26 PIEBALD RATS AND SELECTION. 



nearest individuals in grade to these two produced by the same group 

 of parents are of grade 4|, but some 4| parents of the same generation 

 produced two offspring of grade 5. (See Table 10.) Because of the 

 marked advance in grade of these individuals beyond the ordinary 

 range of variation in the series we called them "mutants, " without 

 wishing then or now to commit ourselves to any particular theory as to 

 their nature or origin. We have used the term and now use it as one of 

 convenience merely. The two "mutant" individuals had the same 

 father and their mothers were sisters. Their pedigree for two genera- 

 tions is as follows : 



The mutant male was mated with the mutant female and also with 

 other females of the plus series, with the results shown in Table 51. In 

 every case the young fall into two distinct groups, one of which varies 

 about the general mean of the plus series (approximately 3f ), while the 

 other varies about the father's grade as a mean (approximately 5^). 



The mutant female had 16 young, 6 in the lower group, mean 3.87, 

 and 10 in the upper group, mean 5.60. (See Table 51, lowest row.) 

 The other females had in all 114 young almost equally divided between 

 the two groups, 58 in the lower group, mean 3.73, and 56 in the upper 

 group, mean 5.45. This result indicates clearly (what the sequel also 

 confirms) that the male mutant transmitted in half his gametes the high 

 grade of pigmentation which he himself manifested, while in the other 

 half of his gametes he transmitted the ordinary condition of the plus 

 race at that time. In other words his "mutant" character behaved as 

 a dominant unit in relation to the ordinary condition of the plus race. 



It is evident that the female mutant was of similar constitution. 

 This being the case, we should expect three-fourths of the offspring of 

 the two mutants to be in the upper group. In reality 10 of their 16 

 young were of this sort. 



The male mutant was mated also with females of the minus series 

 with the results indicated in Table 52. Again, the offspring fall into 

 two distinct groups, a lower and an upper. The lower group should be 

 comparable with the result obtained in Fi when the plus and minus 

 races are crossed with each other. (Compare Table 50.) Such it 

 proves to be. It includes 35 individuals of mean grade —0.49 and 

 standard deviation 0.77. Series 2 of Table 50 is nearly contempo- 

 raneous with this experiment. The Fi offspring in that series were of 

 mean grade — 1 and standard deviation 0.60. 



