CHAPTER XII 
SEX AS AN ADAPTATION 
In what sense may the separation of all the individuals of 
a species into two kinds of individuals, male and female, be 
called an adaptation? Does any advantage result to the species 
that would not come from a non-sexual method of reproduc- 
tion? Many attempts have been made to answer these ques- 
tions, but with what success I shall now try to show. 
There are four principal questions that must be con- 
sidered : — 
I. The different kinds of sexual individuals in the animal 
and plant kingdoms. 
II. The historical question as to the evolution of separate 
sexes. 
III. The factors that determine the sex in each individual 
developing from an egg. 
IV. The question as to whether any advantage is gained 
by having each new individual produced by the union of two 
germ-cells, or by having the germ-cells carried by two kinds 
of individuals. 
While our main problem is concerned with the last of these 
topics, yet there would be little hope of giving a complete an- 
swer to it unless we could get some answer to the first three 
questions. 
Tue DirFeEReNT Kinps oF SExuaL INDIVIDUALS 
Amongst the unicellular animals and plants the fusion of 
two (or more) individuals into a single one is generally re- 
garded as the simplest, and possibly also the most primitive, 
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