THEORY OF EVOLUTION 



67 



amined. Formerly we were taught that eye- 

 less animals arose in caves. This case shows 

 that they may also arise suddenly in glass milk 

 bottles, by a change in a single factor. 



I may recall in this connection that wingless 

 flies (fig. 5 f) also arose in our cultures by a 

 single mutation. We used to be told that 

 wingless insects occurred on desert islands be- 

 cause those insects that had the best developed 

 wings had been blown out to sea. Wlietlier 

 this is true or not, I will not pretend to say, 

 but at any rate wingless insects may also arise, 

 not througli a slow process of elimination, but 

 at a single step. 



The preceding examples have all related 



Fig. 31. Mutant race of fruit fly called har to the right 

 (normal to the left). The eye is a narrow vertical bar, the 

 outline of the original eye is indicated. 



