colors of mammals, such as Professor Castle has descrihecf, are due 

 to different stages of oxidation of a single melanin pigment. If 

 this is the case, then Mendelian inheritance would appear to resolve 

 itself into the inheritance of purely quantitative differences. Some 

 quantitative factor or condition of the germ cell determines that the 

 oxidation of this pigment shall stop in the adult individual at 

 different points in different cases, giving for instance, a black mouse 

 in one case and a yellow one in another. 



A knowledge of the inheritance of purely quantitative differ- 

 ences would therefore seem to be more important than ever before, 

 because many apparently qualitative differences may in the last 

 analysis prove to be purely quantitative. The study of the inherit- 

 ance of the capacity for producing different quantities of pigment, 

 which I have briefly outlined to you, is only begun, but I hope later 

 to get many more data of quantitative inheritance from this sourcu. 

 Needless to say, a knowledge of the inheritance of quantitative differ- 

 ences in man would be valuable from the standpoint of Eugenics. 



