200 MULTIPLE FACTORS 



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 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 

 causing a darker coat had been introduced from the 

 w^ld strain; and such would be the expectation if 

 selection had ehminated 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 Fi were uniform; these 

 inbred gave 3 uniform to 1 hooded in F2. 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 direction, i.e., the F2 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 influ- 

 ence the condition of the color pattern. Plus selec- 

 tion had weeded out some of the ''minus" factors, but 

 crossing with a race in which no selection had been 

 practised brought them back. When the selected 

 plus and minus races were crossed to each other the 

 variability w^as somewhat increased in Fi, and was 

 further increased in F2. The extreme conditions of 

 the grandparents rarely appear in this generation. 

 Again the results are those the theory calls for. 



