THEORY i)V K\'()LrTI().\ 



167 



ble of demon. 'it rating tluit tlie efJ'ects of selec- 

 tion are actually due to modifiers. It has been 



Fli;. S:i 



DrosDphila aiii])el'i]ihil;i with truncate witig's. 



worked out in our laboratory, chiefly by ^NIul- 

 ler. and used in a particular case to demonstrate 

 that selection produced its effect by isolating 

 modifying factors. For example, a mutant 

 tyjie called truncate appeared, cliaracterized 

 by sliorter wings, usually s<iuare at the end. 

 (fig. 83a). The wings varied from those of 

 )iormal length to wings much shorter (fig. 

 88b). For three years the mutant stock was 



