Wilson — Unsound Mendelian Developments. 421 



factors for groups in a set : factors which prevent others from manifesting 

 their effects being regarded as higher or epistatic, and the concealed factors 

 being regarded as lower or hypostatic. In the case of mice " the determiner 

 for grey " is spoken of as epistatic to that for black, and that for black as 

 epistatic to the determiner for chocolate. There are, however, several 

 determinants concerned in the production of each of these colours ; and we 

 can only express the relative positions of the alternative pairs of these 

 determinants, for which purpose the words dominant and recessive are equally 

 appropriate. 



If, on the other hand, the words are used to indicate the relative 

 positions of groups in a set, they can do so only partially. In the set 

 'd XY : S Xi/ : 3a'F: 1 «y, the first group might be called epistatic to the 

 other three, and the second aiid third groups each to the fourth ; but how 

 is the relationship of the two middle groups to each other to be indicated ? 



The writer of this paper is very deeply indebted to his colleague 

 Dr. F. E. Hackett, who was already interested in the mathematics of the 

 subject, for many suggestions and criticisms. 



