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, 
