262 



THE THEORY OF THE GENE 



development function as a male and at a later stage as 

 a female. It remains, then, entirely a question of fact 

 whether evidence can be produced proving that an indi- 

 vidual having the genetic make-up of a male may, under a 



Fig. 142. 

 Spider crab, a, normal male; a', abdomen of normal male from 

 below; h, normal female; h', abdomen of normal female from 

 below; c, parasitized male; c', abdomen of parasitized male from 

 below. (After Geoffrey Smith.) 



different set of conditions, become a functional female, 

 or the reverse. Several such cases have been reported in 

 recent years which call for a careful and unprejudiced 

 scrutiny. 



Environmental Changes. 



It was shown by Giard in 1886 that when the males of 

 crabs are parasitized by other crustaceans, such as Pelto- 



