92 EVOLUTION AND GENETICS 



spines on the thorax are twisted or kinky, which is 

 due to a factor called "forked." The effect is best 

 seen on the thorax, but all spines on the body are 

 similarly modified; even the minute hairs are also 

 affected. The lighter color of the body and antennae 

 is intended to indicate that the character tan is also 

 present. The tan flies are interesting because they 

 have lost the positive heliotropism. As this peculiar- 

 ity of the tan flies is inherited like all the other sex- 

 linked characters, it follows that when a tan female 

 is bred to a wild male all the sons inherit the reces- 

 sive tan color and indifference to light, while the 

 daughters show the dominant sex-linked character 

 of their father, i.e., they are "gray," and go to the 

 light. Hence when such a brood is disturbed the 

 females fly to the light, but the males remain behind. 



One of the first mutants that appeared was called 

 rudimentary on account of the condition of the wings 

 (c) . The same mutation has apj^eared indepen- 

 dently several times. In the drawing (c) the dark 

 body color is intended to indicate "sable." 



In the fourth figure (d) the third and fourth lon- 

 gitudinal veins of the wing are fused into one vein 

 from the base of the wing to the level of the first 

 cross-vein and in addition converge and meet near 

 their outer ends. 



In the fifth figure (e) the wings are shorter and 

 more pointed than in the wild fly. This character is 



