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 efficient 
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 Pieridae structural colours are 
practically absent, while pigmental colours are 
vivid and striking. When, however, we pass to 
such families as the Lycaenidae 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 ? 
