INTRODUCTION 
5 
colour have some connection with physiology, or 
are at least not wholly accounted for when their 
usefulness is proved (see Mr. Beddard’s Animal 
Coloration , and, for the general question, Mr. Bate¬ 
son’s great work on Variation). 
Having now justified the intrusion of the bio¬ 
logist into what appears to be the domain of the 
physicist, we are at liberty to face the phenomena 
of colour as they appear in organisms. As, however, 
the relation of colour phenomena to the theory of 
Natural Selection has had so much attention bestowed 
upon it, we do not propose, at least in the first place, 
to consider it here, but rather to direct attention to 
the chemical, and where possible, to the physio¬ 
logical aspects of the colours and pigments of 
organisms. In a final summary it will be necessary 
to consider the bearing of the facts upon theories. 
