124 MENDELISM chap. 



approaches in pigmentation a bird of the constitution 

 Ppii, 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 pigmenta- 

 tion, and the series is further complicated by the fact 

 that these various grades exhibit a rather different 

 amount of pigmentation 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^ generation depends upon the fact that these two 

 factors interact upon one another, and to different 

 degrees according as the zygote is for one or other 

 or both of them in a homozygous or a heterozygous 

 state. Moreover, certain of these intermediates will 

 breed true to an intermediate condition of the 

 pigmentation. A male of the constitution ffPPII 

 when bred with females of the constitution FfPPIi 

 will produce only males like itself and females like 

 the maternal parent. We have dealt with this case 

 in some detail, because the existence of families 



