28 PIEBALD RATS AND SELECTION. 



tions, and the variability of the group as measured by its standard 

 deviation is also low, namely, 0.19. This indicates that the mutant 

 character is a strongly dominant unit in relation to the ordinary condi- 

 tion of the plus series. 



Table 54 shows the character of the Fa offspring of the original male 

 mutant mated with females of the plus series. The lower group 

 parents, those into which the mutant character did not presumably 

 enter at all, produced 59 offspring recorded in the first part of Table 54. 

 Their mean grade is 3.78 and their standard deviation 0.33. These are 

 very close to the constants of the general plus series, which for genera- 

 tion 10 were 3.73 and 0.36, respectively. 



The second division of Table 54 shows the character of the young 

 produced by the Fi parents of the upper group (Table 51). Such 

 parents are supposed to have received a "mutant" gamete from their 

 father, grade about 5.50, and a plus-series gamete from their mother, 

 grade about 3.75. If they produce gametes of these same two sorts, their 

 offspring should also fall into two corresponding groups; in fact they do. 

 There are 11 offspring of mean grade 3.86 and 79 offspring of mean 

 grade 5.50. As in the previous generation, the two groups do not 

 approach each other in grade. The mean and standard deviation of 

 the lower group of offspring are similar to those of the plus race. The 

 mean of the upper group is about the same as that of their parents 

 (upper group of offspring, Table 51), namely, 5.50, as compared with 

 5.45; their standard deviation is somewhat lower, namely, 0.15, as com- 

 pared with 0.23. This result indicates that the "mutant" character 

 and the hooded character of the plus series segregate from each other in 

 a simple way without modifying each other appreciably. It seems 

 possible that they contain the same modifiers (if modifiers are present) 

 and differ merely by the main unit which we called the hooded character 

 in the early part of this paper. Each contains a different condition of 

 that main unit. Consequently there is no increase of variability in Fa 

 when these two conditions are intercrossed. This we should expect to 

 happen, if they differed by more than a single factor. 



A very different result is obtained from the cross between the mutant 

 and narrow races. Although Fi from that cross was quite variable 

 (see Table 52), Fa is still more variable (see Table 55). The lower 

 group Fi individuals, which resembled Fi between the plus and minus 

 races, produced 61 young (first division of Table 55), which resemble 

 Fa between the plus and minus races. They range in grade from —2 

 to +3j, mean +0.58, standard deviation 1.17. In the two series of 

 crosses between the plus and minus races (Table 50) the means were 

 -fO.24 and +0.72, respectively, and the standard deviations 1.01 and 

 0.87. This indicates, as did the cross with the plus series, that the 

 "lower group" gametes produced by the original mutant male did not 

 differ materially from gametes produced by the ordinary plus race from 

 which the mutant sprang. 



