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THE AMERICAN NATURALIST [Vol.LVI 



those which Mavor has recently produced with X-rays — 

 as to be due to actual gene mutations. But even if they 

 were due to real genie differences, the possibility has in 

 most cases by no means been excluded (1) that these genie 

 differences were present in the stock to begin with, and 

 merely became sorted out unequally, through random 

 segregation; or (2) that other, invisible genie differences 

 were present which, after random sorting out, themselves 

 caused differences in mutation rate between the different 

 lines. Certain recent results by Altenburg and myself 

 suggest that genie differences, affecting mutation rate, 

 may be not uncommon. To guard against either of these 

 possibilities it would have been necessary to test the 

 stocks out by a thorough course of inbreeding beforehand, 

 or else to have run at least half a dozen different pairs 

 of parallel lines of the control and treated series, and to 

 have obtained a definite difference in the same direction 

 between the two lines of each pair; otherwise it can be 

 proved by the theory of ' ' probable error ' ' that the dif- 

 ferences observed may have been a mere matter of ran- 

 dom sampling among genie differences originally present. 

 Accumulating large numbers of abnormal or inferior 

 individuals by selective propagation of one or two of the 

 treated lines— as has been done in some cases— adds noth- 

 ing to the significance of the results. 



At best, however, these genetically unrefined methods 

 would be quite insensitive to mutations occurring at any- 

 thing like ordinary frequency, or to such differences in 

 mutation rate as have already been found in the analytical 

 experiments on mutation frequency. And it seems quite 

 possible that larger differences than these will not easily 

 be hit upon, at least not in the early stages of our investi- 

 gations, in view of the evidence that mutation is ordi- 

 narily due to an accident on an ultramicroscopic scale, 

 rather than directly caused by influences pervading the 

 organism. For the present, then, it appears most prom- 

 ising to employ organisms in which the genetic composi- 

 tion can be controlled and analyzed, and to use genetic 

 methods that are sensitive enough to disclose mutations 

 occurring in the control as well as in the treated individ- 



