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 



