fortuitous variations of the same part. If it be urged 
that the effects of use would modify all the individuals of a 
species, while the fortuitous variations to the amount named 
only apply to a portion of them, it may be replied, that that 
portion is sufficiently large to afford ample materials for 
selection, since it often equals the numbers that can annually 
survive ; while the recurrence in each successive generation of 
a like amount of variation would render possible such a rapid 
adjustment to new conditions that the effects of use or disuse 
would be as nothing in comparison. It follows, that even 
admitting the modifying effects of the environment, and that 
such modifications are inherited, they would yet be entirely 
swamped by the greater effects of fortuitous variation, and the 
far more rapid cumulative results of the selection of such 

Supposed Action of the Environment in Initiating Variations. 

It is, however, urged that the reaction of the environment 
initiates variations, which without it would never arise ; such, 
for instance, as the origin of horns through the pressures and 
irritations caused by butting, or otherwise using the head as a 
weapon or for defence. Admitting, for the sake of argument, 
that this is so, all the evidence we possess shows that, from the 
very first appearance of the rudiment of such an organ, it would 
vary to a greater extent than the amount of growth directly 
produced by use ; and these variations would be subject to 
selection, and would thus modify the organ in ways which use 
alone would never bring about. We have seen that this has 
been the case with the branching antlers of the stag, which 
have been modified by selection, so as to become useful 
in other ways than as a mere weapon ; and the same has 
almost certainly been the case with the variously curved 
and twisted horns of antelopes. In like manner, every con- 
ceivable rudiment would, from its first appearance, be subject 
to the law of variation and selection, to which, thenceforth, 
the direct effect of the environment would be altogether 

A very similar mode of reasoning will apply to the other 
branch of the subject the initiation of structures and organs 


by the action of the fundamental laws of growth. Admitting 
that such laws have determined some of the main divisions of 
the animal and vegetable kingdom, have originated certain 
important organs, and have been the fundamental cause of 
certain lines of development, yet at every step of the process 
these laws must have acted in entire subordination to the law of 
natural selection. No modification thus initiated could have 
advanced a single step, unless it were, on the whole, a useful 
modification ; while its entire future course would be necessarily 
subject to the laws of variation and selection, by which it 
would be sometimes checked, sometimes hastened on, sometimes 
diverted to one purpose, sometimes to another, according as the 
needs of the organism, under the special conditions of its 
existence, required such modification. We need not deny that 
such laws and influences may have acted in the manner 
suggested, but what we do deny is that they could possibly 
escape from the ever-present and all-powerful modifying effects 
of variation and natural selection. 1 

Weisinann's Theory of Heredity. 

Professor August Weismann has put forth a new theory of 
heredity founded upon the "continuity of the germ-plasm," 
one of the logical consequences of which is, that acquired 
characters of whatever kind are not transmitted from parent to 
offspring. As this is a matter of vital importance to the theory 
of natural selection, and as, if well founded, it strikes away the 
foundations of most of the theories discussed in the present 
chapter, a brief outline of Weismann's views must be attempted, 

1 In an essay on "The Duration of Life," forming part of the translation 
of Dr. Weismann's papers already referred to, the author still further 
extends the sphere of natural selection by showing that the average duration 
of life in each species has been determined by it. A certain length of life is 
essential in order that the species may produce offspring sufficient to ensure 
its continuance under the most unfavourable conditions ; and it is shown that 
the remarkable inequalities of longevity in different species and groups may 
be thus accounted for. Yet more, the occurrence of death in the higher 
organisms, in place of the continued survival of the unicellular organisms how- 
ever much they may increase by subdivision, may be traced to the same great 
law of utility for the race and survival of the fittest. The whole essay is of 
exceeding interest, and will repay a careful perusal. A similar idea occurred 
to the present writer about twenty years back, and was briefly noted down at 
the time, but subsequently forgotten. 


although it is very difficult to make them intelligible to persons 
unfamiliar with the main facts of modern embryology. 1 

The problem is thus stated by Weismanri : " How is it 
that in the case of all higher animals and plants a single cell 
is able to separate itself from amongst the millions of most 
various kinds of which an organism is composed, and by 
division and complicated differentiation to reconstruct a new 
individual with marvellous likeness, unchanged in many cases 
even throughout whole geological periods ? " Darwin at- 
tempted to solve the problem by his theory of "Pangenesis," 
which supposed that every individual cell in the body gave off 
gemmules or germs capable of reproducing themselves, and that 
portions of these germs of each of the almost infinite number of 
cells permeate the whole body and become collected in the 
generative cells, and are thus able to reproduce the whole 
organism. This theory is felt to be so ponderously complex 
and difficult that it has met with no general acceptance among 

The fact that the germ-cells do reproduce with wonderful 
accuracy not only the general characters of the species, but 
many of the individual characteristics of the parents or more 
remote ancestors, and that this process is continued from 
generation to generation, can be accounted for, Weismann 
thinks, only on two suppositions which are physiologically 
possible. Either the substance of the parent germ-cell, after 
passing through a cycle of changes required for the construction 
of a new individual, possesses the capability of producing anew 
germ-cells identical with those from which that individual was 
developed, or the new germ-cells arise, as far as their essential 
and characteristic substance is concerned, not at all out of the body 
of the individual, but direct from the parent germ-cell. This latter 
view Weismann holds to be the correct one, and, on this theory, 
heredity depends on the fact that a substance of special mole- 
cular composition passes over from one generation to another. 
This is the " germ-plasm," the power of which to develop itself 
into a perfect organism depends on the extraordinary complica- 
tion of its minutest structure. At every new birth a portion 

1 The outline here given is derived from two articles in Nature, vol. 
xxxiii. p. 154, and vol. xxxiv, p. 629, in which Weismaim's papers are summar- 
ised and partly translated. 


of the specific germ-plasm, which the parent egg-cell contains, 
is not used up in producing the offspring, but is reserved un- 
changed to produce the germ-cells of the following generation. 
Thus the germ-cells so far as regards their essential part the 
germ-plasm are not a product of the body itself, but are 
related to one another in the same way as are a series of 
generations of unicellular organisms derived from one another 
by a continuous course of simple division. Thus the question 
of heredity is reduced to one of growth. A minute portion 
of the very same germ-plasm from which, first the germ-cell, 
and then the whole organism of the parent, were developed, 
becomes the starting-point of the growth of the child. 

The Cause of Variation. 

But if this were all, the offspring would reproduce the 
parent exactly, in every detail of form and structure ; and 
here we see the importance of sex, for each new germ grows 
out of the united germ-plasms of two parents, whence arises a 
mingling of their characters in the offspring. This occurs in each 
generation ; hence every individual is a complex result repro- 
ducing in ever-varying degrees the diverse characteristics of his 
two parents, four grandparents, eight great-grandparents, and 
other more remote ancestors ; and that ever-present individual 
variation arises which furnishes the material for natural 'selec- 
tion to act upon. Diversity of sex becomes, therefore, of primary 
importance as the cause of variation. Where asexual genera- 
tion prevails, the characteristics of the individual alone are 
reproduced, and there are thus no means of effecting the 
change of form or structure required by changed conditions of 
existence. Under such changed conditions a complex organ- 
ism, if only asexually propagated, would become extinct. But 
when a complex organism is. sexually propagated, there is an 
ever-present cause of change which, though slight in any one 
generation, is cumulative, and under the influence of selection 
is sufficient to keep up the harmony between the organism 
and its slowly changing environment. 1 

1 There are many indications that this explanation of the cause of variation 
is the true one. Mr. E. B. Poulton suggests one, in the fact that partheno- 
genetic reproduction only occurs in isolated species, not in groups of related 
species ; as this shows that parthenogenesis cannot lead to the evolution of 


T/ie Non-Heredity of Acquired Characters. 

Certain observations on the embryology of the lower 
animals are held to afford direct proof of this theory of heredity, 
but they are too technical to bo made clear to ordinary 
readers. A logical result of the theory is the impossibility of 
the transmission of acquired characters, since the molecular 
structure of the germ-plasm is already determined within the 
embryo ; and Weismann holds that there are no facts which 
really prove that acquired characters can be inherited, although 
their inheritance has, by most writers, been considered so prob- 
able as hardly to stand in need of direct proof. 

We have already shown, in the earlier part of this chapter, 
that many instances of change, imputed to the inheritance of 
acquired variations, are really cases of selection ; while the very 
fact that use implies usefulness renders it almost impossible to 
eliminate the action of selection in a state of nature. As 
regards mutilations, it is generally admitted that they are not 
hereditary, and there is ample evidence on this point. When 
it was the fashion to dock horses' tails, it was not found that 
horses were born with short tails ; nor are Chinese women 
born with distorted feet ; nor are any of the numerous forms 
of racial mutilation in man, which have in some cases been 
carixtd on for hundreds of generations, inherited. Neverthe- 
less, a few cases of apparent inheritance of mutilations have 
been recorded, 1 and these, if trustworthy, are difficulties in the 
way of the theory. The undoubted inheritance of disease is 
hardly a difficulty, because the predisposition to disease is a 
congenital, not an acquired character, and as such would be the 
subject of inheritance. The often-quoted case of a disease 
induced by mutilation being inherited (Brown-Sequard's 
epileptic guinea-pigs) has been discussed by Professor Weis- 
mann, and shown to be not conclusive. The mutilation itself 
section of certain nerves was never inherited, but 

new forms. Again, iii parthenogenetic females the complete apparatus for 
fertilisation remains unreduced ; but if these varied as do sexually produced 
animals, the organs referred to, being unused, would become rudimentary. 

Even more important is the significance of the " polar bodies," as explained 
by Weismann in one of his Essays ; since, if his interpretation of them be 
correct, variability is a necessary consequence of sexual generation. 

J Darwin's Animals and Plants, vol. ii. pp. 23, 24. 


the resulting epilepsy, or a general state of weakness, de- 
formity, or sores, was sometimes inherited. It is, however, 
possible that the mere injury introduced and encouraged the 
growth of certain microbes, which, spreading through the 
organism, sometimes reached the germ-cells, and thus trans- 
mitted a diseased condition to the offspring. Such a transfer- 
ence of microbes is believed to occur in syphilis and tuberculosis, 
and has been ascertained to occur in the case of the muscardine 
silkworm disease. 1 

The Theory of Instinct. 

The theory now briefly outlined cannot be said to be 
proved, but it commends itself to many physiologists as being 
inherently probable, and as furnishing a good working 
hypothesis till displaced by a better. We cannot, therefore, 
accept any arguments against the agency of natural selection 
which are based upon the opposite and equally unproved 
theory that acquired characters are inherited ; and as this 
applies to the whole school of what may be termed Neo- 
Lamarckians, their speculations cease to have any weight. 

The same remark applies to the popular theory of instructs 
as being inherited habits ; though Darwin gave very little 
weight to this, but derived almost all instincts from spontaneous 
useful variations which, like other spontaneous variations, are 
of course inherited. At first sight it appears as if the acquired 
habits of our trained dogs pointers, retrievers, etc. are 
certainly inherited ; but this need not be the case, because 
there must be some structural or psychical peculiarities, such 
as modifications in the attachments of muscles, increased 
delicacy of smell or sight, or peculiar likes and dislikes, 
which are inherited ; and from these, peculiar habits follow 
as a natural consequence, or are easily acquired. Now, as 
selection has been constantly at work in improving all our 
domestic animals, we have unconsciously modified the structure, 
while preserving only those animals which best served our 
purpose in their peculiar faculties, instincts, or habits. 

1 In. his essay on " Heredity," Dr. Weismann discusses many other cases 
of supposed inheritance of acquired characters, and shows that they can all 
be explained in other ways. Shortsightedness among civilised nations, for 
example, is due partly to the absence of selection and consequent regression 
towards a mean, and partly to its individual production by constant reading. 


Much of the mystery of instinct arises from the persistent 
refusal to recognise the agency of imitation, memory, observa- 
tion, and reason as often forming part of it. Yet there is 
ample evidence that such agency must be taken into account. 
Both Wilson and Leroy state that young birds build inferior 
nests to old ones, and the latter author observes that the best 
nests are made by birds whose young remain longest in the 
nest. So, migration is now well ascertained to be effected by 
means of vision, long flights being made on bright moonlight 
nights when the birds fly very high, while on cloudy nights 
they fly low, and then often lose their way. Thousands 
annually fly out to sea and perish, showing that the instinct 
to migrate is imperfect, and is not a good substitute for reason 
and observation. 

Again, much of the perfection of instinct is due to the 
extreme severity of the selection during its development, any 
failure involving destruction. The chick which cannot break 
the eggshell, the caterpillar that fails to suspend itself properly 
or to spin a safe cocoon, the bees that lose their way or that 
fail to store honey, inevitably perish. So the birds that fail 
to feed and protect their young, or the butterflies that lay 
their eggs on the wrong food-plant, leave no offspring, and 
the race with imperfect instincts perishes. Now, during the 
long and very slow course of development of each organism, 
this rigid selection at every step of progress has led to the 
preservation of every detail of structure, faculty, or habit that 
has been necessary for the preservation of the race, and has 
thus gradually built up the various instincts which seem so 
marvellous to us, but which can yet be shown to be in many 
cases still imperfect. Here, as everywhere else in nature, we 
find comparative, not absolute perfection, with every gradation 
from what is clearly due to imitation or reason up to what 
seems to us perfect instinct that in which a complex action 
is performed without any previous experience or instruction. 1 

1 Weismann explains instinct on similar lines, and gives many interesting 
illustrations (see Essays on Heredity}. He holds "that all instinct is entirely 
due to the operation of natural selection, and has its foundation, not upon 
inherited experiences, but upon variations of the germ." Many interesting 
and difficult cases of instinct are discussed by Darwin in Chapter VIII of the 
Origin of Spevies, which should be read in connection with the above remarks. 

Since this chapter was written my attention has been directed to Mr, 


Concluding Remarks. 

Having now passed in review the more important of the 
recent objections to, or criticisms of, the theory of natural 
selection, we have arrived at the conclusion that in no one 
ease have the writers in question been able materially to 
diminish its importance, or to show that any of the laws or 
Forces to which they appeal can act otherwise than in strict 
subordination to it. The direct action of the environment as 
set forth by Mr. Herbert Spencer, Dr. Cope, arid Dr. Karl 
Semper, even if we admit that its effects on the individual 
ure transmitted by inheritance, are so small in comparison 
with the amount of spontaneous variation of every part of 
the organism that they must be quite overshadowed by the 
latter. And if such direct action may, in some cases, have 
initiated certain organs or outgrowths, these must from their 
very first beginnings have been subject to variation and 
natural selection, and their further development have been 
almost wholly due to these ever-present and powerful causes. 

Francis Goilton's Theory of Heredity (already referred to at p. 417) which 
was published thirteen years ago as an alternative for Darwin's theory of 

Mr. Galton's theory, although it attracted little attention, appears to me 
to be substantially the same as that of Professor Weismann. Gal ton's 
"stirp " is Weismann's "germ-plasm." Galton supposes the sexual elements 
in the offspring to be directly formed from the residue of the stirp not used 
up in the development of the body of the parent Weismann's '* continuity 
of the germ-plasm." Galton also draws many of the same conclusions from 
his theory. He maintains that characters acquired by the individual as the 
result of external influences cannot be inherited, unless such influences act 
directly on the reproductive elements instancing the possible heredity of 
alcoholism, because the alcohol permeates the tissues and may reach the 
sexual elements. He discusses the supposed heredity of effects produced by 
use or disuse, and explains them much in the same manner as does Weismann, 
Galton is an anthropologist, and applies the theory, mainly, to explain the 
peculiarities of hereditary transmission in man, many of which peculiarities 
he discusses and elucidates. Weismann is a biologist, and is mostly concerned 
with the application of the theory to explain variation and instinct, and to 
the further development of the theory of evolution. He has worked it out 
more thoroughly, and has adduced embryologicai evidence in its support ; but, 
the views of both writers are substantially the same, and their theories were 
nrrived at quite independently. The names of Galton and Weismann should 
therefore be associated as discoverers of what may be considered (if finally 
established) the most important contribution to the evolution theory since tho 
appearance of the Origin of tfpecies. 


The same remark applies to the views of Professor Geddes on 
the laws of growth which have determined certain essential 
features in the morphology of plants and animals. The 
attempt to substitute these laws for those of variation and 
natural selection has failed in cases where we can apply a 
definite test, as in that of the origin of spines on trees and 
shrubs ; while the extreme diversity of vegetable structure 
and form among the plants of the same country and of the 
same natural order, of itself affords a proof of the preponder- 
ating influence of variation and natural selection in keeping 
the many diverse forms in harmony with the highly complex 
and ever-changing environment. 

Lastly, we have seen that Professor Weismann's theory of 
the continu