xv MAN 163 



together may, if heterozygous, give the recessive 

 blue, but no individuals of the brown class are to 

 be looked for among the offspring of blues mated 

 together. The blues, however, may carry factors 

 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 harid, the difference 

 between a blue and a green may be a difference 

 dependent upon more than one factor. To what 



