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 



