204 HEREDITARY CHARACTERS 



every individual is capable of fertilising and being fertilised, 

 and performs both functions. At a later stage in evolution 

 we find that the cells that are to be fertilised the ova and 

 the cells that fertilise the sperms are differentiated from 

 each other, although both are produced by the same indi- 

 vidual. Later still we find that although the same individual 

 produces both ova and sperms, these groups of cells become 

 mature at different periods, the sperms at one time, the 

 ova at another. Thus the fertilisation of the ova by sperms 

 produced in the same individual is prevented, and cross- 

 fertilisation is ensured. Among the higher animals we find 

 that the individual produces only sperms or only ova, that 

 is to say, the sexes are separated. 



Now it appears certain these steps in evolution must have 

 been produced by natural selection acting upon variations. It 

 is by the selection of individual characters that racial char- 

 acters have been produced, and we know of no other material 

 upon which selection could work. Sex, then, must certainly 

 have been produced by the selection of individual characters. 

 We have seen that racial characters when crossed, tend to 

 blend, while individual characters tend to be transmitted 

 in an alternative manner. Variation towards producing 

 sperms and ova at different times, and subsequently varia- 

 tions towards producing only sperms or only ova, would at 

 first be transmitted in an alternative manner. The advan- 

 tages of sperms and ova being produced in separate indi- 

 viduals are very obvious, for inbreeding is thus limited to 

 a considerable extent. If such variations, however, tended 

 after a long period of selection to produce characters that 

 blended, hermaphrodites would tend to be produced again, 

 that is, the sexes would blend, and the whole advantage 

 of the sexes being in separate individuals would be lost. 

 It is therefore probable that natural selection has so in- 

 fluenced the sexual characters, as to keep them within 

 the category of individual characters, in so far as their 

 mode of transmission is concerned, and has prevented their 



