Even in the vegetable kingdom we see the same thing: for the tints of foliage are deepest, and the colours of flowers and fruits richest, on those plants which are in the most healthy and vigorous condition.”
Add to all these considerations that “this intensity of coloration becomes most developed during the breeding season, when the vitality is at a maximum,” and we shall be prepared for Mr. Wallace’s summing up of his case:—
“If now we accept the evidence of Mr. Darwin’s most trustworthy correspondents, that the choice of the female, so far as she exerts any, falls upon ‘the most vigorous, defiant, and mettlesome male’; and if we further believe, what is certainly the case, that these are as a rule the most highly-coloured and adorned with the finest developments of plumage, we have a real and not a hypothetical cause at work. For these most healthy, vigorous, and beautiful males will have the choice of the finest and most healthy females; and will be able best to protect and rear those families. Natural Selection, and what may be termed Male Selection, will tend to give them the advantage in the struggle for existence; and thus the fullest and the finest colours will be transmitted, and tend to advance in each succeeding generation.”
By this strong chain of reasoning (to which my brief >résumé of course cannot do justice) Mr. Wallace shows that Darwin needlessly introduced the principle of Sexual Selection into animal courtship; and at the same time furnishes a new confirmation of Darwin’s compliment that he has “an innate genius for solving difficulties.”
What makes Mr. Wallace’s argument the more cogent is the fact that Darwin himself, in speaking of the lowest classes of animals, explains their beauty on the same principles as those which Mr. Wallace applies to the higher animals. Thus he says: “We can, in our ignorance of most of the lowest animals, only say that their bright tints result either from the chemical nature or the minute structure of their tissues, independently of any benefit thus derived.” “It is almost certain that these animals have too imperfect senses, and much too low mental powers, to appreciate each other’s beauty or other attractions, or to feel rivalry.” “Nor is it at all obvious how the offspring from the more beautiful pairs of hermaphrodites would have any advantage over the offspring of the less beautiful, so as to increase in number, unless indeed vigour and beauty generally coincided.” And once more, “The sedentary annelids become duller-coloured, according to M. Quatrefages, after the period of reproduction; and this I presume may be attributed to their less vigorous condition at that time.”
So far we have only considered the origin of animal colours in general. Mr. Wallace, however, has not only made clear the general connection between beautiful and vivid colours and health, but, by utilising his own researches and those of Mr. Bates and other naturalists, he has been able to show to what a great extent we can explain even the particular colours of the various classes of animals. He distinguishes four classes of animal colours—Protective, Warning, Sexual, and Typical.
(1) Protective Colours.—These “are exceedingly prevalent in nature, comprising those of all the white arctic animals, the sandy-coloured desert forms, and the green birds and insects of tropical forests. It also comprises thousands of cases of special resemblance—of birds to the surroundings of their nests, and especially of insects to the bark, leaves, flowers, or soil on or amid which they dwell. Mammalia, fishes, and reptiles, as well as mollusca, present similar phenomena; and the more the habits of animals are investigated, the more numerous are found to be the cases in which their colours tend to conceal them, either from their enemies or from the creatures they prey upon.”
(2) Warning Colours.—In this class, on the other hand, the object is not to conceal the animal, but to make it conspicuous. Certain species of gorgeously-coloured butterflies, e.g. are never eaten by birds, spiders, lizards, or monkeys, who eagerly feed on other butterflies. “The reason simply is that they are not fit to eat, their juices having a powerful odour and taste that is absolutely disgusting to all these animals. Now we see the reason of their showy colours and slow flight. It is good for them to be seen and recognised, for then they are never molested; but if they did not differ in form and colouring from other butterflies, or if they flew so quickly that their peculiarities could not be easily noticed, they would be captured, and though not eaten, would be maimed or killed.”
Mimicry is the name given to a second and still more marvellous class of Warning Colours. They belong to defenceless creatures which so closely resemble other brightly-coloured but nauseous or dangerous animals that they are mistaken for the latter, and therefore left alone. E.G. “Wasps are imitated by moths, and ants by beetles; and even poisonous snakes are mimicked by harmless snakes, and dangerous hawks by defenceless cuckoos.”
(3) Typically-coloured animals are those species which are brilliantly coloured in both sexes, “and for whose particular colours we can assign no function or use.” This group “comprises an immense number of showy birds, such as Kingfishers, Barbets, Toucans, Lories, Tits, and Starlings; among insects most of the largest and handsomest butterflies,” etc. “It is a suggestive fact that all the brightly-coloured birds mentioned above build in holes or form covered nests, so that the females do not need that protection during the breeding season which I believe to be one of the chief causes of the dull colour of female birds when their partners are gaily coloured.”
(4) Sexual Colours, comprising those cases in which the sexes differ, and with which Darwin’s theory of Sexual Selection is directly concerned. Through no direct fault of his own, Darwin leaves on his readers the impression—which has become almost a commonplace of conversation—that it is the general rule among animals for the males of each species to be more ornamented than the females. The truth is, however, that “with the exception of butterflies, the sexes are almost alike in the great majority of insects. The same is the case in mammals and reptiles; while the chief departure from the rule occurs in birds, though even here in very many cases the law of sexual likeness prevails.”
The reason why I have devoted so much space to Mr. Wallace’s colour theories is to emphasise the truth contained in this last sentence; the fact, namely, that even if Sexual Selection were accepted as an active principle, it would account in only a very limited number of cases for the personal beauty of animals, and the reader of Mr. Wallace’s Tropical Nature and his Contributions to the Theory of Natural Selection cannot fail to be convinced that Sexual Selection does not even hold good in this limited number of cases, but that “the primary cause of sexual diversity of colour is the need of protection, repressing in the female those bright colours which are normally produced in both sexes by general laws.”
Incidentally Mr. Wallace mentions as an additional function of colour the fact that it may serve as a means of recognition to the sexes. “This view affords us an explanation of the curious fact that among butterflies the females of closely-allied species in the same locality sometimes differ considerably, while the males are much alike; for, as the males are the swiftest, and by far the highest flyers, and seek out the females, it would evidently be advantageous for them to be able to recognise their true partners at some distance off.”
To me it seems that this function of colour is, next to Protection, its most important object, and that Mr. Wallace does not give it sufficient prominence. He says, in speaking of Typical Colours, that we can assign “no function or use for them.” But why should they not serve the sexes as a means of recognition at at a distance? especially as colours can be recognised at a greater distance than forms. Many years before Darwin and Mr. Wallace wrote on this subject, Schopenhauer’s genius anticipated this view of the matter. “The extremely varied and vivid colours of the feathers of tropical birds,” he wrote, “have been explained in a very general way, with reference to their efficient cause, as due to the strong effect of the tropical light. As their final cause I would suggest that these brilliant plumes are the gala uniforms by means of which the species, which are so numerous there and often belonging to the same genus, recognise each other; so that every male finds his female. The same is true of the butterflies
