INTRODUCTION 
ance to the organisms—whence we have all the 
recent developments of colour theories. For a 
historical sketch of the way in which many theories 
as to the meaning of colour in organisms have been 
built up around the central doctrine of Natural 
Selection, reference should be made to Mr. Wallace’s 
Darwinism. 
It is, however, of some importance to note that 
the interest in the phenomena of colour thus mani¬ 
fested has been very largely polemical in nature. 
The supreme test of the value of a new scientific 
hypothesis is that it explains series of phenomena 
hitherto inexplicable, and the eagerness with which 
the colours of organisms have been discussed and in¬ 
vestigated has been chiefly stimulated by the hope 
of adding fresh arguments to those already brought 
forward in support of Darwin’s theory. We do not 
propose at present to discuss the question of how 
far this hope has been realised, but shall rather pass 
on to consider a third aspect of the colours of 
animals which has been as yet little dwelt upon. 
This is the meaning of the colour in the functional 
economy of the organism. The problems of colour 
have of late years been so exclusively considered 
from the outside—in connection with the relation 
of the organism to its environment—that we are 
apt to forget that the colours must also have a 
meaning and a justification in the physiological 
processes of the individual. The present method of 
treating the subject is in large part the result of that 
habit of regarding organisms as mere bundles of 
qualities which has been so keenly criticised by a 
recent writer. If we once realise that the colour of 
