i6o COLOUR IN NATURE chap. 



life the fatty bodies contained deposits of urates — 

 that is, salts of uric acid — which were apparently 

 never eliminated, not even at the moment of 

 emergence from the pupa. In the Lepidoptera, on 

 the other hand, which are more perfectly organised 

 animals, we find that these useless waste products 

 are perhaps in part eliminated at the time of the 

 emergence from the pupa state, and in part utilised 

 as pigments. The wings are non -vital parts of the 

 body, at a distance from the essential organs ; we 

 can therefore readily understand that associated 

 with the advance in structure from the Orthoptera 

 to the butterflies we should have a removal of 

 waste products from the fatty body, to be in part 

 eliminated, in part stored up in the wings. Further, 

 if in vertebrates some of the most important advances 

 are associated with the development of a more eiificient 

 kidney, if the steps from fish to snake and from 

 snake to bird are accompanied by a diminished 

 deposition of waste products in the skin, is it not 

 possible that a similar process occurs in butterflies .? 

 That is, may not the occurrence of a large amount of 

 bright pigment in the wings of a butterfly indicate 

 a relatively low degree of specialisation ? We have 

 seen that in the Pieridse structural colours are 

 practically absent, while pigmental colours are 

 vivid and striking. When, however, we pass to 

 such families as the Lycsenidae and the Apaturidae, 

 we find that pigmental colours decrease in im- 

 portance, and structural colours are responsible 

 for the major part of the effect. Is it not possible 

 that this marks a genuine advance ? 



