H4 Eye-Colours [CH. vi 



variety of the Canary. The heredity of these characters 

 has not yet been investigated by precise methods. Fanciers 

 however are agreed that some very remarkable phenomena 

 of sex-limitation occur, which will be described in con- 

 nection with other instances of the same kind. 



The only other example I know of a race which com- 

 bines an albino eye with external colour is the breed of 

 cats called " Siamese." These animals, which breed per- 

 fectly true, were introduced from Siam, where they have 

 been kept for an indefinite period as pets of the royal 

 household. Like the Himalayan rabbit, Siamese cats are 

 born almost white, but the fur becomes a curious fawn with 

 darker chocolate points on the ears and extremities. The 

 eyes are more or less deficient in pigment throughout life. 

 Some pigment however is formed in the eye, as is 

 evidenced by the fact that the iris has a blue colour and is 

 partially opaque. 



From the little that is known respecting this group of 

 cases it is clear that the genetics of eye-pigments offer 

 many features of interest. We have seen first that varia- 

 tions in the colour of the iris may exhibit simple Mendelian 

 heredity. Secondly when the general pigmentation of the 

 eye is considered, it appears that the presence or absence 

 of such pigment may follow Mendelian rules ; but there is 

 an inter-dependence between the factor or factors which 

 produce the pigmentation of the eye and those which 

 govern the pigmentation of the fur or feathers, such that 

 the eye-colour must be regarded in certain cases (e.g. mice) 

 as determined by two, or perhaps more, complementary 

 factors. The actual nature of the pigment in the eye is 

 also variable, black being replaced by chocolate in several 

 types destitute of black in their hair. Lastly, there is a 

 noticeable association of chocolate pigment, in hair or 

 feathers, with the more or less non-pigmented eye, as in the 

 Himalayan rabbit, the Siamese cat, and the Cinnamon 

 canary. 



The peculiar phenomena connected with Albinism in 

 Man are mentioned later in connection with human heredity 

 in general. 



