1 68 MENDELISM chap. 



which are capable of modifying the brown. Just 

 as the pale pink -tinged sweet -pea (PI. IV., 9) 

 when mated with a suitable white gives only deep 

 purples, so an eye with very little brown pigment 

 mated with certain blues produces progeny of a 

 deep brown, far darker than either parent. The 

 blue may carry a factor which brings about intensi- 

 fication of the brown pigment. There are doubtless 

 other factors which modify the brown when present, 

 but we do not yet know enough of the inheritance 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 classi- 

 fication of eye-colours, to which some anthropologists 

 attach considerable 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, starting 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 gseen 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 

 Mendelian analysis has disclosed the factors upon 

 which the numerous varieties depend. 



