MULTIPLE FACTORS 261 



allelomorphs; the new types, like the old, were 

 how^ever highly stable genetically, and not to be 

 confounded in any sense with ''fluctuating factors." 

 In addition to variation due to these mutations 

 in the chief factor, differences in modifying factors 

 have also been established between bar-eyed lines, 

 in the experiments of ]\Iatoon and Zeleny, of 

 Zeleny, and of May. In certain of these cases the 

 stocks were evidently heterozygous for the modifiers 

 from the beginning, but in other cases mutation 

 seems to have occurred during the course of the 

 experiment. 



