256 
COLOUR IN NATURE 
CHAP. 
melanin pigment is perhaps explicable enough in 
view of the great prevalence of these pigments in 
Vertebrates, but what are we to say of the lipo- 
chromes ? Is their presence in the feathers in some 
families and apparent absence in others a sign of 
the greater primitiveness of the first or not? If 
their presence in feathers is associated with the 
amount of oil in these structures, why are they 
absent from the hair of mammals, which is also very 
oily? We have also to consider the curious fact 
that, while the muscles of fishes may be coloured by 
red lipochromes, those of birds are not so coloured, 
and the fat of birds is apparently always (?) coloured 
with yellow, and not with the red lipochromes. Are 
the reds formed from the yellow during the process 
of the development of the feathers ? These and 
many other similar questions are suggested by the 
study of the pigments of birds, and some at least 
might be answered by a careful study of the pig¬ 
ments even of the species of a genus. To say that 
the coloration is in each case produced by natural 
selection obviously helps us little, for it can hardly 
be supposed that the insignificant colour patches pro¬ 
duced by turacin can have been of such supreme 
importance as to determine the development of a 
new pigment, while similarly in the birds of paradise 
a red colour is sometimes due to zoorubin and some¬ 
times not. 
Pigments of Birds’ Eggs 
The colours of the egg-shells in birds are, as is 
well known, often beautiful and varied. Rare as 
