292 COLOUR IN NATURE chap. 



variety, or derivative of melanin," a somewhat inter- 

 esting conclusion. 



Pigment and Waste Products 



The question as to whether the pigments of 

 mammals are to be regarded as products of destruc- 

 tive metabolism is one which has considerable 

 bearing on the general question. In considering 

 particular cases of pigmentation we have again and 

 again come across suggestions to the effect that the 

 pigments of organisms are effete substances incapable 

 of serving directly useful purposes, which may be 

 stored up in the cutaneous tissues, and so give rise 

 to coloration. In considering these suggestions in 

 a little more detail, we may, in the first place, 

 provisionally exclude cases like that of the Lepi- 

 doptera, where the pigments, in some cases at least, 

 are definitely excretory substances. Our immediate 

 concern is not with these, but with the numerous 

 kinds of pigment which are different from the 

 ordinary waste products of the organism in which 

 they occur, and which have not been proved to have a 

 genetic connection with these. Such pign^ents have 

 not infrequently been described as waste products, 

 and it is necessary for us to consider how far this is 

 justifiable. 



Experimental Evidence 



In the first place, it is interesting to note that 

 the suggestions have been usually made in connec- 



