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) 
