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THE AMEBIC AN NATURALIST 



[Vol. L 



F 2 hoodeds less than crosses between Irish and plus. More 

 directly, the plus race should be more modified by the 

 Irish, the minus race more modified by the wild. Ob- 

 servation of Table I will show that these results are 

 realized. 



As already noted in the case of crosses between the 

 plus and wild races, this table shows that in other crosses 

 the different generations of the selected races are differ- 

 ently modified. After long selection there is more modi- 

 fication as the result of crossing. This generalization is 

 supported by all the averages and all the standard devia- 

 tions in crosses involving the wild race ; it is supported 

 by all but one average and by all but one standard devia- 

 tion in crosses involving the Irish race. If selection is 

 sorting out different groups of factors in the plus and 

 minus races, crosses made after many selections bring 

 together groups of factors more diverse than when crosses 

 are made after only a few selections. The greater the 

 diversity in the numbers of plus or minus factors in the 

 animals crossed, the more extensive will be the segrega- 

 tion in the second generation. More extended segrega- 

 tion may be expressed by increased variability and by 

 more pronounced modification of the averages of the F 2 

 hoodeds. 



14. The behavior of the " mutant" in crosses with the 

 plus and minus races gives clear support to the multiple 

 factor hypothesis. Castle (Castle and Phillips, :14, p. 

 29) has clearly demonstrated this point. The " mutant " 

 is a suddenly appearing, quantitatively increased stage 

 of the hooded character, that is controlled by a Mien- 

 delian factor. Crossed with the race from which it 

 sprang, the extracted individuals show no change from 

 the uncrossed race, either as to averages or variability; 

 crossed with the other race, modifications were found, 

 equalling those obtained when the two races were crossed 

 together. The newly discovered factor acts independ- 

 ently of the other factors, is not modified by them, and 

 does not modify them. Being the one difference between 



