CHAPTER XI 



The function, production, and determination of sex Sex alternating in the same 

 individual Experiments on the determination of sex Wilson's theory of 

 the determination of sex The Mendelian interpretation of the determi- 

 nation of sex Archdall Reid's theory that Mendelian characters are non- 

 sexual traits reproduced in the sexual mode A suggested explanation 

 Potentiality of producing secondary sexual characters present in both sexes 

 Sex is a character which is probably transmitted alternatively like 

 individual variations Conclusions. 



THE function, production, and determination of sex are prob- 

 lems so intimately connected with heredity that they must 

 necessarily occupy an important place in any work upon 

 this subject. The word sex is generally used to denote 

 certain morphological differences found uniformly in indi- 

 viduals of the same species, but we find that it is only in 

 the later stages of evolution that such differences exist. 

 Where the production of a new individual necessitates the 

 fusion of cells derived from two pre-existing individuals, it 

 is obvious that a mingling of the germ plasms must occur. 

 We have seen that in some unicellular forms, periods of con- 

 jugation occur at intervals in the life-history of the race, 1 

 but in these organisms there is no sex. Any individual is 

 capable of fertilising any other individual. We find that 

 something of a similar nature happens in much more highly 

 differentiated organisms. In some snails,' for instance, the 

 individuals are hermaphrodites, and every individual pro- 

 duces both ova and sperms. The effect of this mingling of 

 the germplasm was considered in detail in Chapter VIII., 

 and it is not necessary to do more here than point out that 

 the conclusion arrived at was that the elimination of useless 

 variations is thus ensured. 



1 See pp. 2, 3, 29. 

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