50 HEREDITY AND EUGENICS 



quently associated with a patchy or piebald coat. 

 Both conditions nia}^ arise when self colour is raated 

 with w^hite, and in some cases it may be looked upon 

 as a phenomenon of disintegration following on the 

 quantitative dilution of a factor for pigmentation. 

 It resembles in this respect the striping of flowers 

 (see p. 58). Because factors may be diluted and 

 disintegrated in this w^ay by crossing, it is not neces- 

 sar}' to assume, as Bond does, that there were originally 

 independent factors for each eye and subordinate 

 factors independently controlling different areas of 

 the iris. There is no evidence that factors have been 

 built up in this way. They appear rather to originate 

 as germinal changes or new conditions of equilibrium, 

 which may later become modified by crossing or 

 otherwise. 



A number of observations on the e^^e colours of 

 birds and their inheritance are recorded b}^ the same 

 writer (Bond, 191 9). His studies were chiefly of 

 pigeons and fowls, although references are made to 

 many other species. The pigment granules producing 

 eye colour may be black, brown, ^^ellow, ruby, or 

 pearl. The '' bull " eye owes its black colour, as 

 in the white fantail pigeons, to the absence of pig- 

 ment from the anterior surface of the iris. The pos- 

 terior uveal pigment shines through the translucent 

 tissues of the iris and gives the eye its colour, as in 

 blue human eyes. Also, as with blue eyes in man, the 

 "bull" eye of the chick is retained in the adult. 

 The ruddy glow of this e3"e is due to the plexus 

 of bloodvessels. (A similar t3^pe of eye occurs in 

 guinea-pigs of the type which Castle calls red-eyed 

 silver agouti). But in most birds with dark or black 

 eyes, the colour is due to the presence of anterior 

 iris pigment. In the rock pigeon {Columba livia) the 

 iris colour is yellow or orange, while in other pigeons 

 it may be white or red, and in the stock dove (C. cenas) 



