128 MEXDELISM chap. 



various combinations of the two factors P and /, being 

 either PPIi, PPIL PpIL or Ppli, and in each of these 

 cases the pigment is more or less intense according to the 

 constitution of the bird. Thus a bird of the constitution 

 PPIi approaches in pigmentation a bird of the constitu- 

 tion Ppli, while a bird of the constitution PpII has but 

 little more pigment than the unpigmented bird. In this 

 way we have seven distinct grades of pigmentation, and 

 the series is further complicated by the fact that these 

 various grades exhibit a rather different amount of pig- 

 mentation according as they occur in a male or a female 

 bird, for, generally speaking, the female of a given grade 

 exhibits rather more pigment than the corresponding 

 male. The examination of a number of birds bred in this 

 way might quite well suggest that in this case we were 

 dealing with a character which could break up, as it 

 were, to give a continuous series of intergrading forms 

 between the two extremes. With the constant handling 

 of large numbers it becomes possible to recognise most of 

 the different grades, though even so it is possible to make 

 mistakes. Nevertheless, as breeding tests have amply 

 shown, we are dealing with but two interacting factors 

 which segregate cleanly from one another according to 

 the strict Mendelian rule. The approach to continuity 

 in variation exhibited by the F 2 generation depends upon 

 the fact that these two factors interact upon one another, 

 and to different degrees according as the zygote is for one 



