MULTIPLE FACTORS 197 



shown in Fig. 61. The dark pigment covers the 

 head and extends as a stripe down the back. The 

 extent of the hood and the breadth of the dorsal 

 band are so variable that in one direction, 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). The 

 steady progress in the plus direction that took place 

 during 13 generations is shown on page 98, and in 

 the minus direction on page 199. 



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 pigmentation, 

 since in all such cases the process of selection in a 

 heterogeneous population sorts out some of the fac- 

 tors 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 Phillips 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. This result 

 shows that there is one chief factor (which is re- 

 cessive) for hooded coat. However, the F 2 hooded 



