68 EXPERIMENTAL EVOLUTION LECT. 



the first of which is that colour is more abundant 

 when the genital functions are most active ; and the 

 second, that colour is also in relation with the fatty 

 layer underlying the skin, being less abundant when 

 this layer is thicker, and vice versa. Recent observa- 

 tions, and the general experience of breeders, go to 

 show that there is much truth in this view, as the 

 breeders have remarked that the best breeds for flesh 

 and fat the Southdown for instance have less colour 

 than the other breeds of the same origin ; and the same 

 obtains among cattle, and among domestic birds, 

 where the white, or faintly coloured individuals are 

 known to be less vigorous than the others. It is also 

 commonly observed that the cause which induces 

 albinism generally affects reproductive functions in a 

 marked manner, and that partial or total sterility is 

 induced : in fact one may say that the absence of 

 pigment and the sterility are correlated expressions 

 of the same constitutional change. Such is the case, 

 for instance, with the Fredericksberg breed of horses. 

 These horses are very peculiar in their colour, which is 

 pure white, although they are not albinos. They are 

 sterile inter se, and in order to preserve the breed, 

 which is used by the royal family of Denmark, they 

 are mated with grey or black mares, and among the 

 progeny pure white horses are obtained. The sterility 

 between like and like is not complete, as in some cases 



