CHAPTER XV 
THE RELATION OF FACTS TO THEORIES 
General Summary—Theories as to Origin of Colour : Poulton, 
Wallace, Eimer, Cunningham, Simroth — Criticism of 
Natural Selection — Criticism of Other Theories—Con¬ 
clusion. 
We have now completed our general survey of the 
colours and colouring-matters of organisms. We 
have seen that these colours are due either to definite 
pigments deposited in the tissues, or to optical effects 
produced by the structure of these tissues. We have 
discussed the chemical, and, so far as is known, the 
physiological nature of some of the chief pigments, 
and described the appearances presented by the most 
striking optical colours. Finally, we have rapidly 
surveyed the colour phenomena presented by the 
most familiar plants and animals. That the survey 
as a whole is halting and incomplete must be obvious 
to all. We have seen that it is as yet impossible to 
give a definite physiological explanation of the origin 
of pigment; that it is practically impossible to classify 
pigments in a logical manner ; that most of the 
problems connected with the subject are entirely 
