160 ANIMAL LIFE AND HUMAN PROGRESS 



This is what is meant by " puiity of the gametes," a con- 

 ception which is the very essence of Mendel's discovery. But, 

 one may ask, the purity of the gametes for what ? In the 

 case of the Rosecombs it stands for something which makes 



Fig. 2. — Diagram showing" the result of mating together two heterozygous 

 birds. Each produces an equal number of "black" and of "white" 

 gametes, and the result of bringing together two such series of gametes is 

 a generation of chicks in which the blacks are three times as numerous as 

 the whites. As indicated in the figure, the blacks are constitutionally of^ 

 two sorts. The true-breeding blacks, viz. those formed by the union of 

 two " black " gametes, form one third of the total blacks produced. 



the difference between black plumage and white plumage. 

 We may suppose that the gametes of the blacks possess a 

 something, possibly of the nature of an enzyme, or the precursor 

 of an enzyme, which brings about the change, while the gametes 



