48 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 

 Pieridse 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 



