ch. xv THE RELATION OF FACTS TO THEORIES 301 
unsolved. What is the meaning of the great series 
of lipochromes in the economy of animals ? How 
do they arise, and why are they sometimes introduced, 
and in other cases synthetically formed ? Why 
should they so frequently occur in pairs, and what is 
the relation between the red and the yellow series ? 
These are only a few of the unanswered questions 
which make one at times doubtful whether it is not 
still too soon to attempt a synthetic survey of the 
biological aspects of colour. It is, however, notice¬ 
able that if the physiology of pigments and colour is 
still in an embryonic condition, yet the speculative 
side of the subject has attained rank and rapid 
growth. It is impossible to conclude a work of this 
description without some reference to theories, but 
we should pass to the consideration of these with a 
full consciousness of the blanks in knowledge. 
Theories as to Origin of Colour in 
Organisms 
1. The Darwinian Theory .—So far we have con¬ 
sidered organisms as if they were isolated objects, 
uninfluenced by their surroundings; it is, however, one 
of the most striking characteristics of modern scientific 
thought that organisms are no longer looked upon 
as independent creations, but as linked to one another 
by the closest of relations. Their colours are often 
their most striking external features ; we must ask 
what effect these colours have upon their relations to 
other organisms. Now it is a matter of common 
observation that the colours of some animals corre¬ 
spond so closely to the colours of the objects among 
