Sex as an Adaptation 437 



In order that half of the individuals shall become males 

 and half females it is necessary to assume that in some 

 individuals the male element dominates and in others the 

 female, and since each fertilized egg contains both male 

 and female elements, it is necessary to assume that either 

 the egg or the spermatozoon contains the dominating ele- 

 ment. 



Castle supposes that in hermaphroditic organisms the two 

 characters " exist in the balanced relationship in which they 

 were received from the parents," but, as has just been stated, 

 in unisexual forms one or the other sex dominates, except of 

 course in those rare cases, as in the bees and ants, where 

 half of the body may bear the characters of one sex, and the 

 other half that of the other sex. 



In parthenogenetic species the female character is supposed 

 to be uniformly stronger, so that it dominates in every contest, 

 " for the fertilized egg in such species develops invariably 

 into a female." Under certain circumstances, as Castle 

 points out, the parthenogenetic female produces both males 

 and females, and this is also true in the occasional develop- 

 ment of the unfertilized egg of the silkworm moth, and of 

 the gypsy moth, in which both male and female individuals 

 are produced by parthenogenesis. These facts show that 

 even in unfertilized eggs both sexes are potentially present ; 

 but this might be interpreted to mean that some eggs are 

 male and some female, rather than that each egg has the 

 possibility of both kinds of development. If, however, one 

 polar body is retained in these parthenogenetic eggs, then ex 

 hypothese each egg would contain the potentialities of both 

 sexes (if the polar body were of the opposite sex character). 

 It seems necessary to make this assumption because in some 

 parthenogenetic forms males and females may be produced 

 later by each individual, as in the aphids, and this could not 

 occur if we assume that some parthenogenetic eggs are purely 

 male and some female. 



