MULTIPLE FACTORS 261 
allelomorphs; the new types, like the old, were 
however 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 Matoon 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. 
