252 MULTIPLE FACTORS 



extent of pigmentation of the hooded coat, but had 

 httle 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, w^ere bred to wild gray rats: the Fi 

 were uniform; these inbred gave 3 uniform to 1 

 hooded in Fo. 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, i.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 Fi, 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 



