No. 602] SHORTER ARTICLES AND DISCUSSIONS 113 



ously to the same wild male in the same cage. The 10 young of 

 this family may have been produced either by full brother and sister, 



F, young were produced by brother-sister matings. 



TABLE 158 



Table 158 shows the classification of the young obtained by crossing 



It will be observ^ed that the F, young (table 158) which are white- 

 spotted are in no case hooded. Their range of variation does not fall 

 beyond that of the uncrossed mutant race. It is certain, therefore, 

 that the " mutant " condition is not hooded plus an independent Mende- 

 lian modifier. It is a changed form of white-spotting, alternative to the 

 form of spotting found in the race from which it was derived (the plus- 

 selection series, generation 10). It is, without much doubt, also alter- 

 native to the self condition of wild rats, though fluctuation in grade ob- 

 scures the segregation, which may, very likely, be imperfect. This 

 senses to confirm the general conclusion that throughout the entire series 

 of experiments with the hooded pattern of rats we are dealing with 

 quantitative variations in one and the same genetic factor. 



Conclusion 



I have now presented the evidence which has led me to reject 

 the hypothesis formerly held tentatively that modifying factors 

 were largely concerned in changes produced in the hooded pat- 

 tern of rats under repeated selection. This evidence seems to 

 me to admit of only one consistent interpretation, that a single 

 variable genetic factor was concerned in the original hooded 

 race, that a changed condition of this same factor was produced 

 in the minus race, and another changed condition in the plus 

 race, and a third appeared in the mutant race. All are allelo- 

 morphs of each other, and of the non-hooded or self condition 

 found in wild rats, yet all tend to modify each other in crosses. 

 The character has a high degree of genetic stability, yet is sub- 



