298 COLOUR IN NATURE chap. 



Most have been conducted by the method of sections, 

 and during the course of preparation of the objects, 

 the lipochromes, and the numerous other unstable or 

 soluble pigments, are completely removed or destroyed, 

 so that of these the investigators have nothing to 

 say. It is obvious, however, that it is these brightly 

 coloured substances which give rise to the most 

 striking of the phenomena of coloration. Practically 

 any substance occurring in opaque granules may give 

 rise to dull brownish colours, and so may be termed 

 a pigment, but does this help us as to the origin of 

 the bright blue of many jelly-fish, the gorgeous red 

 of some Crustacea, the bright colours of fishes and 

 birds ? These may be " waste products," but there 

 is yet no proof of it ; they may be reserves ; they may 

 be comparable to the production of aniline dyes in 

 the coal-gas industry, i.e. by-products (Durham), but 

 there is as yet little certainty. It is possible that 

 some of the difficulties may be solved by a careful 

 study of the chsetopterin group of pigments, for the 

 members of it are widely distributed, tend to occur 

 in connection with endodermic (digestive) organs, and 

 under artificial conditions give rise to brightly coloured 

 derivatives, but the investigations have still to be 

 made. 



Again, the method of study by means of injec- 

 tions has obviously its limitations as a method of 

 determining the physiological value of pigments. 

 Thus Cudnot found that introduced pigments in the 

 case of the Crustacea were eliminated by the excre- 

 tory organs, or by the hepatic cells and the faeces ; 

 they were never stored up in the epidermal tissues, 

 and yet the Crustacea are remarkable for the pro- 



