INTRODUCTION 3 



ance to the organisms — whence we have all the 

 recent developments of colour theories. For a 

 historical sketch of the way in which many theories 

 as to the meaning of colour in organisms have been 

 built up around the central doctrine of Natural 

 Selection, reference should be made to Mr. Wallace's 

 Darwinism. 



It is, however, of some importance to note that 

 the interest in the phenomena of colour thus mani- 

 fested has been very largely polemical in nature. 

 The supreme test of the value of a new scientific 

 hypothesis is that it explains series of phenomena 

 hitherto inexplicable, and the eagerness with which 

 the colours of organisms have been discussed and in- 

 vestigated has been chiefly stimulated by the hope 

 of adding fresh arguments to those already brought 

 forward in support of Darwin's theory. We do not 

 propose at present to discuss the question of how 

 far this hope has been realised, but shall rather pass 

 on to consider a third aspect of the colours of 

 animals which has been as yet little dwelt upon. 

 This is the meaning of the colour in the functional 

 economy of the organism. The problems of colour 

 have of late years been so exclusively considered 

 from the outside — in connection with the relation 

 of the organism to its environment — that we are 

 apt to forget that the colours must also have a 

 meaning and a justification in the physiological 

 processes of the individual. The present method of 

 treating the subject is in large part the result of that 

 habit of regarding organisms as mere bundles of 

 qualities which has been so keenly criticised by a 

 recent writer. If we once realise that the colour of 



