132 HEREDITY [cH. 



are destroyed by parasites \ Cases of this kind there- 

 fore bring us back to the question of 'secondary sexual 

 characters ' and sexual dimorphism which were briefly 

 touched upon at the beginning of this chapter. 



In discussing the determination of sex, the nature 

 of sex itself must be sharply distinguished from the 

 secondary characters which are normally associated 

 with it. The essential difference between the sexes 

 is the presence of testes in the male and ovaries in 

 the female ; the associated characters, which may or 

 may not be directly connected with reproduction, are 

 certainly sometimes and may perhaps always be 

 clearly separated into a distinct category. It has 

 been seen that the evidence is conclusive that sex 

 often, and probably always, is determined by the 

 germ-cells, i.e. from the moment of fertilisation if not 

 before. But it is equally clear that the characters 

 associated with one or the other sex are not de- 

 termined in the same way ; their presence or the 

 extent to which they are developed may be to some 

 extent dependent upon environment. Tliis is shown 

 by the case of the worker-bee ; by the effects of cas- 

 tration in preventing the appearance of distinctively 

 sexual characters ; by the appearance in old or 



1 In this particular instance it has been found that a male crab 

 affected by the parasite may even produce eggs, and it must be supposed 

 that femaleness is present in a recessive condition, so to speak, in the 

 male, and is caused to appear by the action of the parasite (Sacculina). 



