MULTIPLE FACTORS 251 



called plus, the rat is all black, except for a white 

 stripe on the belly, and in the other direction, 

 minus, the only black present is on the head. 



Two selections were carried out: one in the plus 

 direction (toward the darker type), the other in the 

 minus direction (toward the lighter type). In the 

 case of both lines steady progress in the direction 

 of selection took place during the thirteen gener- 

 ations of the main experiment. 



This progress in the direction of selection would 

 be expected if the race were not at the start pure 

 for factors that determine the amount of pigmen- 

 tation, since in all such cases the process of selection 

 in a heterogeneous population sorts out some of the 

 factors from others. Selection in most cases creates 

 nothing that is not already present, but separates 

 existing factors. 



There are several ways in which the composition 

 of the rats after their selection can be tested, and 

 some of these tests Castle and Philli])s have made. 

 When light-colored rats from the minus series were 

 bred to wild or to Irish rats that had a uniformly 

 (or nearly uniformly) dark coat, all the offspring 

 had practically completely colored coats. When 

 these were inbred they gave 3 uniform to 1 hooded 

 coat. Tliis result shows that there is one chief 

 factor (which is recessive) for hooded coat. How- 

 ever, the Fo hooded rats differed more among 

 themselves than did those from the grandparental 

 strain of hooded rats, which shows that other 

 factors were involved as well, that modified the 



