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 



I. 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 



