II CAUSES OF COLOUR-VARIATION 67 



great deal to have some information concerning the 

 phenomena which accompany it, and especially the 

 chemical and biological phenomena, and it is import- 

 ant also to be able to detect such phenomena even in 

 cases where no external or morphological differences 

 are obvious. Later on, I shall have to revert to these 

 matters, and for the present it is sufficient to show 

 that even in cases where the external variations seem 

 so weak as to be generally considered of no real 

 account by naturalists, very marked chemical varia- 

 tions underlie the slight morphological differences. 



Upon the whole, then, if a steady and careful 

 investigation were made of the chemical differences 

 which accompany colour variation, doubtless we should 

 see that the latter is of great importance. 



Concerning the causes of colour variation very 

 little is known. Mr. A. R. Wallace has given a 

 detailed discussion of the matter in his recent book on 

 Darwinism, and emphasizes the protective value of 

 colours and their consequent relation to natural 

 selection. This factor has doubtless been of much 

 importance, but on the other hand, as Wallace indeed 

 admits, colour variations have much to do also with 

 the general constitution, and some relationship between 

 vigour, or weakness, and colour is commonly recog- 

 nized. In 1823, Heusinger already contended that the 

 quantity of colour in animals is subject to two laws : 



F 2 



