284 COLOUR IN NATURE chap. 



Those associated with the presence of melanin pig- 

 ment are here as elsewhere less inexplicable than 

 those due to lipochromes. They are certainly far 

 more conspicuous in the males than in the females, 

 and may therefore be ascribed more or less directly 

 to the greater vitality of the male, which expresses 

 itself in more rapid metabolism and increased pro- 

 duction of pigment. But with the lipochromes the 

 case is different ; their constant association with fats 

 makes it difficult to regard them as products of 

 destructive metabolism. Again, though in many 

 cases they are only conspicuous in coloration in the 

 males, yet the peculiar case of the green and red 

 parrots (p. 252) seems to show that in some instances 

 they may be more abundant in the females. It may, 

 of course, be suggested that the lipochromes in the 

 female are largely used up in the colouring of the 

 yolk, while in the male they may colour the plumage ; 

 but it is still difficult to account for the virtual 

 absence of lipochrome from many families, as the 

 crows, the humming-birds, and so on — families cer- 

 tainly highly specialised in other respects. Nor can 

 we regard the presence of lipochrome as indicating 

 want of specialisation in view of the fact that, as 

 in the birds of Paradise, they may be absent or 

 present within the limits of a family, without obvious 

 differences in the amount of specialisation. In view 

 of the absence of lipochrome from the cuticular 

 structures of mammals the question is of much 

 interest. The absence of derivatives of waste 

 products in the cuticular structures of birds as 

 compared with insects is, of course, probably to 

 be associated with the well -developed, excretory. 



