THE ''mutant" series. 27 



The upper group of offspring (Table 52) result, we may suppose, 

 from a mutant gamete (grade about 5|) united with a narrow series 

 gamete (grade about —2). This group includes 31 individuals varying 

 closely about grade 4|, and with a standard deviation of only 0.31. 

 The lower average grade of this group (4.43) compared with the similar 

 group of Table 51, which had a mean of 5.47, shows the influence of 

 the minus-series gamete upon the heterozygote in lowering its grade by 

 about 1. Whether the plus-series gametes have any effect upon the 

 grade of the heterozygotes recorded in the upper group of Table 51 is 

 not certain, because a homozygous group of mutants has not yet been 

 established. It may be observed, however, that one individual in the 

 upper group of Table 51 was of grade 6 (colored all over), and it is pos- 

 sible that homozygous "mutants," when obtained, will approximate 

 that grade, as most wild rats do. Further, a comparison of Tables 51 

 and 53 shows that mutant heterozygotes formed by crosses with the 

 plus series are of slightly lower mean grade than the offspring of the two 

 mutants, among which should occur both homozygous and heterozygous 

 mutants. It seems probable, therefore, that homozygous mutants will 

 be found to be of somewhat higher grade than heterozygous ones. 



The question early suggested itself to our minds, will these "mutants" 

 prove to be mutants in the sense of De Vries? Will they prove to be 

 more stable than the modifications ordinarily secured by selection in our 

 experiments? To test this matter, we have raised two additional gen- 

 erations of offspring from the two mutants and have bred a second 

 generation of offspring from each of the four groups of Fi offspring 

 recorded in Tables 51 and 52, derived from matings with the plus and 

 minus races respectively. 



The Fa descendants of the two original mutants proved very similar 

 to the Fi descendants. (See Table 53.) They fall as before into two 

 groups, an upper and a lower. The former includes 30 individuals of 

 mean grade 5.52, the latter 2 of mean grade 3.37. As the parents of 

 this generation were taken wholly from the upper group of offspring of 

 generation Fi, and as theoretically that group should contain 2 hetero- 

 zygous individuals to one which is homozygous for the "mutant" char- 

 acter, it is to be expected that in F2 more than three-fourths of the 

 offspring will fall in the upper group. For any pair, one member of 

 which is homozygous for the mutant character, should produce only 

 offspring falling in the upper group; and offspring falling in the lower 

 group should be produced only by pairs both members of which are 

 heterozygous. 



The upper group in F2 should contain a larger proportion of homozy- 

 gous mutants than in Fi, and since the parents of F3 were chosen from 

 this upper group of F2 offspring, it is not surprising that the 11 F3 off- 

 spring recorded up to this time all fall in the upper group. The mean 

 of this upper group is remarkably constant through the three genera- 



