No. 600] PIEBALD BATS AND MULTIPLE FACTOBS 



729 



tion, therefore, of heterozygosity in the races (Castle and 

 Phillips, :14, p. 30). Muller ( :14, p. 573) repeats Castle's 

 suggestion that the further increase in the variability in 

 the second generation of this cross argues for the sup- 

 position that the two races differ in regard to several fac- 

 tors. This is a logical interpretation uncontested by the 

 facts, though of course it is not proved. On the other 

 hand, it has been proved, by the crosses between the two 

 races, that they are not distinguished by just one Men- 

 delian unit. Now if the increased variability in the F t 

 of the cross between the plus and the minus races be due 

 to heterozygosis in these races, and if selection is reduc- 

 ing this heterozygosis, crosses made after the races had 

 been selected for a longer time should give less variabil- 

 ity in Fj. The figures show this to be the case. If there 

 is less variability in F l7 in later generations there should 

 correspondingly be less variability in the F 2 . We find: 



Fj from cross plus by minus, generations 5 and 6 = S. D. 0.71; generation 

 10 = 0.60 



F, from cross plus by minus, generations 5 and 6 = S. D. 1.01 ; generation 

 10 = 0.87 



11. The reductions in the averages of both the plus and 

 minus races after crossing with wild or Irish, first led 

 Castle to consider a factorial interpretation (Castle and 

 Phillips, :14, p. 25). Muller ( :14, p. 574) has fully re- 

 stated the bearing of this on the multiple factor theory. 

 The cross has apparently undone selection to some ex- 

 tent by restoring some of the factors that had been se- 

 lected out in forming the two races; the cross has int- 

 creased the heterozygosity of the extracted hooded rats, 

 returning plus factors to the minus race and minus fac- 

 tors to the plus race. 



In the light of the above interpretation, the conver- 

 sion of the minus race into the plus race by means of a 

 cross is significant. Selection for increase in pigmenta- 

 tion was started from extracted hooded rats from a cross 

 of minus with wild. The first generation of this selec- 

 tion made as sudden an advance as the first generation of 

 selected plus rats did at the beginning of the experiment. 



