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 



