160 ANIMAL LIFE AND HUMAN PROGRESS 



This is what is meant by "purity 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 heteroz 

 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 



