THE " mutant" series. 29 



The second division of Table 55 shows the character of the F 2 young 

 produced by the upper group of Fi offspring recorded in Table 52. It 

 consists of two groups, a lower and an upper. The lower represents 

 the extracted minus race, the upper represents the extracted dominants 

 or mutants, whether homozygous or heterozygous. The former group 

 has an average of +0.75 and a standard deviation of 1.03, which values 

 are close to the corresponding constants of Series 2, Table 50, the 

 latest of the plus-minus crosses, in which the mean was +0.72 and the 

 standard deviation 0.87. 



The upper group offspring of Table 55, second division, the homo- 

 zygous and heterozygous mutants, number 68; they have a mean grade 

 of 4.77 as compared with 4.43 in Fi, which consisted exclusively of 

 heterozygotes. This shows the extracted homozygotes to be of higher 

 grade than the heterozygotes. The highest grade mutant among the 

 31 Fi young, all of which were heterozygotes, was of grade 5, but among 

 the 68 F 2 young are 16 of higher grade than 5. We expect one-third of 

 these 68 individuals to be homozygotes. Now all of the F 2 mutants 

 from the cross of mutant with plus race (Table 54) were of grade 5 or 

 higher, only 2 in 79 being as low as 5, and 13 of the 79 being of grade 5f , 

 a grade not attained at all in F 2 from the mutant-minus cross (Table 55) . 

 This result shows us that the cross with the minus race does affect per- 

 manently the mutant character, lowering its grade even in homozygous 

 mutants extracted from the cross. It also increases the variability of 

 the mutants, for the standard deviation of the mutant group in Table 

 55 is 0.44, whereas in Table 54 (mutant-plus F 2 ), in a like number of 

 individuals, it was 0.15, or only about one-third as great. 



That the variability of the mutants is unaffected by a cross with the 

 plus race, but that it is increased by a cross with the minus race, and 

 that, further, the mean of the mutants is affected little or none by a cross 

 with the plus race, but that it is lowered by a cross with the minus race- 

 these several facts are all conformable with the hypothesis that the 

 change in variability due either to crossing or to selection results from modi- 

 fying factors which, as they are independent of the main factor concerned, 

 are probably transmitted in a different part or component of the germ- 

 cell than that factor. For if the mutant and the plus race are alike as 

 regards the modifiers, but differ only in the main factor, then no change 

 in variability should result from intercrossing them, but only alterna- 

 tive conditions as regards the main factor. This is the observed result. 

 But if the mutant and the minus race differ not only in the main factor, 

 but also in modifiers which are independent of it, then, when they are 

 crossed, we may expect that through independent segregation of main 

 factor and modifiers the extracted minus race will be raised in grade, 

 while the extracted mutants are lowered, and both will become more 

 variable. This also is the observed result. 



One objection may be offered to this interpretation, namely, that 

 the increased variability is not delayed until F 2 , but is already in evi- 



