So much, then, for the Duke’s premiss—namely, that “every modification of structure must have been functionless at first, when it began to appear.” This premiss is clearly opposed to observable fact. But now, the second position is that, even if this were not so, the Duke’s conclusion would not follow. This conclusion, it will be remembered, is, that if incipient structures are useless, it necessarily follows that natural selection can have had no part whatever in their inception. Now, this is a conclusion which does not “necessarily” follow. Even if it be granted that there are structures which in their first beginnings are not of any use at all for any purpose, it is still possible that they may owe their origin to natural selection—not indeed directly, but indirectly. This possibility arises from the occurrence in nature of a principle which has been called the Correlation of Growth.
Mr. Darwin, who has paid more attention to this matter than any other writer, has shown, in considerable detail, that all the parts of any given organism are so intimately bound together, or so mutually dependent upon each other, that when one part is caused to change by means of natural selection, some other parts are very likely to undergo modification as a consequence. For example, there are several kinds of domesticated pigeons and fowls, which grow peculiar wing-like feathers on the feet. These are quite unlike all the other feathers in the animal, except those of the wing, to which they bear a very remarkable resemblance. Mr. Darwin records the case of a bantam where these wing-like feathers were nine inches in length, and I have myself seen a pigeon where they reproduced upon the feet a close imitation of the different kinds of feathers which occupy homologous positions in the wing—primaries, secondaries, and tertiaries all being distinctly repeated in their proper anatomical relations. Furthermore, in this case, as in most cases where such wing-feathers occur upon the feet, the third and fourth toes were partly united by skin; and, as is well known, in the wing of a bird the third and fourth digits are completely united by skin; “so that in feather-footed pigeons, not only does the exterior surface support a row of long feathers, like wing-feathers [which, as just stated, may in some cases be obviously differentiated into primaries, secondaries and tertiaries], but the very same digits which in the wing are completely united by skin become partially united by skin in the feet; and thus by the law of correlated variation of homologous parts, we can understand the curious connexion of feathered legs and membrane between the two outer toes[46].” The illustration is drawn from the specimen to which I have referred.
Fig. 117.—Feather-footed pigeon. Drawn from nature.
Many similar instances of the same law are to be met with throughout organic nature; and it is evident that in this principle we find a conceivable explanation of the origin of such adaptive structures as could not have been originated by natural selection acting directly upon themselves: they may have been originated by natural selection developing other adaptive structures elsewhere in the organism, the gradual evolution of which has entailed the production of these by correlation of growth. And, if so, when once started in this way, these structures, because thus accidentally useful, will now themselves come under the direct action of natural selection, and so have their further evolution determined with or without the correlated association which first led to their inception.
Of course it must be understood that in thus applying the principle of correlated growth, to explain the origin of adaptive structures where it is impossible to explain such origin by natural selection having from the first acted directly upon these structures themselves, Darwinists do not suppose that in all—or even in most—cases of correlated growth the correlated structures are of use. On the contrary, it is well known that structures due to correlated growth are, as a rule, useless. Being only the by-products of adaptive changes going on elsewhere, in any given case the chances are against these correlated effects being themselves of any utilitarian significance; and, therefore, as a matter of fact, correlated growths appear to be usually meaningless from the point of view of adaptation. Still, on the doctrine of chances, it is to be expected that sometimes a change of structure which has thus been indirectly produced by correlation of growth might happen to prove useful for some purpose or another; and in as many cases as such indirectly produced structures do prove useful, they will straightway begin to be improved by the direct action of natural selection. In all such cases, therefore, we should have an explanation of the origin of such a structure, which is the only point that we are now considering.
I think, then, that all this effectually disposes of the doctrine of “prophetic germs.” But, before leaving the subject, I should like to make one further statement of greater generality than any which I have hitherto advanced. This statement is, that we must remember how large a stock of meaningless structures are always being produced in the course of specific transmutations, not only by correlation of growth, which we have just been considering, but also by the direct action of external conditions, together with the constant play of all the many and complex forces internal to organisms themselves. In other words, important as the principle of correlation undoubtedly is, we must remember that even this is very far from being the only principle which is concerned in the origination of structures that may or may not chance to be useful. Therefore, it is not only natural selection when operating indirectly through the correlation of growth that is competent to produce new structures without reference to utility. In all the complex action and reaction of internal and external forces, new variations are perpetually arising without any reference to utility, either present or future. Among all this multitude of promiscuous variations, the chances must be that some percentage will prove of some service, either from the first moment of their appearance, or else after they have undergone some amount of development. Such development prior to utility may be due, either to correlation of growth, to the structure having previously performed some other function, as already explained, or else to a continued operation of the causes which were concerned in the first appearance of originally useless characters. In a series of chapters which will be devoted to the whole question of utility in the next volume, I shall hope to give very good reasons for concluding that useless characters are not only of highly frequent occurrence, but are due to a variety of other causes besides correlation of growth. And, if so, the possibility of originally useless characters happening in some cases to become, by increased development, useful characters, is correspondingly increased. Among a hundred varietal or specific characters which are directly produced in as many different species by a change of climate, for example, some five or six may be potentially useful: that is to say, characters thus adventitiously produced in an incipient form may only require to be further developed by a continuance of the same causes as first originated them, in order that some percentage of the whole number shall become of some degree of use. Those professed followers of Darwin, therefore, who without any reason—or, as it appears to me, against all reason—deny the possibility of useless specific characters in any case or in any degree (unless correlated with useful characters), are playing into the hands of Darwin’s critics by indirectly countenancing the difficulty which we are now considering. For, if correlation of growth is unreasonably supposed to be the only possible cause of the origin of incipient structures which are not useful from the first moment of their inception, clearly the field is greatly narrowed as regards the occurrence of incipient characters sufficient in amount—and, still more, in constancy of appearance and persistency of transmission—to admit of furnishing material for the working of natural selection. But in the measure that incipient characters—whether varietal or specific—are recognised as not always or “necessarily” useful from the moment of their inception, and yet capable of being developed to a certain extent by the causes which first led to their occurrence, in that measure is this line of criticism closed. For of all the variations which thus occur, it is only those which afterwards prove of any use that are laid hold upon and wrought up by natural selection into adaptive structures, or working organs. And, therefore, what we see in organic nature is the net outcome of the development of all the happy chances. So it comes that the appearance presented by organic nature as a whole is that of a continual fulfilment of structural prophecies, when, in point of fact, if we had a similar record of all the other variations it would be seen that possibly not one such prophecy in a thousand is ever destined to be fulfilled.
Here, then, I feel justified in finally taking leave of the difficulty from the uselessness of incipient organs, as this difficulty has been presented, in varying degrees of emphasis, by the Duke of Argyll, Mr. Mivart, Professors Nägeli, Bronn, Broca, Eimer, and, indeed, by all other writers who have hitherto advanced it. For, as thus presented, I think I have shown that it admits of being adequately met. But now, I must confess, to me individually it does appear that behind this erroneous presentation of the difficulty there lies another question, which is deserving of much more serious attention. For although it admits of being easily shown—as I have just shown—that the difficulty as ordinarily presented fails on account of its extravagance, the question remains whether, if stated with more moderation, a real difficulty might not be found to remain.