20 



EVOLUTION AND GENETICS 



one or another of these mutations. Many different 

 kinds of changes have taken place in the wings and 

 several of these involve the size of the wings. If we 

 arrange the latter arbitrarily in the order of their 

 size there will be an almost complete series begin- 



FiG. 6. — Male and female vinegar fly, Dro- 

 sophila melanogaster. 



ning with normal wings and ending with those of 

 aj^terous flies. Several of these types are re^^resented 

 in figure 7. The order in which these mutations oc- 

 curred bears no relation to their size ; each originated 

 independently from the wild type. 



JNlutations have occurred involving the pigmen- 

 tation of the body and wings. The head and thorax 

 of the wild Drosophila melanogaster are grayish yel- 

 low, the abdomen is banded with yellow and black, 

 and the wings are gray. There have appeared in our 

 cultures several kinds of darker tyj)es ranging to 

 almost black flies and to lighter types that are pale 

 yellow. If put in line a series may be made from 

 the darkest flies at one end to the light yellow flies 



