Sex as an Adaptation 439 



The preceding discussion will show how far we still are 

 from knowing what factors determine sex. Castle's argu- 

 ment well illustrates how many assumptions must be made 

 in order to make possible the view that sex is a predeter- 

 mined quality of each germ-cell. Even if these assumptions 

 were admissible, we still return to the old idea that the fer- 

 tilized egg has both possibilities, and something determines 

 which shall dominate. Until we have ascertained definitely 

 by experimental work whether the sex in some forms can be 

 determined by external conditions, it is almost worthless to 

 speculate further. Whatever decision is reached, the conclu- 

 sion will have an immediate bearing on the question to be 

 next discussed. Meanwhile, we can at least examine some 

 of the theories that have been advanced as to what advan- 

 tage, if any, has been gained by having the individuals of 

 many classes divided into two kinds, male and female. 



Sex as a Phenomenon of Adaptation 



Of what advantage is it to have the individuals of many 

 species separated into males and females ? It is obviously 

 a disadvantage from the point of view of propagation to have 

 half of the individuals incapable of producing young, and the 

 other half also incapable of doing so, as a rule, unless the 

 eggs are fertilized by the other sex. Is there any compen- 

 sation gained because each new individual arises from two 

 parents instead of from one ? Many answers have been 

 attempted to these questions. 



At the outset it should be recognized that we are by no 

 means forced to assume, as is so often done, that because 

 there is this separation of the sexes it must have arisen on 

 account of its advantage to the species. Whether the result 

 may be of some benefit regardless of how it arose, may 

 be an entirely different question. It would be extremely 

 difficult to weigh the relative advantages (if there are any) 



