18, 19. DISMORPHIA ASTYNOME, MALE AND FEMALE, ALSO BELONGING TO THE FAMILY OF 'WHITES,' AND MIMICKING IMMUNE HELICONIIDS; A WHITE SPOT ON THE POSTERIOR WING OF THE MALE IS ALL THAT REMAINS OF THE ORIGINAL 'WHITE' COLORATION. 20. ELYMNIAS PHEGEA, W. AFRICA, OF THE FAMILY SATYRIDES, MIMICS THE FOREGOING SPECIES. 21. ACRÆA GEA, AN IMMUNE W. AFRICAN SPECIES. 22. DANAIS GENUTIA, AN IMMUNE DANAID FROM CEYLON. 23. PLYMNIAS UNDULARIS, FEMALE, ONE OF THE MIMICS OF FIG. 22. THE MALE, WHICH IS QUITE DIFFERENT, IS FIGURED ON PLATE III (FIG. 24). _To face Plate II_ [Illustration: PLATE II. LONDON: EDWARD ARNOLD.] We see, then, that the need for protection in butterflies has a great influence on their external appearance, especially as regards their colour and marking. First, because the resting insect frequently has the visible surfaces sympathetically coloured, and also, because there are numerous species, indeed whole families, which contain nauseous, perhaps even actually poisonous, juices, and these have been subject to a double process of selection, directed towards the increase of the nauseousness, and at the same time towards acquiring as conspicuous a dress as possible. Thus the whole surface of these butterflies became gaily coloured, and often--as in many of the tropical nocturnal Lepidoptera which fly by day, the Agaristidæ, Euschemidæ, and Glaucopidæ--quite glaringly bright. We thus understand the striking or at least readily recognizable colours of the Heliconiidæ, the Euplœæ, the Danaidæ, and the Acræidæ. Finally, the unpalatable species influence many others which are edible, since the latter strive to resemble an immune species; and how considerable the variations and colour transformations thus induced can be is shown by the Whites of the genus _Perhybris_ (Pl. II, Figs. 16 and 17) and _Archonias_, in which the male has wholly or partially retained the primitive dress of the Whites, and in which, side by side with wholly mimetic species, other species occur in which both sexes exhibit the garb of the Whites unaltered. Such cases tell decidedly against the often expressed view that mimetic species must have had from the outset a great resemblance to the model; they show rather that very great deviations in form, but more especially in colour, have been brought about solely by the necessity for mimetic adaptation, and that they have come about only slowly and step by step, as the different grades of resemblance to the model in different species of the same genus clearly show. Lepidoptera are by no means the only insects which exhibit the phenomenon of mimicry, nor are insects the only animals in which it occurs; and unpleasant taste and odour are not the only protective characters; there are many others, as, for instance, among insects, the hardness of the chitinous cuticle.