No. 519] THE IMPERFECTION OF DOMINANCE 135 



striking fact is that its activity is so feeble that it cannot 

 prevent the development of the tail. In the case of the 

 second cock, however, the inhibitor is stronger and be- 

 haves more nearly according to Mendelian expectation. 

 Now, if a character can be so feeble as to fail completely 

 in development in the heterozygote and even in the homo- 

 zygote it will give the impression of non-inheritability ; 

 and I have little doubt that many cases in which there is 

 apparently no or only a slight inheritance are due to a 

 weak determiner. I could cite a considerable number of 

 cases of this sort in my experience, but I refer for an ac- 

 count of them to a book that is being published for me by 

 the Carnegie Institution of Washington. Still one other 

 lesson may be drawn and that is the apparent variability 

 in what I call the potency of determiners. The evidence 

 for this potency seems to me to be strong, as it certainly 

 is important for an interpretation of all non-Mendelian 

 cases of heredity. By the aid of the facts of imper- 

 fection in dominance and the hypothesis of varying 

 potency of determiners the territory to which the prin- 

 ciple of the segregation of determiners is applicable 

 becomes greatly extended. 



