OLD TYPES AND NEW 159 



of an increasing number of dominants so that after 

 only a few generations the chances are much reduced 

 that recessives will appear, which means the practical 

 purity of the strain. 



5. THE FACTOR HYPOTHESIS 



It has been ascertained within the last decade 

 that some characters require more than a single de- 

 terminer to bring them to expression. The idea of 

 compound determiners for a single character may be 

 termed the factor hypothesis of heredity. The con- 

 verse is also true, that certain single determiners 

 may control more than one character. For instance, 

 the determiner for gray hair in rats also produces a 

 lighter color on the belly. 



Mendel, whose experiments led him to believe that 

 each character depends upon only a single deter- 

 miner for the reason that he worked on characters 

 severally belonging to different parts of the plant, 

 was apparently unaware of the existence, in certain 

 cases at least, of compound determiners. 



These compound factors may be arranged in va- 

 rious categories. For example, there may be, 



(1) A complementary factor which is added to a 

 dissimilar factor in order that a particular character 

 may appear ; 



(2) A supplementary factor which is added to a % 

 dissimilar factor with the result that a character is 

 modified in some way ; 



(3) A cumulative factor which, when added to 



