Page:Darwinism by Alfred Wallace 1889.djvu/278

 closely allied forms—those that require minute examination to discriminate them as distinct species—are most generally found in separate areas, and are what are termed representative forms.

The extension we have now given to the theory of mimicry is important, since it enables us to explain a much wider range of colour phenomena than those which were first imputed to mimicry. It is in the richest butterfly region in the world—the Amazon valley—that we find the most abundant evidence of the three distinct sets of facts, all depending on the same general principle. The form of mimicry first elucidated by Mr. Bates is characterised by the presence in each locality of certain butterflies, or other insects, themselves edible and belonging to edible groups, which derived protection from having acquired a deceptive resemblance to some of the inedible butterflies in the same localities, which latter were believed to be wholly free from the attacks of insectivorous birds. Then came the extension of the principle, by Dr. F. Müller, to the case of species of distinct genera of the inedible butterflies resembling each other quite as closely as in the former cases, and like them always found in the same localities. They derive mutual benefit from becoming, in appearance, one species, from which a certain toll is taken annually to teach the young insectivorous birds that they are uneatable. Even when the two or more species are approximately equal in numbers, they each derive a considerable benefit from thus combining their forces; but when one of the species is scarce or verging on extinction, the benefit becomes exceedingly great, being, in fact, exactly apportioned to the need of the species.

The third extension of the same principle explains the grouping of allied species of the same genera of inedible butterflies into sets, each having a distinct type of coloration, and each consisting of a number of species which can hardly be distinguished on the wing. This must be useful exactly in the same way as in the last case, since it divides the inevitable toll to insectivorous birds and other animals among a number of species. It also explains the fact of the great similarity of many species of inedible insects in the same locality—a similarity which does not obtain to anything