THEORY OF EVOLUTION 65 



There has appeared in our cultures a fly in 

 which the third division of the thorax with its 

 appendages has changed into a segment like 

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



FIG. 29. Mutant race of fruit fly with intercalated dupli- 

 cate mesothorax on dorsal side. 



normal mesothorax and its wings are imper- 

 fectly developed, but the bristles 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 



