178 MENDELISM CHAP. 



heritance of the various shades to justify any statement 

 other than that the heredity of the pigment in front of 

 the iris behaves as though it were due to a Mendelian 

 factor. 



Even this fact is of considerable importance, for it at 

 once suggests that the present systems of classification of 

 eye-colours, to which some anthropologists attach con- 

 siderable weight, are founded on a purely empirical and 

 unsatisfactory basis. Intensity of colour is the criterion 

 at present in vogue, and it is customary to arrange the 

 eye-colours in a scale of increasing depth of shade, start- 

 ing with pale greys and ending with the deepest browns. 

 On this system the lighter greens are placed among the 

 blues. But we now know that blues may differ from the 

 deep browns in the absence of only a single factor, while, 

 on the other hand, the difference between a blue and a 

 green may be a difference dependent upon more than one 

 factor. To what extent eye-colour may be valuable as a 

 criterion of race it is at present impossible to say, but if it 

 is ever to become so, it will only be after a searching Men- 

 delian analysis has disclosed the factors upon which the 

 numerous varieties depend. 



A discussion of eye-colour suggests reflections of another 

 kind. It is difficult to believe that the markedly different 

 states of pigmentation which occur in the same species 

 are not associated with deep-seated chemical differences 

 influencing the character and bent of the individual. 



