XVII 



MAN 205 



and the work of Hurst and others has clearly demon- 

 strated 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 pigment is entirely 

 absent, and as the little blood-vessels are not ob- 

 scured 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 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 



