THEORY OF EVOLUTION 



65 



Tliere has aj^jjeared in our cultures a fly in 

 which the third division of the thorax with its 

 ajijiendages lias changed into a segment like 

 the second (fig. 29). It is smaller than the 



Fig. J9. Mutant race of fruit fly with intercalated dupli- 

 cate niesotliorax on dor'-al side. 



normal mesothorax and its wings are imper- 

 fectly developed, but the l^ristles on the upper 

 surface may have the typical arrangement of 

 the normal mesothorax. The mutant shows 

 how great a change may result from a single 

 factor difference. 



A factor that causes duplication in the legs 



