124 INTRODUCTION TO SEXUAL PHYSIOLOGY 



then in itself be directlv favoured by natural selection (see 

 p. 6). 



We have seen, further, that some of the characteristics 

 possessed and inherited by organisms are dominant in the 

 Mendelian sense, and some recessive, and the perpetuation of 

 useful variations which are also dominant would be favoured by 

 conjugation. Here, again, we have a hint as to the way in which 

 the process of conjugation, once having started, may have tended 

 to continue, for the cell multiplication which follows conjugation 

 was originally no different from the ordinary division which 

 took place when the cell had attained a certain size. 



As evolution proceeded certain cells were specially set apart 

 to fulfil the function of conjugation, and these became differ- 

 entiated into motile cells or spermatozoa, and inert cells, provided 

 with an additional supply of nutriment, or ova. Among some 

 of the Protozoa we find already the beginnings of a gametic 

 differentiation into two kinds of conjugating cells {e.g. Vorticella, 

 Volvox). 



Among the lower animals, as we have seen, the ova and 

 spermatozoa are often produced by the same individual (herma- 

 phroditism), and some forms are alternatively male and female, 

 producing spermatozoa at some times and ova at others, as 

 with the oyster. Amongst the higher animals, hermaphroditism 

 of this sort is not uncommon, and the cliange from female to male 

 in the progress of individual life is known to occur, even among 

 vertebrates, in isolated cases. It has been suggested that the 

 sexual characters of the male may be present in a latent stage 

 in every female, and the evidence shows that under certain 

 circumstances, a sex reversal may be brought about. Con- 

 versely it is possible that female characters are latent in the 

 male, since the change from male to female can also occur (see 

 p. 129). 



Nevertheless, among dioecious animals there is usually a clearly 

 defined distinction between individuals which are male and those 

 which are female, and this distinction extends to the chromosome 

 constitution of the body cells. Moreover, it has been shown 

 very clearly, more particularly by Morgan and his school of 

 workers, that there are corresponding differences in the chromo- 

 some constitution of the germ cells. It has been found that in 

 most species the gametes of one sex or the other are dimorphic, 



