MULTIPLE FACTORS 255 
a visible effect only in the presence of this chief 
factor for hoodedness. The F, from the crosses to 
self-color indicate that such modifiers are really 
present in the rats, since the hooded F, are far more 
variable than their hooded ancestors. The under- 
standing of this point is so important that similar 
relations of the same sort may be cited. If a choco- 
late mouse (7.e., one that carries the factors for black 
and for cinnamon) is mated to a white mouse carry- 
ing the factors for gray (instead of those for black 
and cinnamon) the F, generation will be gray. In 
the F, there are three colored mice to one white 
one, but there are several sorts of colored mice. 
Color of any kind is dependent on the action of a 
factor allelomorphic to white, hence the 3 : 1 ratio, 
but this classification ignores the occurrence of 
several kinds of colored mice which are due to 
differences in other factors determining what kind 
of color will develop. 
There are several cases in Drosophila that il- 
lustrate the same point. Eosin is a light eye color. 
Another factor called cream produces no effect on 
other eye colors, but makes eosin still lighter. A 
male pure for cream and for eosin bred to a red 
female gives red eye color in F;. The Fy’s inbred 
give three reds to one light eye color, but among 
the lights three different but overlapping kinds 
may be detected. Here, as in Castle’s case, there 
is a chief factor (eosin) for reduced pigmentation, 
which must be present if any reduction in the color 
occurs at all, and another factor (cream) that 
