In order that the force of this argument may not be misapprehended, it is necessary to bear in mind that it is in no way affected by cases where a structure or an instinct is of primary benefit to its possessor, and then becomes of secondary benefit to some other species on account of the latter being able in some way or another to utilise its action. Of course organic nature is full of cases of this kind; but they only go to show the readiness which all species display to utilise for themselves everything that can be turned to good account in their own environments, and so, among other things, the structures and instincts of other animals. For instance, it would be no answer to Darwin’s challenge if any one were to point to a hermit-crab inhabiting the cast-off shell of a mollusk; because the shell was primarily of use to the mollusk itself, and, so far as the mollusk is concerned, the fact of its shell being afterwards of a secondary use to the crab is quite immaterial. What Darwin’s challenge requires is, that some structure or instinct should be shown which is not merely of such secondary or accidental benefit to another species, but clearly adapted to the needs of that other species in the first instance—such, for example, as would be the case if the tail of a rattle-snake were of no use to its possessor, while serving to warn other animals of the proximity of a dangerous creature; or, in the case of instincts, if it were true that a pilot-fish accompanies a shark for the purpose of helping the shark to discover food. Both these instances have been alleged; but both have been shown untenable. And so it has proved of all the other cases which thus far have been put forward.
Perhaps the most remarkable of all the allegations which ever have been put forward in this connexion are those that were current with regard to instincts before the publication of Darwin’s work. These allegations are the most remarkable, because they serve to show, in a degree which I do not believe could be shown anywhere else, the warping power of preconceived ideas. A short time ago I happened to come across the 8th edition of the Encyclopædia Britannica, and turned up the article on “Instinct” there, in order to see what amount of change had been wrought with regard to our views on this subject by the work of Darwin—the 8th edition of the Encyclopædia Britannica having been published shortly before The Origin of Species by means of Natural Selection. I cannot wait to give any lengthy quotations from this representative exponent of scientific opinion upon the subject at that time; but its general drift may be appreciated if I transcribe merely the short concluding paragraph, wherein he sums up his general results. Here he says:—
It thus only remains for us to regard instinct as a mental faculty, sui generis, the gift of God to the lower animals, that man in his own person, and by them, might be relieved from the meanest drudgery of nature.
Now, here we have the most extraordinary illustration that is imaginable of the obscuring influence of a preconceived idea. Because he started with the belief that instincts must have been implanted in animals for the benefit of man, this writer, even when writing a purely scientific essay, was completely blinded to the largest, the most obvious, and the most important of the facts which the phenomena of instinct display. For, as a matter of fact, among all the many thousands of instincts which are known to occur in animals, there is no single one that can be pointed to as having any special reference to man; while, on the other hand, it is equally impossible to point to one which does not refer to the welfare of the animal presenting it. Indeed, when the point is suggested, it seems to me surprising how few in number are the instincts of animals which have proved to be so much as of secondary or accidental benefit to man, in the same way as skins, furs, and a whole host of other animal products are thus of secondary use to him. Therefore, this writer not only failed to perceive the most obvious truth that every instinct, without any single exception, has reference to the animal which presents it; but he also conceived a purely fictitious inversion of this truth, and wrote an essay to prove a statement which all the instincts in the animal kingdom unite in contradicting.
This example will serve to show, in a striking manner, not only the distance that we have travelled in our interpretation of organic nature between two successive editions of the Encyclopædia Britannica, but also the amount of verification which this fact furnishes to the theory of natural selection. For, inasmuch as it belongs to the very essence of this theory that all adaptive characters (whether instinctive or structural) must have reference to their own possessors, we find overpowering verification furnished to the theory by the fact now before us—namely, that immediately prior to the enunciation of this theory, the truth that all adaptive characters have reference only to the species which present them was not perceived. In other words, it was the testing of this theory by the facts of nature that revealed to naturalists the general law which the theory, as it were, predicted—the general law that all adaptive characters have primary reference to the species which present them. And when we remember that this is a kind of verification which is furnished by millions of separate cases, the whole mass of it taken together is, as I have before said, overwhelming.
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 analogy 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 larvæ 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].