It is somewhat remarkable that the enormous importance of this argument in favour of natural selection as a prime factor of organic evolution has not received the attention which it deserves. Even Darwin himself, with his characteristic reserve, has not presented its incalculable significance; nor do I know any of his followers who have made any approach to an adequate use of it in their advocacy of his views. In preparing the present chapter, therefore, I have been particularly careful not to pitch too high my own estimate of its evidential value.

That is to say, I have considered, both in the domain of structures and of instincts, what instances admit of being possibly adduced _per contra_, or as standing outside the general law that adaptive structures and instincts are of primary use only to their possessors. In the result I can only think of two such instances. These, therefore, I will now dispose of.

The first was pointed out, and has been fully discussed, by Darwin himself. Certain species of ants are fond of a sweet fluid that is secreted by aphides, and they even keep the aphides as we keep cows for the purpose of profiting by their "milk." Now the point is, that the use of this sweet secretion to the aphis itself has not yet been made out.

Of course, if it is of no use to the aphis, it would furnish a case which completely meets Darwin's own challenge. But, even if this supposition did not stand out of a.n.a.logy with all the other facts of organic nature, most of us would probably deem it prudent to hold that the secretion must primarily be of some use to the aphis itself, although the matter has not been sufficiently investigated to inform us of what this use is. For, in any case, the secretion is not of any vital importance to the ants which feed upon it: and I think but few impartial minds would go so far to save an hypothesis as to maintain, that the Divinity had imposed this drain upon the internal resources of one species of insect for the sole purpose of supplying a luxury to another. On the whole, it seems most probable that the fluid is of the nature of an excretion, serving to carry off waste products. Such, at all events, was the opinion at which Darwin himself arrived, as a result of observing the facts anew, and in relation to his theory.

The other instance to which I have alluded as seeming at first sight likely to answer Darwin's challenge is the formation of vegetable galls.

The great number and variety of galls agree in presenting a more or less elaborate structure, which is not only foreign to any of the uses of plant-life, but singularly and specially adapted to those of the insect-life which they shelter. Yet they are produced by a growth of the plant itself, when suitably stimulated by the insects' inoculation--or, according to recent observations, by emanations from the bodies of the larvae which develop from the eggs deposited in the plant by the insect.

Now, without question, this is a most remarkable fact; and if there were many more of the like kind to be met with in organic nature, we might seriously consider whether the formation of galls should not be held to make against the ubiquitous agency of natural selection. But inasmuch as the formation of galls stands out as an exception to the otherwise universal rule of every species for itself, and for itself alone, we are justified in regarding this one apparent exception with extreme suspicion. Indeed, I think we are justified in regarding the peculiar pathological effect produced in the plant by the secretions of the insect as having been in the first instance accidentally beneficial to the insects. Thus, if any other effect than that of a growing tumour had been produced in the first instance, or if the needs of the insect progeny had not been such as to have derived profit from being enclosed in such a tumour, then, of course, the inoculating instinct of these animals could not have been developed by natural selection. But, given these two conditions, and it appears to me there is nothing very much more remarkable about an accidental correlation between the effects of a parasitic larva on a plant and the needs of that parasite, than there is between the similarly accidental correlation between a hydated parasite and the nutrition furnished to it by the tissues of a warm-blooded animal. Doubtless the case of galls is somewhat more remarkable, inasmuch as the morbid growth of the plant has more concern in the correlation--being, in many instances, a more specialized structure on the part of a host than occurs anywhere else, either in the animal or vegetable world. But here I may suggest that although natural selection cannot have acted upon the plant directly, so as to have produced galls ever better and better adapted to the needs of the insect, it may have so acted upon the plants indirectly _though the insects_. For it may very well have been that natural selection would ever tend to preserve those individual insects, the quality of whose emanations tended to produce the form of galls best suited to nourish the insect progeny; and thus the character of these pathological growths may have become ever better and better adapted to the needs of the insects. Lastly, looking to the enormous number of relations and inter-relations between all organic species, it is scarcely to be wondered at that even so extraordinary an instance of correlation as this should have arisen thus by accident, and then have been perfected by such an _indirect_ agency of natural selection as is here suggested[37].

[37] Note B.

The third general cla.s.s of facts which tell so immensely in favour of natural selection as an important cause of organic evolution, are those of domestication. The art of the horticulturist, the fancier, the cattle-breeder, &c., consists in producing greater and greater deviations from a given wild type of plant or animal, in any particular direction that may be desired for purposes either of use or of beauty.

Cultivated cereals, fruits, and flowers are known to have been all derived from wild species; and, of course, the same applies to all our domesticated varieties of animals. Yet if we compare a cabbage rose with a wild rose, a golden pippin apple with a crab, a toy terrier with any species of wild dog, not to mention any number of other instances, there can be no question that, if such differences had appeared in nature, the organisms presenting them would have been ent.i.tled to rank as distinct species--or even, in many cases, as distinct genera. Yet we know, as a matter of fact, that all these differences have been produced by a process of artificial selection, or pairing, which has been continuously practised by horticulturists and breeders through a number of generations. It is the business of these men to note the individual organisms which show most variation in the directions required, and then to propagate from these individuals, in order that the progeny shall inherit the qualities desired. The results thus become c.u.mulative from generation to generation, until we now have an astonishing manifestation of useful qualities on the one hand, and of beautiful qualities on the other, according as the organisms have been thus bred for purposes of use or for those of beauty.

Now it is immediately obvious that in these cases the process of artificial selection is precisely a.n.a.logous to that of natural selection (and of s.e.xual selection which will be considered later on), in all respects save one: the utility or the beauty which it is the aim of artificial selection continually to enhance, is utility or beauty in relation to the requirements or to the tastes of man; whereas the utility or the beauty which is produced by natural selection and s.e.xual selection has reference only to the requirements or the tastes of the organisms themselves. But, with the exception of this one point of difference, the processes and the products are identical in kind.

Persevering selection by man is thus proved to be capable of creating what are virtually new specific types, and this in any required direction. Hence, when we remember how severe is the struggle for existence in nature, it becomes impossible to doubt that selection by nature is able to do at least as much as artificial selection in the way of thus creating new types out of old ones. Artificial selection, indeed, notwithstanding the many and marvellous results which it has accomplished, can only be regarded as but a feeble imitation of natural selection, which must act with so much greater vigilance and through such immensely greater periods of time. In a word, the proved capabilities of artificial selection furnish, in its best conceivable form, what is called an argument _a fortiori_ in favour of natural selection.

Or, to put it in another way, it may be said that for thousands of years mankind has been engaged in making a gigantic experiment to test, as it were by antic.i.p.ation, the theory of natural selection. For, although this prolonged experiment has been carried on without any such intention on the part of the experimenters, it is none the less an experiment in the sense that its results now furnish an overwhelming verification of Mr. Darwin's theory. That is to say, they furnish overwhelming proof of the efficacy of the selective principle in the modification of organic types, when once this principle is brought steadily and continuously to bear upon a sufficiently long series of generations.

In order to furnish ocular evidence of the value of this line of verification, I have had the following series of drawings prepared.

Another and equally striking series might be made of the products of artificial selection in the case of plants; but it seems to me that the case of animals is more than sufficient for the purpose just stated.

Perhaps it is desirable to add that considerable care has been bestowed upon the execution of these portraits; and that in every case the latter have been taken from the most typical specimens of the artificial variety depicted. Those of them which have not been drawn directly from life are taken from the most authoritative sources; and, before being submitted to the engraver, they were all examined by the best judges in each department. In none of the groups, however, have I aimed at an exhaustive representation of all the varieties: I have merely introduced representatives of as many as the page would in each case accommodate.

[Ill.u.s.tration: FIG. 91.--Pigeons. Drawn from life (prize specimens).]

[Ill.u.s.tration: FIG. 92.--Pigeons, continued. Drawn from life (prize specimens).]

[Ill.u.s.tration: FIG. 93.--Fowls. Drawn from life (prize specimens).]

[Ill.u.s.tration: FIG. 94.--Fowls, continued. Drawn from life (prize specimens).]

[Ill.u.s.tration: FIG. 95.--Pair of j.a.panese Fowls, long-tailed breed.

Drawn from stuffed specimens in the British Museum.]

[Ill.u.s.tration: FIG. 96.--Canaries. Drawn from life (prize specimens).]

[Ill.u.s.tration: FIG. 97.--Sebastopol, or Frizzled Goose. Drawn from a photograph.]

[Ill.u.s.tration: FIG. 98.--The Dingo, or wild dog of Australia, 1/10 nat. size. Drawn from life (_Zoological Gardens_).]

[Ill.u.s.tration: FIG. 99.--Dogs. Drawn from life (prize specimens).]

[Ill.u.s.tration: FIG. 100.--Dogs, continued. Drawn from life (prize specimens).]

[Ill.u.s.tration: FIG. 101.--The Hairless Dog of j.a.pan, 1/10 nat. size.

Drawn from a photograph kindly lent for the purpose by the proprietor.]

[Ill.u.s.tration: FIG. 102.--The skull of a Bull-dog compared with that of a Deerhound. Drawn from nature.]

[Ill.u.s.tration: FIG. 103. Rabbits. Drawn from life (prize specimens).]

[Ill.u.s.tration: FIG. 104.--Horses. Drawn from life (prize specimens).]

[Ill.u.s.tration: FIG. 105.--Sheep. The ill.u.s.trations are confined to British breeds. Drawn from life (prize specimens).]

[Ill.u.s.tration: FIG. 106.--Cattle. The ill.u.s.trations are confined to British breeds. Drawn from life (prize specimens).]

[Ill.u.s.tration: FIG. 107.--Wild Boar contrasted with a modern Domesticated Pig. Drawn from life (_Zoological Gardens_, and prize specimen).]

The exigencies of s.p.a.ce have prevented, in some of the groups, strict adherence to a uniform scale--with the result that contrasts between different breeds in respect of size are not adequately rendered. This remark applies especially to the dogs; for although the artist has endeavoured to draw them in perspective, unless the distance between those in the foreground and those in the background is understood to be more considerable than it appears, an inadequate idea is given of the relative differences of size. The most instructive of the groups, I think, is that of the Canaries; because the many and great changes in different directions must in this case have been produced by artificial selection in so comparatively short a time--the first mention of this bird that I can find being by Gesner, in the sixteenth century.

Now, it is surely unquestionable that in these typical proofs of the efficacy of artificial selection in the modification of specific types, we have the strongest conceivable testimony to the power of natural selection in the same direction. For it thus appears that wherever mankind has had occasion to operate by selection for a sufficiently long time--that is to say, on whatever species of plant or animal he chooses thus to operate for the purpose of modifying the type in any required direction,--the results are always more or less the same: he finds that all specific types lend themselves to continuous deflection in any particulars of structure, colour, &c., that he may desire to modify.

Nevertheless, to this parallel between the known effects of artificial selection, and the inferred effects of natural selection, two objections have been urged. The first is, that in the case of artificial selection the selecting agent is a voluntary intelligence, while in the case of natural selection the selecting agent is Nature herself; and whether or not there is any counterpart of man's voluntary intelligence in nature is a question with which Darwinism has nothing to do. Therefore, it is alleged, the a.n.a.logy between natural selection and artificial selection fails _ab initio_, or at the fountain-head of the causes which are taken by the a.n.a.logy to be respectively involved.

The second objection to the a.n.a.logy is, that the products of artificial selection, closely as they may resemble natural species in all other respects, nevertheless present one conspicuous and highly important point of difference: they rarely, if ever, present the physiological character of mutual infertility, which is a character of extremely general occurrence in the case of natural species, even when these are most nearly allied.

I will deal with these two objections in the next chapter, where I shall be concerned with the meeting of all the objections which have ever been urged against the theory of natural selection. Meanwhile I am engaged only in presenting the general arguments which support the theory, and therefore mention these objections to one of them merely _en pa.s.sant_.

And I do so in order to pledge myself effectually to dispose of them later on, so that for the purposes of my present argument both these objections may be provisionally regarded as non-existent; which means, in other words, that we may provisionally regard the a.n.a.logy between artificial selection and natural selection as everywhere logically intact.

To sum up, then, the results of the foregoing exposition thus far, what I hold to be the three princ.i.p.al, or most general, arguments in favour of the theory of natural selection, are as follows.

First, there is the _a priori_ consideration that, if on independent grounds we believe in the theory of evolution at all, it becomes obvious that natural selection _must_ have had _some_ part in the process. For no one can deny the potent facts of heredity, variability, the struggle for existence, and survival of the fittest. But to admit these facts is to admit natural selection as a principle which must be, at any rate, one of the factors of organic evolution, supposing such evolution to have taken place. Next, when we turn from these _a priori_ considerations, which thus show that natural selection _must_ have been concerned to some extent in the process of evolution, we find in organic nature evidence _a posteriori_ of the extent to which this principle _has_ been thus concerned. For we find that among all the countless millions of adaptive structures which are to be met with in organic nature, it is an invariable rule that they exist in relation to the needs of the particular species which present them: they never have any primary reference to the needs of other species. And as this extraordinarily large and general fact is exactly what the theory of natural selection would expect, the theory is verified by the fact in an extraordinarily cogent manner. In other words, the fact goes to prove that in _all_ cases where adaptive structures or instincts are concerned, natural selection must have been either the sole cause at work, or, at the least, an influence controlling the operation of all other causes.

Lastly, an actually experimental verification of the theory has been furnished on a gigantic scale by the operations of breeders, fanciers, and horticulturists. For these men, by their process of selective acc.u.mulation, have empirically proved what immense changes of type may thus be brought about; and so have verified by antic.i.p.ation, and in a most striking manner, the theory of natural selection--which, as now so fully explained, is nothing more than a theory of c.u.mulative modifications by means of selective breeding.

So much, then, by way of generalities. But perhaps the proof of natural selection as an agency of the first importance in the trans.m.u.tation of species may be best brought home to us by considering a few of its applications in detail. I will therefore devote the rest of the present chapter to considering a few cases of this kind.

There are so many large fields from which such special ill.u.s.trations may be supplied, that it is difficult to decide which of them to draw upon.

For instance, the innumerable, always interesting, and often astonishing adaptations on the part of flowers to the fertilising agency of insects, has alone given rise to an extensive literature since the time when Darwin himself was led to investigate the subject by the guidance of his own theory. The same may be said of the structures and movements of climbing plants, and in short, of all the other departments of natural history where the theory of natural selection has led to the study of the phenomena of adaptation. For in all these cases the theory of natural selection, which first led to their discovery, still remains the only scientific theory by which they can be explained. But among all the possible fields from which evidences of this kind may be drawn, I think the best is that which may be generically termed defensive colouring. To this field, therefore, I will restrict myself. But, even so, the cases to be mentioned are but mere samples taken from different divisions of this field; and therefore it must be understood at the outset that they could easily be multiplied a hundred-fold.

_Protective Colouring._

A vast number of animals are rendered more or less inconspicuous by resembling the colours of the surfaces on which they habitually rest.

Such, for example, are grouse, partridges, rabbits, &c. Moreover, there are many cases in which, if the needs of the creature be such that it must habitually frequent surfaces of different colours, it has acquired the power of changing its colour accordingly--e. g. cuttle-fish, flat-fish, frogs, chameleons, &c. The physiological mechanism whereby these adaptive changes of colour are produced differs in different animals; but it is needless for our purposes to go into this part of the subject. Again, there are yet other cases where protective colouring which is admirably suited to conceal an animal through one part of the year, would become highly conspicuous during another part of it--namely, when the ground is covered with snow. Accordingly, in these cases the animals change their colour in the winter months to a snowy white: witness stoats, mountain hares, ptarmigan, &c. (Fig. 108.)

[Ill.u.s.tration: FIG. 108.--Seasonal changes of colour in Ptarmigan (_Lagopus mutus_). Drawn from stuffed specimens in the British Museum, 1/6 nat. size, with appropriate surroundings supplied.]

Now, it is sufficiently obvious that in all these cla.s.ses of cases the concealment from enemies or prey which is thus secured is of advantage to the animals concerned; and, therefore, that in the theory of natural selection we have a satisfactory theory whereby to explain it. And this cannot be said of any other theory of adaptive mechanisms in nature that has ever been propounded. The so-called Lamarckian theory, for instance, cannot be brought to bear upon the facts at all; and on the theory of special creation it is unintelligible why the phenomena of protective colouring should be of such general occurrence. For, in as far as protective colouring is of advantage to the species which present it, it is of corresponding disadvantage to those other species against the predatory nature of which it acts as a defence. And, of course, the same applies to yet other species, if they serve as prey. Moreover, the more minutely this subject is investigated in all its details, the more exactly is it found to harmonise with the naturalistic interpretation[38].