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 of different zones and latitudes” (_Welt als Wille u. V._, ii. 381).