THEORY OF EVOLUTION 115 



becomes black with age. Most of the other 

 mutations in eye color that have occurred tend 

 to give a lighter color: this one, which is also 

 recessive, makes the eye darker. 



In the fifth figure (e) the color of the dark- 

 est fly is due to a factor called ebony, which is 

 an allelomorph of sooty. 



In the sixth figure (f ) the wings are beaded, 

 i.e., the margin is defective at intervals, giving 

 a beaded-like outline to the wings. This con- 

 dition is very variable and much affected by 

 other factors that influence the shape of the 

 wings. The lighter eye color of the drawing 

 may be taken to represent pink. 



In the seventh figure (g) the wings are 

 curled up over the back. This is a recessive 

 character. 



Group IV 



Only two mutants have been obtained that do 

 not belong to any of the preceding groups; 

 these are put together in Group IV. It has 

 been shown that they are linked to each other 

 and the linkage is so close that it has thus far 

 been impossible to obtain the double recessive. 



