The Project Gutenberg ebook of Darwinism (1889), by Alfred Russel Wallace

help of an ordinary divided rule or a pair of compasses the variation of

the different parts can be ascertained and compared just as if the

specimens themselves were before the reader, but with much greater ease.
In my lectures on the Darwinian theory in America and in this country I

used diagrams constructed on a different plan, equally illustrating the

large amount of independent variability, but less simple and less

intelligible. The present method is a modification of that used by Mr.

Francis Galton in his researches on the theory of variability, the upper

line (showing the variability of the body) in Diagrams 4, 5, 6, and 13,

being laid down on the method he has used in his experiments with

sweet-peas and in pedigree moth-breeding.[25] I believe, after much

consideration, and many tedious experiments in diagram-making, that no

better method can be adopted for bringing before the eye, both the

amount and the peculiar features of individual variability.

_Variations of the Habits of Animals._

structure which have been already described, are the changes of habits

which often occur in certain individuals or in whole species, since

these must necessarily depend upon some corresponding change in the

brain or in other parts of the organism; and as these changes are of

great importance in relation to the theory of instinct, a few examples

of them will be now adduced.
The Kea (Nestor notabilis) is a curious parrot inhabiting the mountain

ranges of the Middle Island of New Zealand. It belongs to the family of

Brush-tongued parrots, and naturally feeds on the honey of flowers and

the insects which frequent them, together with such fruits or berries as

are found in the region. Till quite recently this comprised its whole

diet, but since the country it inhabits has become occupied by Europeans

it has developed a taste for a carnivorous diet, with alarming results.

It began by picking the sheepskins hung out to dry or the meat in

process of being cured. About 1868 it was first observed to attack

living sheep, which had frequently been found with raw and bleeding

wounds on their backs. Since then it is stated that the bird actually

burrows into the living sheep, eating its way down to the kidneys, which

form its special delicacy. As a natural consequence, the bird is being

destroyed as rapidly as possible, and one of the rare and curious

members of the New Zealand fauna will no doubt shortly cease to exist.

The case affords a remarkable instance of how the climbing feet and

powerful hooked beak developed for one set of purposes can be applied to

another altogether different purpose, and it also shows how little real

stability there may be in what appear to us the most fixed habits of

life. A somewhat similar change of diet has been recorded by the Duke of

Argyll, in which a goose, reared by a golden eagle, was taught by its

foster-parent to eat flesh, which it continued to do regularly and

apparently with great relish.[26]
Change of habits appears to be often a result of imitation, of which Mr.

Tegetmeier gives some good examples. He states that if pigeons are

reared exclusively with small grain, as wheat or barley, they will

starve before eating beans. But when they are thus starving, if a

bean-eating pigeon is put among them, they follow its example, and

thereafter adopt the habit. So fowls sometimes refuse to eat maize, but

on seeing others eat it, they do the same and become excessively fond of

it. Many persons have found that their yellow crocuses were eaten by

sparrows, while the blue, purple, and white coloured varieties were left

untouched; but Mr. Tegetmeier, who grows only these latter colours,

found that after two years the sparrows began to attack them, and

thereafter destroyed them quite as readily as the yellow ones; and he

believes it was merely because some bolder sparrow than the rest set the

example. On this subject Mr. Charles C. Abbott well remarks: "In

studying the habits of our American birds--and I suppose it is true of

birds everywhere--it must at all times be remembered that there is less

stability in the habits of birds than is usually supposed; and no

account of the habits of any one species will exactly detail the various

features of its habits as they really are, in every portion of the

territory it inhabits."[27]

nest-building of some common chaffinches which were taken to New Zealand

and turned out there. He says: "The cup of the nest is small, loosely

put together, apparently lined with feathers, and the walls of the

structure are prolonged for about 18 inches, and hang loosely down the

side of the supporting branch. The whole structure bears some

resemblance to the nests of the hangnests (Icteridae), with the

exception that the cavity is at the top. Clearly these New Zealand

chaffinches were at a loss for a design when fabricating their nest.

They had no standard to work by, no nests of their own kind to copy, no

older birds to give them any instruction, and the result is the abnormal

structure I have just described."[28]

instincts of animals are subject to variation; and had we a sufficient

number of detailed observations we should probably find that these

variations were as numerous, as diverse in character, as large in

amount, and as independent of each other as those which we have seen to

characterise their bodily structure.

_The Variability of Plants._
The variability of plants is notorious, being proved not only by the

endless variations which occur whenever a species is largely grown by

horticulturists, but also by the great difficulty that is felt by

botanists in determining the limits of species in many large genera. As

examples we may take the roses, the brambles, and the willows as well

illustrating this fact. In Mr. Baker's _Revision of the British Roses_

(published by the Linnean Society in 1863), he includes under the single

species, Rosa canina--the common dog-rose--no less than twenty-eight

named _varieties_ distinguished by more or less constant characters and

often confined to special localities, and to these are referred about

seventy of the _species_ of British and continental botanists. Of the

genus Rubus or bramble, _five_ British species are given in Bentham's

_Handbook of the British Flora_, while in the fifth edition of

Babington's _Manual of British Botany_, published about the same time,

no less than _forty-five_ species are described. Of willows (Salix) the

same two works enumerate _fifteen_ and _thirty-one_ species

respectively. The hawkweeds (Hieracium) are equally puzzling, for while

Mr. Bentham admits only seven British species, Professor Babington

describes no less than thirty-two, besides several named varieties.
A French botanist, Mons. A. Jordan, has collected numerous forms of a

common little plant, the spring whitlow-grass (Draba verna); he has

cultivated these for several successive years, and declares that they

preserve their peculiarities unchanged; he also says that they each come

true from seed, and thus possess all the characteristics of true

species. He has described no less than fifty-two such species or

permanent varieties, all found in the south of France; and he urges

botanists to follow his example in collecting, describing, and

cultivating all such varieties as may occur in their respective

districts. Now, as the plant is very common almost all over Europe and

ranges from North America to the Himalayas, the number of similar forms

over this wide area would probably have to be reckoned by hundreds if

not by thousands.
The class of facts now adduced must certainly be held to prove that in

many large genera and in some single species there is a very large

amount of variation, which renders it quite impossible for experts to

agree upon the limits of species. We will now adduce a few striking

cases of individual variation.
The distinguished botanist, Alp. de Candolle, made a special study of

the oaks of the whole world, and has stated some remarkable facts as to

their variability. He declares that on the same branch of oak he has

noted the following variations: (1) In the length of the petiole, as one

to three; (2) in the form of the leaf, being either elliptical or

obovoid; (3) in the margin being entire, or notched, or even pinnatifid;

(4) in the extremity being acute or blunt; (5) in the base being sharp,

blunt, or cordate; (6) in the surface being pubescent or smooth; (7) the

perianth varies in depth and lobing; (8) the stamens vary in number,

independently; (9) the anthers are mucronate or blunt; (10) the fruit

stalks vary greatly in length, often as one to three; (11) the number of

fruits varies; (12) the form of the base of the cup varies; (13) the

scales of the cup vary in form; (14) the proportions of the acorns vary;

(15) the times of the acorns ripening and falling vary.

described and named, and these are most numerous in the best-known

species. Our British oak (Quercus robur) has twenty-eight varieties;

Quercus Lusitanica has eleven; Quercus calliprinos has ten; and Quercus

coccifera eight.
A most remarkable case of variation in the parts of a common flower has

been given by Dr. Hermann Müller. He examined two hundred flowers of

Myosurus minimus, among which he found _thirty-five_ different

proportions of the sepals, petals, and anthers, the first varying from

four to seven, the second from two to five, and the third from two to

ten. Five sepals occurred in one hundred and eighty-nine out of the two

hundred, but of these one hundred and five had three petals, forty-six

had four petals, and twenty-six had five petals; but in each of these

sets the anthers varied in number from three to eight, or from two to

nine. We have here an example of the same amount of "independent

variability" that, as we have seen, occurs in the various dimensions of

birds and mammals; and it may be taken as an illustration of the kind

and degree of variability that may be expected to occur among small and

little specialised flowers.[29]

variation occurs; and I have myself gathered in one locality flowers

varying from 7/8 inch to 1-3/4 inch in diameter; the bracts varying from

1-1/2 inch to 4 inches across; and the petaloid sepals either broad or

narrow, and varying in number from five to ten. Though generally pure

white on their upper surface, some specimens are a full pink, while

others have a decided bluish tinge.
Mr. Darwin states that he carefully examined a large number of plants of

Geranium phaeum and G. pyrenaicum (not perhaps truly British but

frequently found wild), which had escaped from cultivation, and had

spread by seed in an open plantation; and he declares that "the

seedlings varied in almost every single character, both in their flowers

and foliage, to a degree which I have never seen exceeded; yet they

could not have been exposed to any great change of their

conditions."[30]

collected by Mr. Darwin and have been copied from his unpublished

MSS.:--
"De Candolle (_Mem. Soc. Phys. de Genève_, tom. ii. part ii. p. 217)

states that Papaver bracteatum and P. orientale present indifferently

two sepals and four petals, or three sepals and six petals, which is

sufficiently rare with other species of the genus."

the unilocular ovarium is free, but M. Dubury (_Mem. Soc. Phys. de

Genève_, tom. ii. p. 406) has often found individuals in Cyclamen

hederaefolium, in which the base of the ovary was connected for a third

part of its length with the inferior part of the calyx."
"M. Aug. St. Hilaire (Sur la Gynobase, _Mem. des Mus. d'Hist. Nat._,

tom. x. p. 134), speaking of some bushes of the Gomphia oleaefolia,

which he at first thought formed a quite distinct species, says: 'Voilà

donc dans un même individu des loges et un style qui se rattachent

tantôt a un axe vertical, et tantôt a un gynobase; donc celui-ci n'est

qu'un axe veritable; mais cet axe est deprimé au lieu d'être vertical."

He adds (p. 151), 'Does not all this indicate that nature has tried, in

a manner, in the family of Rutaceae to produce from a single

multilocular ovary, one-styled and symmetrical, several unilocular

ovaries, each with its own style.' And he subsequently shows that, in

Xanthoxylum monogynum, 'it often happens that on the same plant, on the

same panicle, we find flowers with one or with two ovaries;' and that

this is an important character is shown by the Rutaceae (to which

Xanthoxylum belongs), being placed in a group of natural orders

characterised by having a solitary ovary."
"De Candolle has divided the Cruciferae into five sub-orders in

accordance with the position of the radicle and cotyledons, yet Mons. T.

Gay (_Ann. des Scien. Nat._, ser. i. tom. vii. p. 389) found in sixteen

seeds of Petrocallis Pyrenaica the form of the embryo so uncertain that

he could not tell whether it ought to be placed in the sub-orders

'Pleurorhizée' or 'Notor-hizée'; so again (p. 400) in Cochlearia

saxatilis M. Gay examined twenty-nine embryos, and of these sixteen were

vigorously 'pleurorhizées,' nine had characters intermediate between

pleuro-and notor-hizées, and four were pure notor-hizées."
"M. Raspail asserts (_Ann. des Scien. Nat._, ser. i. tom. v. p. 440)

that a grass (Nostus Borbonicus) is so eminently variable in its floral

organisation, that the varieties might serve to make a family with

sufficiently numerous genera and tribes--a remark which shows that

important organs must be here variable."

_Species which vary little._

in animals and plants, do not prove that all species vary to the same

extent, or even vary at all, but, merely, that a considerable number of

species in every class, order, and family do so vary. It will have been

observed that the examples of great variability have all been taken from

common species, or species which have a wide range and are abundant in

individuals. Now Mr. Darwin concludes, from an elaborate examination of

the floras and faunas of several distinct regions, that common, wide

ranging species, as a rule, vary most, while those that are confined to

special districts and are therefore comparatively limited in number of

individuals vary least. By a similar comparison it is shown that species

of large genera vary more than species of small genera. These facts

explain, to some extent, why the opinion has been so prevalent that

variation is very limited in amount and exceptional in character. For

naturalists of the old school, and all mere collectors, were interested

in species in proportion to their rarity, and would often have in their

collections a larger number of specimens of a rare species than of a

species that was very common. Now as these rare species do really vary

much less than the common species, and in many cases hardly vary at all,

it was very natural that a belief in the fixity of species should

prevail. It is not, however, as we shall see presently, the rare, but

the common and widespread species which become the parents of new forms,

and thus the non-variability of any number of rare or local species

offers no difficulty whatever in the way of the theory of evolution.

_Concluding Remarks._
We have now shown in some detail, at the risk of being tedious, that

individual variability is a general character of all common and

widespread species of animals or plants; and, further, that this

variability extends, so far as we know, to every part and organ, whether

external or internal, as well as to every mental faculty. Yet more

important is the fact that each part or organ varies to a considerable

extent independently of other parts. Again, we have shown, by abundant

evidence, that the variation that occurs is very large in

amount--usually reaching 10 or 20, and sometimes even 25 per cent of the

average size of the varying part; while not one or two only, but from 5

to 10 per cent of the specimens examined exhibit nearly as large an

amount of variation. These facts have been brought clearly before the

reader by means of numerous diagrams, drawn to scale and exhibiting the

actual variations in inches, so that there can be no possibility of

denying either their generality or their amount. The importance of this

full exposition of the subject will be seen in future chapters, when we

shall frequently have to refer to the facts here set forth, especially

when we deal with the various theories of recent writers and the

criticisms that have been made of the Darwinian theory.
A full exposition of the facts of variation among wild animals and

plants is the more necessary, because comparatively few of them were

published in Mr. Darwin's works, while the more important have only been

made known since the last edition of _The Origin of Species_ was

prepared; and it is clear that Mr. Darwin himself did not fully

recognise the enormous amount of variability that actually exists. This

is indicated by his frequent reference to the extreme slowness of the

changes for which variation furnishes the materials, and also by his use

of such expressions as the following: "A variety when once formed must

again, _perhaps after a long interval of time_, vary or present

individual differences of the same favourable nature as before"

(_Origin_, p. 66). And again, after speaking of changed conditions

"affording a better chance of the occurrence of favourable variations,"

he adds: "_Unless such occur natural selection can do nothing_"

(_Origin_, p. 64). These expressions are hardly consistent with the fact

of the constant and large amount of variation, of every part, in all

directions, which evidently occurs in each generation of all the more

abundant species, and which must afford an ample supply of favourable

variations whenever required; and they have been seized upon and

exaggerated by some writers as proofs of the extreme difficulties in the

way of the theory. It is to show that such difficulties do not exist,

and in the full conviction that an adequate knowledge of the facts of

variation affords the only sure foundation for the Darwinian theory of

the origin of species, that this chapter has been written.

1874.]
[Footnote 18: _Proceedings of the Entomological Society of London_,

1875, p. vii.]
[Footnote 19: _Ann. des Sci. Nat._, tom. xvi. p. 50.]
[Footnote 20: See _Winter Birds of Florida_, p. 206, Table F.]
[Footnote 21: See Table I, p. 211, of Allen's _Winter Birds of

Florida_.]

Mammals, _Bull. U.S. Geol. and Geog. Survey_, vol. ii. p. 314 (1876).]

24.]
[Footnote 26: _Nature_, vol. xix. p. 554.]

258.]

The facts of variation and artificial selection--Proofs of the

generality of variation--Variations of apples and

melons--Variations of flowers--Variations of domestic

animals--Domestic pigeons--Acclimatisation--Circumstances

favourable to selection by man--Conditions favourable to

variation--Concluding remarks.

Having so fully discussed variation under nature it will be unnecessary

to devote so much space to domesticated animals and cultivated plants,

especially as Mr. Darwin has published two remarkable volumes on the

subject where those who desire it may obtain ample information. A

general sketch of the more important facts will, however, be given, for

the purpose of showing how closely they correspond with those described

in the preceding chapter, and also to point out the general principles

which they illustrate. It will also be necessary to explain how these

variations have been increased and accumulated by artificial selection,

since we are thereby better enabled to understand the action of natural

selection, to be discussed in the succeeding chapter.

_The facts of Variation and Artificial Selection._
Every one knows that in each litter of kittens or of puppies no two are

alike. Even in the case in which several are exactly alike in colours,

other differences are always perceptible to those who observe them

closely. They will differ in size, in the proportions of their bodies

and limbs, in the length or texture of their hairy covering, and notably

in their disposition. They each possess, too, an individual

countenance, almost as varied when closely studied as that of a human

being; not only can a shepherd distinguish every sheep in his flock, but

we all know that each kitten in the successive families of our old

favourite cat has a face of its own, with an expression and

individuality distinct from all its brothers and sisters. Now this

individual variability exists among all creatures whatever, which we can

closely observe, even when the two parents are very much alike and have

been matched in order to preserve some special breed. The same thing

occurs in the vegetable kingdom. All plants raised from seed differ more

or less from each other. In every bed of flowers or of vegetables we

shall find, if we look closely, that there are countless small

differences, in the size, in the mode of growth, in the shape or colour