118 TROPISMS 



which is not heliotropic. This lack of heliotropic response 

 is linked with a peculiar color — "tan" — by which the 

 mutant is characterized. The character "tan" is sex 

 linked. The daughters inherit the factor for the character 

 from their fathers but do not show the character, while 

 the sons inherit the factor from their mothers and do 

 show the character. The lack of heliotropic reaction in 

 this mutant is apparently not due to any structural 

 defect in the eye (McEwen 549 ). 



Keeping successive generations of flies in the dark 

 does not influence their heliotropism. F. Payne 550 ' 551 

 raised sixty-nine successive generations of Drosophila 

 in the dark, but the reaction of the insects to light (as 

 well as their eyes) remained entirely normal. 



