Darwin, and After Darwin, Volume 2 of 3 / Post-Darwinian Questions: Heredity and Utility

by George John Romanes

II. Food.

Although, as yet, little is definitely known on the subject, there can be no doubt that in the case of many animals differences of food induce differences of colour within the life-time of individuals, and therefore independently of natural selection.

Thus, sundry definite varieties of the butterfly Euprepia caja can be reared according to the different nourishment which is supplied to the caterpillar; and other butterflies are also known on whose colouring and markings the food of the caterpillar has great influence[114].

Again, I may mention the remarkable case communicated to Darwin by Moritz Wagner, of a species of Saturnia, some pupae of which were transported from Texas to Switzerland in 1870. The moths which emerged in the following year were like the normal type in Texas. Their young were supplied with leaves of Juglans regia, instead of their natural food, J. nigra; and the moths into which these caterpillars changed were so different from their parents, both in form and colour, "that they were reckoned by entomologists as a distinct species[115]."

With regard to mollusks, M. Costa tells us that English oysters, when turned down in the Mediterranean, "rapidly became like the true Mediterranean oyster, altered their manner of growth, and formed prominent diverging rays." This is most probably due to some change of food. So likewise may be the even more remarkable case of Helix nemoralis, which was introduced from Europe to Virginia a few years ago. Under the new conditions it varied to such an extent that up to last year no less than 125 varieties had been discovered. Of these 67, or more than half, are new—that is, unknown in the native continent of the species[116].

In the case of Birds, the Brazilian parrot Chrysotis festiva changes the green in its feathers to red or yellow, if fed on the fat of certain fishes; and the Indian Lori has its splendid colouring preserved by a peculiar kind of food (Wallace). The Bullfinch is well known to turn black when fed on hemp seeds, and the Canary to become red when fed on cayenne pepper (Darwin). Starting from these facts, Dr. Sauermann has recently investigated the subject experimentally; and finds that not only finches, but likewise other birds, such as fowls, and pigeons, are subject to similar variations of colour when fed on cayenne pepper; but in all cases the effect is produced only if the pepper is given to the young birds before their first moult. Moreover, he finds that a moist atmosphere facilitates the change of colour, and that the ruddy hue is discharged under the influence either of sunlight or of cold. Lastly, he has observed that sundry other materials such as glycerine and aniline dyes, produce the same results; so there can be no doubt that organic compounds probably occur in nature which are capable of directly affecting the colours of plumage when eaten by birds. Therefore the presence of such materials in the food-stuffs of birds occupying different areas may very well in many cases determine differences of colouration, which are constant or stable so long as the conditions of their production are maintained.

III. Sexual Selection.

Passing on now to causes of specific change which are internal, or comprised within the organisms themselves, we may first consider the case of Sexual Selection.

Mr. Wallace rejects the theory of sexual selection in toto, and therefore nothing that can be said under this head would be held by him to be relevant. Many naturalists, however, believe that Darwin was right in the large generalization which he published under this title; and in so far as any one holds that sexual selection is a true cause of specific modification, he is obliged to believe that innumerable specific characters—especially in birds and mammals—have been produced without reference to utility (other, of course, than utility for sexual purposes), and therefore without reference to natural selection. This is so obvious that I need not pause to dilate upon it. One remark, however, may be useful. Mr. Wallace is able to make a much more effective use of his argument from "necessary instability" when he brings it against the Darwinian doctrine of sexual selection, than he does when he brings it against the equally Darwinian doctrine of specific characters in general not being all necessarily due to natural selection. In the latter case, it will be remembered, he is easily met by showing that the causes of specific change other than natural selection, such as food, climate, &c., may be quite as general, persistent, and uniform, as natural selection itself; and therefore in this connexion Mr. Wallace's argument falls to the ground. But the argument is much more formidable as he brings it to bear against the theory of sexual selection. Here he asks, What is there to guarantee the uniformity and the constancy of feminine taste with regard to small matters of embellishment through thousands of generations, and among animals living on extensive areas? And, as we have seen in Part 1, it is not easy to supply an answer. Therefore this argument from the "necessary instability of character" is of immeasurably greater force as thus applied against Darwin's doctrine of sexual selection, than it is when brought against his doctrine that all specific characters need not necessarily be due to natural selection. Therefore, also, if any one feels disposed to attach the smallest degree of value to this argument in the latter case, consistency will require him to allow that in the former case it is simply overwhelming, or in itself destructive of the whole theory of sexual selection. And, conversely, if his belief in the theory of sexual selection can survive collision with this objection from instability, he ought not to feel any tremor of contact when the objection is brought to bear against his scepticism regarding the alleged utility of all specific characters. For assuredly no specific character which is apparent to our eyes can be supposed to be so refined and complex (and therefore so presumably inconstant and unstable), as are those minute changes of cerebral structure on which a psychological preference for all the refined shadings and many pigments of a complicated pattern must be held ultimately to depend. For this reason, then, as well as for those previously adduced, if any one agrees with Darwin in holding to the theory of sexual selection notwithstanding this objection from the necessary instability of unuseful embellishments, a fortiori he ought to disregard the objection altogether in its relation to useless specific characters of other kinds.