THEORY OF P:A OLU TION 



167 



ble of demonstrating tliat the effects of selec- 

 tion are actually due to modifiers. It has been 



Fig. 88 a. I)roso])liila ainpelophila with truncate wings. 



worked out in our laboratory, cliiefiy by ]Mul- 

 ler, and used in a particular case to demonstrate 

 that selection pi-oduced its effect by isolating 

 modifying factors. For example, a mutant 

 i type called truncate appeared, characterized 

 by sliorter wings, usually square at the end, 

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



I normal lengtli to wings much shorter (fig. 

 83b). For three years the mutant stock was 



