4 COLOUR IN NATURE 
an organism is not an isolated characteristic forced 
upon it, as it were, from without, but may be merely 
the outward expression of its constitution, we may 
surely hope not only to be delivered from many 
laborious hypotheses as to the use of colour in 
particular cases, but also may perhaps learn some¬ 
thing of the physiology of colour. The subject is 
at least now sufficiently important to merit con¬ 
sideration from a purely physiological standpoint. 
We thus see that there are three reasons why it 
is desirable that the Biologist should concern himself 
with Colour in organisms. The first is the con¬ 
spicuousness of colour phenomena in a merely 
objective survey of animals and plants ; the second 
is the relation of these colours to current theories 
of evolution ; and the third is their importance in 
comparative physiology. 
The order of arrangement of these three is not 
purely formal, but to some extent corresponds to 
the historical order in which the subject has been 
studied. Before Darwin, the colours of organisms 
were chiefly studied as convenient marks by means 
of which the organisms could be recognised. When 
Darwin put forward his theory, which is based on 
the supposition that all specific characters are of 
supreme importance, it was a natural deduction 
from the constancy of many colours that these must 
be of great importance, and so we have all the 
modern theories of colour (see Mr. Poulton’s Colours 
of Animals). Again, now that the theory of Natural 
Selection is no longer the centre of men’s thoughts, 
and search is being made for a deeper analysis, it is 
recognised that it is probable that the phenomena of 
