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. 



The Different Kinds 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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