78 Eye Colour in Birds 



segregation occurred with a regrouping of the black and yellow characters, 

 with the result that both were found associated with white and game colour 

 and with fluffy and close-feather pattern. The sex limitation also persists 

 in the F^ generation, the black over yellow or triplex pattern being asso- 

 ciated with the female and the yellow or duplex pattern with the male sex. 

 In this case a secondary male sex character, yellow eye colour, is brought 

 about by the loss of an epistatic character, black pigment, uncovering 

 as it were a hypostatic character, yellow pigment. The ultimate causes 

 of the shedding of this epistatic character must probably be sought for 

 in the metabolic processes associated with the development of sex. The 

 metabolism which develops yellow pigment granules instead of black in 

 connective tissue cells, and also brings about the deposition of yellow 

 pigment granules in striated muscle cells, is no doubt associated with 

 the functional activity of certain endocrinous glands among which we 

 must include the primary sex glands. 



The Triplex Eye or Eye with double anterior pigment in Wild Birds. 



The black over yellow or triplex arrangement of eye colour in the 

 Silky Fowl is a normal example of epistatic character, black, overlying 

 a hypostatic character, yellow pigment. Attention has already been 

 directed to the fact that in the adult male Herring Gull the opposite 

 condition is found. In this case a layer of bright yellow cells covers the 

 anterior surface of the iris and overlies a layer of black branched pigment 

 cells. Thus yellow becomes epistatic to black in the adult male bird. 

 As we have already seen, this result is brought about partly by the 

 removal, by atrophy, of the black cells, and partly by the migration of 

 these cells to a deeper layer of the iris, thus uncovering the yellow pig- 

 ment cells. This arrangement also constitutes a sex limited character in 

 other species. In the adult female Herring Gull the brown colour which 

 is common to nestling birds of both sexes persists through life, while in 

 the male when fully mature the iris assumes a yellow colour. 



These changes in eye-colour dependent on sex and age acquire 

 additional interest from the fact that we have here a demonstration of 

 an actual process going on in the tissues of the zygote during ontogeny, 

 of which the hidden counter-part has previously taken place in the fac- 

 torial constitution of the germ cell at an earlier stage. 



The "Ruby " Eye in Birds. 



We may first deal with the Ruby eye as it occurs in pigeons and 

 doves, because the factors concerned in the production of this iris colour 

 are not the same in all species. 



