356 THE RECAPITULATION THEORY 



plants devices of the most varied and even extra- 

 ordinary kind are adopted to ensure that such 

 cross-fertilisation occurs. The essence of the act 

 of cross-fertilisation, which is already established 

 among Protozoa, consists in combination of the 

 nuclei of two cells, male and female, derived from 

 different individuals. The nature of the process is 

 of such a kind that two individual cells are alone 

 concerned in it ; and it may I think be reasonably 

 argued that the reason why animals commence 

 their existence as eggs *>., as single cells is 

 because it is in this way only that the advantage 

 of cross-fertilisation can be secured, an advantage 

 admittedly of the greatest importance, and to secure 

 which natural selection would operate powerfully. 



The occurrence of parthenogenesis, either occa- 

 sionally or normally, in certain groups is not I 

 think a serious objection to this view. There are 

 very strong reasons for holding that partheno- 

 genetic development is a modified form, derived 

 from the sexual method. Moreover, the view 

 advanced above does not require that cross-fertilisa- 

 tion should be essential to individual development, 

 but merely that it should be in the highest degree 

 advantageous to the species, and hence leaves room 

 for the occurrence, exceptionally, of parthenogenetic 

 development. 



If it be objected that this is laying too much 

 stress on sexual reproduction, and on the advantage 

 of cross-fertilisation, then it may be pointed out in 

 reply that sexual reproduction is the character- 

 istic and essential mode of multiplication among 



