xv MAN 177 



has clearly demonstrated the existence of one Mendelian 

 factor in operation here. Eyes are of many colours, and 

 the colour depends upon the pigment in the iris. Some 

 eyes have pigment on both sides of the iris — on the side 

 that faces the retina as well as on the side that looks out 

 upon the world. Other eyes have pigment on the retinal 

 side only. To this class belong the blues and clear greys ; 

 while the eyes with pigment in front of the iris also are 

 brown, hazel, or green in various shades according to the 

 amount of pigment present. In albino animals the pig- 

 ment is entirely absent, and as the little blood-vessels are 

 not obscured the iris takes on its characteristic pinkish- 

 red appearance. The condition in which pigment is 

 present in front of the iris is dominant to that in which 

 it is absent. Greens, browns, or hazels mated together 

 may, if heterozygous, give the recessive blue, but no in- 

 dividuals of the brown class are to be looked for among 

 the offspring of blues mated together. The blues, how- 

 ever, 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 cer- 

 tain blues produces progeny of a deep brown, far darker 

 than either parent. The blue may carry a factor which 

 brings about intensification of the brown pigment. 

 There are doubtless other factors which modify the brown 

 when present, but we do not yet know enough of the in- 



