COLOUR IN NATURE 
CHAP. 
cuttles. The question whether there is any genetic 
connection between the dark pigments of animals 
belonging to different orders is one of great interest, 
but one which it is at present impossible to answer. 
Dark pigments are mostly very stable and insoluble; 
where they have been analysed they have been 
usually found to contain nitrogen, and in some cases 
sulphur as well. They are probably of no further 
use in metabolism, and seem often to tend to increase 
in the course of development, and to be more abundant 
in dominant than in weak species. Many have 
regarded them as directly waste products increasing 
with increase of metabolism, a question which we 
shall discuss later. 
Pale brown pigments may arise in so many 
different ways that nothing of a general nature can 
be profitably said of them. In plants and perhaps 
in some insects tannin may play the part of a brown 
pigment. 
White as a pigmental colour is rare. Often 
purely structural, it is sometimes due to the depo¬ 
sition of fat in the subcutaneous tissues. In the 
Pieridae among butterflies white is due to uric acid 
which here plays the part of a white pigment. 
The Spectroscopic Characters of Pigments 
The pigments with which we have been concerned 
in this chapter are recognised by their colour, their 
reaction to various chemical reagents, and finally by 
their spectroscopic characters. Most people are 
familiar, at least by hearsay, with the important part 
which has been played by the spectroscope in 
