THE EVOLUTION OF SEX 



31 



is directly above, the abdomen of the female is twisted 

 almost upward. But if the male is below her, she emits 

 her light without turning the body. In the firefly the 

 evidence that the phosphorescent organ is of use in 

 bringing the sexes together seems well established. 



Fig. 18. — Male and female firefly. 



Whether all secondary sexual organs are useful in 

 mating is a question that must be referred to a later 

 chapter. 



THE SEXUAL INSTINCTS 



Side by side with the evolution of these many kinds 

 of structural difference the sexual instincts have evolved. 

 It is only in the lowest forms that the meeting of the 

 egg and sperm is left to chance. The instincts that 

 bring the males and females together at the mating 

 season, the behavior of the individuals at this time in 



