102 HERBERT E. DURHAM. 
Leydig (45) shows that calcareous concretions and bodies of 
the nature of urates may give rise to coloration; now the 
latter class of substances are certainly waste products. Also 
F. G. Hopkins (No. 34) has shown that the yellow colour of 
certain butterflies’ wings is due to the presence of a salt 
of “lepidotic acid,’ a substance very closely related to uric 
acid. 
If a comparison may be allowed, we might depart from the 
classical comparison of an animal to a coal-burning steam- 
engine, and liken it broadly to the present coal-gas industry. 
Here we find great economy exhibited, for though we obtain 
motion (by the gas-engine), light, and heat directly from the 
main product—coal gas—we also meet with by-products 
which arise in the course of manufacture; strictly speaking 
they are “waste products,” yet nowadays they are of such 
value that they are said to be as profitable and lucrative as the 
coal gas itself. We may take the coal-tar colours, or 
“aniline dyes,” as the instance most allied to the matter in 
hand, for they are used to some extent to make individuals 
attractive to one another, or more suited to their environment 
(cf. the “invisible colours” of soldiers’ uniforms during active 
service). 
Eisig (27) discusses the relation of pigmentation by effete 
matters to natural selection. 
The fact that such processes of cutaneous excretion occur, 
gives a tangible appreciation of the differences which are asserted 
to exist between human beings of different complexions ; thus it 
is known that the exhibition of large doses of mercury (Hut- 
chinson) is invariably well borne by dark-haired persons ; now 
in them the pigment excretion through the hairs is far more 
active than it is amongst the blond, and this indicates that at 
any rate one particular form of excretory activity is greater. 
This leads us to a subject which has not yet been mentioned 
—the pathological formation and deposition of pigment. 
Pathologically, pigment may be formed as, for instance, in 
the melanotic tumours, in which the production appears to be 
local (List, 48); in Addison’s disease, where the cutaneous 
