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 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 recessive) for hooded coat. How- 
ever, the F, 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 
