THEORY OF EVOLUTION 113 



carries a pair of wings that do not completely 

 unfold. On this new mesothorax the character- 

 istic arrangement of the bristles is shown. Thus 

 at a single step a typical region of the body 

 has doubled. The character is recessive. 

 The size of the adult fly of D. ampelophila 



I 





FIG. 55. Group III. (See text.) 



varies greatly according to the amount of 

 nourishment obtained by the larva. After the 

 fly emerges its size remains nearly constant, 

 as in many insects. Two races have, how- 



