252 MULTIPLE FACTORS 
extent of pigmentation of the hooded coat, but had 
little effect on the uniform coat. The range of 
variation was extended in the direction of the 
darker coat, showing that modifying factors caus- 
ing a darker coat had been introduced from the 
wild strain; and such would be the expectation if 
selection had eliminated from the domesticated 
strain some of the factors making for the darker 
coat that had been present in the original impure 
population. Conversely the darker hooded rats, 
plus series, were bred to wild gray rats: the F, 
were uniform; these inbred gave 3 uniform to 1 
hooded in F,. The range of variation of the latter 
was again greater than that present in the dark 
hooded rats which had not been outcrossed, but 
now the range extended rather in the minus direc- 
tion, z.e., the F, hooded rats were on the whole 
lighter than their dark hooded grandparents. The 
result is what the multiple factor hypothesis calls 
for, if the wild or Irish rats contain factors that 
influence the condition of the color pattern. Plus 
selection had weeded out some of the ‘‘minus” 
factors, but crossing with a race in which no se- 
lection had been practised brought them back. 
When the selected plus and minus races were 
crossed to each other the variability was somewhat 
increased in F,;, and was further increased in F,. 
The extreme conditions of the grandparents rarely 
appear in this generation. Again the results are 
those the theory calls for. 
The test of reversing the direction of selection 
