In attacking this problem we must ask ourselves, What are the purposes that colouration, and, especially, decoration, can alone subserve? We can only conceive it of use in three ways: first, as protection from its enemies; second, as concealment from its prey; third, as distinctive for its fellows. To the third class may be added a sub-class--attractiveness to the opposite sex. The first necessity would seem to be distinctness of species; for, unless each species were separately marked, it would be difficult for the sexes to discriminate mates of their own kind, in many instances; and this is, doubtless, the reason why species _are_ differently coloured. But protective resemblance, as in _Kallima_,[7] the Leaf-butterfly, and mimicry, as in _D. niavius_ and _P. merope_,[8] sometimes so hide the specific characters that this process seems antagonistic to the prime reason for colouration, by rendering species less distinct. Now, doubtless, protective colouring could not have been so wonderfully developed _if the organ of sight were the only means of recognition_. But it is not. Animals possess other organs of recognition, of which, as everyone knows, smell is one of the most potent. A dog may have forgotten a face after years of absence, but, once his cold nose has touched your hand, the pleased whine and tail-wagging of recognition, tells of awakened memories. Even with ourselves, dulled as our senses are, the odour of the first spring violet calls up the past; as words and scenes can never do. What country-bred child forgets the strange smell of the city he first visits? and how vividly the scene is recalled in after years by a repetition of that odour! But insects, and, it may be, many other creatures, possess sense organs whose nature we know not. The functions of the antennæ and of various organs in the wings, are unknown; and none can explain the charm by which the female Kentish Glory, or Oak Egger moths lure their mates. You may collect assiduously, using every seduction in sugars and lanterns, only to find how rare are these insects; but if fortune grant you a virgin female, and you cage her up, though no eye can pierce her prison walls, and though she be silent as the oracles, she will, in some mysterious way, attract lovers; not singly, but by the dozen; not one now and another in an hour, but in eager flocks. Many butterflies possess peculiar scent-pouches on their wings, and one of these, a _Danais_, is mimicked by several species. It is the possession of these additional powers of recognition that leaves colouration free to run to the extreme of protective vagary, when the species is hard pressed in the struggle for life. [Illustration: Plate II. MIMICRY.]