THEORY OF EVOLUTION 167 



ble of demonstrating that the effects of selec- 

 tion are actually due to modifiers. It has been 



FIG. 83 a. Drosophila ampelophila with truncate wings. 



worked out in our laboratory, chiefly by Mul- 

 ler, and used in a particular case to demonstrate 

 that selection produced its effect by isolating 

 modifying factors. For example, a mutant 

 type called truncate appeared, characterized 

 by shorter wings, usually square at the end, 

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

 normal length to wings much shorter (fig. 

 83b ) . For three years the mutant stock was 



