CO > 00
S< OU_1 62954 >m
OSMANIA / UNrVERSITY LIBRARY
- - : ~ //>/?/
Call No. $9 / '* / '*' Accession No. /
A '
Title
Author -' 1
\
This book should be returned on or before the date
last marked below.
DAKWINISM
AN EXPOSITION OF THE
THEORY OF NATUKAL SELECTION
WITH SOME OF ITS APPLICATIONS
BY
ALFRED RUSSEL WALLACE
LL.D., F.L.S., KICJ.
WITH A PORTRAIT OF THE AUTHOR, MAP AND ILLUSTRATION?
THF HYDERABAD BOOK D^POT
CHADERGHAT HYDERABAD DIM.
ILontion
MACMILLAN AND CO.
AND NEW YORK
1890
First Edition published May 1889
/? mtcd A ugust and October 1889 ; 181,0
PEEFACE TO FIRST EDITION
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
vi PREFACE
are urged with much force and more confidence, and for the
most part by the modern school of laboratory naturalists, to
whom the peculiarities and distinctions of species, as such,
their distribution and their affinities, have little interest as
compared with the problems of histology and embryology,
of physiology and morphology. Their work in these depart-
ments is of the greatest interest and of the highest importance,
but it is not the kind of work which, by itself, enables one to
form a sound judgment on the questions involved in the
action of the law of natural selection. These rest mainly on
the external and vital relations of species to species in a state
of nature on what has been well termed by Semper the
"physiology of organisms," rather than on the anatomy or
physiology of organs.
It has always been considered a weakness in Darwin's
work that he based his theory, primarily, on the evidence of
variation in domesticated animals and cultivated plants. I
have endeavoured to secure a firm foundation for the theory
in the variations of organisms in a state of nature ; and as
the exact amount and precise character of these variations is
of paramount importance in the numerous problems that
arise when we apply the theory to explain the facts of nature,
I have endeavoured, by means of a series of diagrams, to
exhibit to the eye the actual variations as they are found to
exist in a sufficient number of species. By doing this, not
only does the reader obtain a better and more precise idea of
variation than can be given by any number of tabular state-
ments or cases of extreme individual variation, but we obtain
a basis of fact by which to test the statements and objections
usually put forth on the subject of specific variability ; and it
will be found that, throughout the work, I have frequently to
appeal to these diagrams and the facts they illustrate, just as
Darwin was accustomed to appeal to the facts of variation
among dogs and pigeons.
PREFACE vii
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.
vin PREFACE
I thus take up Darwin's earlier position, from which he some-
what receded in the later editions of his works, on account
of criticisms and objections which I have endeavoured to show
are unsound. Even in rejecting that phase of sexual selection
depending on female choice, I insist on the greater efficacy
of natural selection. This is pre-eminently the Darwinian
doctrine, and I therefore claim for my book the position of
being the advocate of pure Darwinism.
I wish to express my obligation to Mr. Francis Darwin for
lending me some of his father's unused notes, 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
CONTENTS
CHAPTER 1
WHAT ARE " SPECIES " AND WHAT IS MEANT BY THEIR
" ORIGIN "
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
CHAPTEE II
THE STRUGGLE FOR EXISTENCE
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
CHAPTEE III
THE VARIABILITY OF SPECIES IN A STATE OF NATURE
Importance of variability Popular ideas regarding it Variability of the
lower animals Tho variability of insects Variation among lizards
CONTENTS
Variation among birds Diagrams of bird -variation Number of
varying individuals Variation in the mammalia Variation in
internal organs Variations in the skull Variations in the habits of
animals The variability of plants Species which vary little Con-
cluding remarks ..... Pages 41-82
CHAPTER IV
VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED
/ PLANTS
The facts of variation and artificial selection Proofs of the generality of
variation Variations of apples and melons Variations of flowers
Variations of domestic animals Domestic, pigeons Acclimatisation
Circumstances favourable to selectjoii_by man Conditions favour-
able to variation- Concluding remarks . , . 83*101
CIIAPTElt V
NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE
FITTEST
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
CHAPTER VI
DIFFICULTIES AND OBJECTIONS
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
CONTENTS xi
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
CHAPTER VII
ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES
AND THE USUAL STERILITY OF THEIR HYBRID OFFSPRING
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
CHAPTEK VIII
THE ORIGIN AND USES OF COLOUR IN ANIMALS
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
xii CONTENTS
CHAPTER IX
WARNING COLORATION AND MIMICRY
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
CHAPTER X
COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX
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
CHAPTER XI
THE SPECIAL COLOURS OF PLANTS: THEIR ORIGIN
AND PURPOSE
The general colour relations of plants Colours of fruits The meaning of
nuts Edible or attractive fruits The colours of flowers Modes of
securing cross-fertilisation The interpretation of the facts Summary
CONTENTS xili
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
CHAPTER XII
THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS
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
CHAPTER XIII
THE GEOLOGICAL EVIDENCES OF EVOLUTION
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
xiv CONTENTS
CHAPTER XIV
FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION
AND HEREDITY
Fundamental difficulties and objections Mr. Herbert Spencer's factors
of organic evolution Disuse and effects of withdrawal of natural
selection Supposed effects of disuse among wild animals Difficulty
as to co-adaptation of 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
CHAPTER XV
DARWINISM APPLIED TO MAN
General identity of human and animal structure Rudiments and varia-
tions showing relation of man to other mammals The embryonic
development of man and other mammalia Diseases common to man
and the lower animals The animals most nearly allied to man
The brains of man and apes External differences of man and apes-
Summary of the animal characteristics of man The geological
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
LIST OF ILLUSTRATIONS
PORTRAIT OF AUTHOR . ... Frontispiece
MAP SHOWING THE 1000-FATHOM LINE . . To face page 349
FTO. PAOtt
1. DIAGRAM OF VARIATIONS OF LACKRTA MURALIS . . 47
2. ,, VARIATION OF LIZARDS . . . , . 48
3. ,, VARIATION OF WINGS AND TAIL OF BIRDS . 53
4. ,, VARIATION OF DOLICHONYX ORYZIVORUS . 55
5. ,, VARIATION OF AGEL^EUS PHQENICEUS . . 56
fi. ,, VARIATION OF CARDINALLS VIRGINIANUS . 58
7. ,, VARIATION OF TARSUS AND TOES . . 60
8. ,, VARIATION OF BIRDS IN LEYDEN MUSEUM . 61
9. ,, VARIATION OF ICTERUS BALTIMORE . . 63
10. VARIATION OF AGEUEUS PIKENICEUS . . 64
11. ,, CURVES OF VARIATION . . . .64
12. ,, VARIATION OF CARDINALLS VIRGINIANUS . 65
13. ,, VARIATION OF SCIURUS CAROLINENSIS . . 67
14. ,, VARIATION OF SKULLS OF WOLF . . 70
15. ,, VARIATION OF SKULLS OF UKSUH LABIATUS . 72
16. VARIATION OF SKULLS OF Sus CRISTATUS . 73
17. PRIMULA VRRIS (Cowslip), From Darwin's Forms of Floivers . 157
18. GAZELLA SOSMMERRINGI (to show recognition marks) . . 219
19. RECOGNITION MARKS OF AFRICAN PLOVERS (from Seebohm's
Charadriadcc ....... 221
xvi LIST OF ILLUSTRATIONS
20. RECOGNITION OF (EDICNEMUS VERMICULATUS AND (E. SENEGA-
LENSIS (from Seebolim's Charadriarfcc) , . . 223
21. RECOGNITION OF CURSORIUS CHALCOPTERUS AND C. GALLICUS
(from Seebolim's Charadrladcc) , . . 224
22. RECOGNITION OF SCOLOPAX MEGA LA AND S. STENURA (from
Seebolim's Cfiaradrindru) ..... 225
23. METHONA PSIDII AND LEPTAUS OIIISE . . . .241
24. OPTHALMIS LINGER AND ARTAX\ SIMULANS (from tlie Official
Narrative of the Voyage of the Challenger) . . . 247
25 WINGS OF ITUNA ILIONE AND THYJUDIA MRGISTO (from Pro-
ceedings of the Entomological Society) .... 251
26. MYGNIMIA AVICULUS AND COLO BO RHOMBUS FASCIATIPENNIS . 259
27. MIMICKING INSECTS FROM THE PHILIPPINES (from Semper's
Animal Life) . . . . . . .260
28. MALVA SYLVESTRIS AND M. HOTUNDIFOLIA (from Lubbock's
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
31. HUMMING-BIRD FERTILISING MARCGRAVIA NEPENTHOIDES . 320
32. DIAGRAM OF MEAN HEIGHT OF LAND AND DEPTH OF OCEANS 345
33. GEOLOGICAL DEVELOPMENT OF THE HORSE TRIBE (from Huxley's
American Addresses') ...... 388
34. DIAGRAM ILLUSTRATING THE GEOLOGICAL DISTRIBUTION OF
PLANTS (from Ward's Sketch of Palwobotany) . . . 402
35. TilANSFORMATION OF AllTEMIA SALINA TO A. MlLHAUSENII
(from Semper's Animal Life) ..... 426
36. BRANCHIPUS STAGNALIS AND ARTEMIA SALINA (from Semper's
Animal Life) . . . . . . .427
37. CHIMPANZEE (TROGLODYTES NIGER) .... 454
CHAPTER I
WHAT ARE "SPECIES," AND WHAT IS MEANT BY
THEIR " ORIGIN "
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
B
DARWINISM CHAP.
produce fertile individuals, and^ which reproduce themselves
by generation^ in such a manner that we may from analogy
suppose them all to have sprung from one
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
WHAT ARE SPECIES
" 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.
DARWINISM CHAP.
nature and origin of species. They render it clear that,
notwithstanding the vast knowledge and ingenious reasoning
of Lamarck, and the more general exposition of. the subject by
the author of the Vestiges of Creation, the first step had not
been taken towards a satisfactory explanation of the deriva-
tion of any one species from any other. Such eminent
naturalists as 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
WHAT ARE SPECIES
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
DARWINISM CHAP.
animals and plants which live together in any one country,
and to give some rational account of the phenomena presented
by their distribution in different parts of the world. And,
lastly, we may expect it to explain many difficulties and to
harmonise many incongruities in the excessively complex
affinities and relations of living things. All this the Darwinian
theory undoubtedly does. It shows us how, by means of
some of the most universal and 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
WHAT ARE SPECIES
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
10 DARWINISM CHAP.
Species should be carefully considered. It is as follows :
"Although much remains obscure, and will long remain
obscure, I can entertain no doubt, after the most deliberate
and dispassionate judgment of which I am capable, that the
view which most naturalists until recently entertained and
which I formerly entertained namely, that each species has
been independently created is 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
obscure."
The Darwinian Theory.
As it will be necessary, in the following chapters, to set
forth a considerable body of facts in almost every department
of natural history, in order to establish the fundamental
propositions on which the theory of natural selection rests,
I propose to give a preliminary statement of what the theory
really is, in order that the reader may better appreciate the
necessity for discussing so many details, and may thus feel a
more enlightened interest in them. Many of the facts to be
adduced are so novel and so curious that they are sure to be
appreciated by every one who takes an interest in nature, but
unless the need of them is clearly seen it may be thought that
time is being wasted on mere curious details and strange facts
which have little bearing on the question at issue.
The theory of natural selection rests on two main classes
of facts which apply to all organised beings without exception,
and which thus take rank as fundamental principles or laws.
The first is, the power of rapid multiplication in a geometrical
progression ; the second, that the offspring always vary slightly
from the parents, though generally very closely resembling
I WHAT ARE SPECIES 13
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
variations.
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
animals.
CHAPTER II
THE STRUGGLE FOR EXISTENCE
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
CHAP, ii THE STRUGGLE FOR EXISTENCE 15
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,
16 DARWINISM CUA
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
soil.
These few illustrations sufficiently show that all the plants
of a country are, as De Candolle says, at war with each other,
each one struggling to occupy ground at the expense of its
neighbour. But, besides 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
tubers.
In animals, it is the eggs or the very young that suffer most
from their various enemies ; in plants, the tender seedlings
when they first appear above the ground. To illustrate this
latter point Mr. Darwin cleared and dug a piece of ground
three feet long and two feet wide, and then marked all the
seedlings of weeds and other plants which came up, noting
what became of them. The total number was 357, and out
of these no less than 295 were destroyed by slugs and insects.
The direct strife of plant with plant is almost equally fatal
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.
THE STRUGGLE FOR EXISTENCE 17
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.
C
18 DARWINISM CHAP.
so as to keep out cattle. On ascertaining this, Mr. Darwin
was so much surprised that he searched among the heather in
the unenclosed parts, and there he found multitudes of little
trees and seedlings which had been perpetually browsed down
by the cattle. In one square yard, at a point about a hundred
yards from one of the old clumps of firs, he counted thirty-
two little trees, and one of them had twenty-six rings of
growth, showing that it had for many years tried to raise its
head above the stems of the heather and had failed. Yet
this heath was very extensive and very barren, and, as Mr.
Darwin remarks, no one 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
THE STRUGGLE FOR EXISTENCE 19
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.
20 DARWINISM CHAP.
Such cases as the above may perhaps be thought excep-
tional, but there is good reason to believe that they are by no
means rare, but are illustrations of what is going on in every
part of the world, only it is very difficult for us to trace out
the complex reactions that are everywhere occurring. The
general impression of the ordinary observer seems to be that
wild animals and plants live peaceful lives and have
few troubles, each being exactly suited to its place and
surroundings, and therefore having no difficulty in maintain-
ing itself. Before showing that this view is, everywhere
and always, demonstrably 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-
ii THE STRUGGLE FOR EXISTENCE 21
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.
22 DARWINISM CHAP.
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
ii THE STRUGGLE FOR EXISTENCE 23
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.
24 DARWINISM CHAP.
river sometimes rose 30 feet in eight hours, doing immense
destruction, and the abundance of the larger carnivora and
large reptiles on its banks, he goes on : " But it was among
the flora that the principle of natural selection was most
prominently displayed. In such a district overrun 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.
THE STRUGGLE FOR EXISTENCE 25
Increase of Organisms in a Geometrical Ratio.
The facts which ha.ve 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
26 DARWINISM CHAP.
two or more broods a year, ten will be below the average of
the year's increase. Such birds as these often live from fifteen
to twenty years in confinement, and we cannot suppose them to
live shorter lives in a state of nature, if unmolested ; but to
avoid possible exaggeration we will take only ten years as the
average duration of their lives. Now, if we start with a single
pair, and these are allowed to live and breed, unmolested, till
they die at the end of ten years, as they might do if turned
loose into a good-sized island with ample vegetable and insect
food, but no other competing or destructive birds or quadrupeds
their numbers would amount to more than twenty millions.
But we know very well 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
ir THE STRUGGLE FOR EXISTENCE 27
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
28 DARWINISM CHAP.
of America where open pastures offered suitable conditions.
Asses, about fifty years after their introduction, ran wild and
multiplied so amazingly in Quito, that the Spanish traveller
Ulloa describes them as being a nuisance. They grazed
together in great herds, defending themselves with their
mouths, and if a horse strayed among 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.
ii THE STRUGGLE FOR EXISTENCE 29
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
grasses.
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
30 DARWINISM CHAP.
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
ii THE STRUGGLE FOR EXISTENCE 31
American naturalist, Alexander Wilson, will be read with
interest:
" 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
maturity.
32 DARWINISM CHAP.
under these flying and fluttering millions, from the frequent
fall of large branches, broken down by the weight of the
multitudes above, and which in their descent often destroyed
numbers of the birds themselves ; while the clothes of those
engaged in traversing the woods were completely covered
with the excrements of the pigeons.
" These circumstances were related to me by many of the
most respectable part of the community in that quarter, and
were confirmed in part by what I myself witnessed. 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
THE STRUGGLE FOR EXISTENCE 33
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
D
34 DARWINISM
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 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.
40 DARWINISM CHAP, n
is heartrending. We have a horror of all violent and sudden
death, because we think of the life full of promise cut short,
of hopes and expectations unfulfilled, and of the grief of
mourning relatives. But all this is quite out of place in the
case of animals, for whom a violent and a sudden death is in
every way the best. Thus the poet's picture of
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
deaths.
On the whole, then, we conclude that the popular idea of
the struggle for existence entailing misery and pain on the
animal world is the very reverse of the truth. What it
really brings about, is, the maximum of life and of the enjoy-
ment of life with the minimum of suffering and pain. Given
the necessity of death and reproduction and without these
there could have been no progressive development of the
organic world, and it is difficult even to imagine a system
by which a greater balance of happiness could have been
secured. And this view was evidently that of Darwin himself,
who thus concludes his chapter on the struggle for existence :
" When we reflect on this struggle, we may console ourselves
with the full belief that the war of nature is not incessant,
that no fear is felt, that death is generally prompt, and that
the vigorous, the healthy, and the happy survive and
multiply."
CHAPTER III
THE VARIABILITY OF SPECIES IN A STATE OF NATURE
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
42 DARWINISM CHAP.
have been carefully described, so that we possess a consider-
able mass of information on the subject. Utilising this in-
formation we will now endeavour to give some idea of the
nature and extent of variation in the species of animals arid
plants.
It is very commonly objected that the widespread and
constant variability which is admitted to be a characteristic of
domesticated animals and cultivated plants is largely due to
the unnatural conditions of their existence, and that we have
no proof of any corresponding amount of variation occurring
in a state of nature. Wild animals and plants, it is said, are
usually stable, and 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
in VARIABILITY OF SPECIES IN A STATE OF NATURE 43
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.
44 DARWINISM CHAP.
subject to great variation, so that there is much uncertainty as
to the number of species ; and variations are especially frequent
in the 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.
in VARIABILITY OF SPECIES IN A STATE OF NATURE 45
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.
46 DARWINISM CHAP.
wings, on which generic and family distinctions are often
established, are also subject to variation. The Rev. R. P.
Murray, in 1872, laid before the Entomological Society
examples of such variation in six species of butterflies, and
other cases have been since described. The 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
DIAGRAM OF VARIATION
47
7 3 5 7 p 77
Head.
^
n _/,.
^"
^l -*
***
body.
x
Mean length. Sin.
/
\
Nech _
\
/
^x
/
\
Af era/7 length. 1. 18in.
Fore Leg.
--*,
^x*'
X
^ ,
/
^s,
/
Mean length. 1.05in.
Hind Leg. .
X
^-^
"^x
/
\
/
/
^^s
^
\
/
/
\
Mean length. 1.90in,
Toeof Hind Foot
/
\
s
^-^
^*^
~"
^^
Mean length. 0. 70in.
357
The lengths in the table are given in millimetres, which are here reduced
to inches for the means.
Fio. 1. -Variations of Lacerta muralis.
48 DARWINISM
.Neck
.Body
Lacerta ocellata
^^^^^H Tail
..Neck
Lacerta uiridis ' J! y . .
14. Hind Legs
' ' '...Tail
M Neck
** Body
Lacerta agilis
M- ///'/
