THEORY OF EVOLUTION 



113 



carries a pair of wings that do not completelj^ 

 unfold. On this new mesothorax the character- 

 istic aiTangenient 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 



varies greatly according to the amount of 

 nourishment obtained by the larva. After the 

 flv emerges its size remains nearly constant, 

 as in manv insects. Two races have, how- 



