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 er 
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. 
