30 



PIEBALD RATS AND SELECTION. 



dence to some extent in FI. The same thing was observable in the 

 crosses of the plus and minus series (Table 50). From that table, 

 Series 1, it will be observed that when the plus and minus races had 

 standard deviations of 0.49 and 0.50, respectively, their FI offspring had 

 a standard deviation of 0.71, an increase by nearly one-half; F 2 showed 

 a further increase to 1.01. In series 2, Table 50, the uncrossed races 

 (generation 10) had standard deviations of 0.36 and 0.24; their F : off- 

 spring had a standard deviation of practically twice this, namely 0.60; 

 F 2 showed a further increase to 0.87. 



At the time of the mutant-minus race crosses, the minus race (genera- 

 tion 10) had a standard deviation of 0.24, the plus race of 0.36. FI 

 (lower group) had a standard deviation of 0.77, and F 2 of 1.17. F l 

 mutants (upper group) had a standard deviation of 0.31 which rose in 

 F 2 to 0.44. These various facts will perhaps be better grasped if pre- 

 sented in tabular form: 



TABLE C. 



The mutant-plus cross, it will be observed, shows no increase of vari- 

 ability either in FI or in F 2 , but crosses involving the minus race show 

 increase of variability both in FI and in F 2 . Interpreted on a Mendelian 

 basis, this means that the mutant and plus races on the one hand and the 

 minus race on the other hand differ by more than a single factor. If 

 they differed by only a single factor, then crosses between them should 

 bring no increase of variability, either in FI or in F 2 . This appears to 

 be true as regards the mutant and plus races when crossed with each 

 other. But if the races crossed differ by more than one factor, and if, 

 further, neither parent is homozygous as regards the factors in which 

 they differ, then we may expect an increase in variability both in FI and 

 in F 2 . This is exactly what we observe when the minus race is crossed 

 with either the plus race or its derivative, the mutant race. 



If we suppose that the plus race and the minus race differ from each 

 other by certain "modifiers," we can not suppose that the plus and the 

 mutant races differ by these same modifiers. They differ in some other 

 single respect; perhaps that in which they differ is the main hooded 

 factor. Are we, then, to suppose that the plus and the minus races do 

 not differ as regards this same main factor? This can not be stated, but 



