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 pigments 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- 
