246 ANIMAL PEDIGREES 



of cross-fertilisation can be secured, an advantage 

 admittedly of the greatest importance, and to 

 secure which natural selection would operate power- 

 fully. 



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 partheno- 

 genetic 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 characteristic 

 and essential mode of multiplication among Metazoa : 

 that it occurs in all Metazoa, and that when 

 asexual reproduction, as by budding, occurs, this 

 merely alternates with the sexual process which, 

 sooner or later, becomes essential. 



If the fundamental importance of sexual repro- 

 duction to the welfare of the species be granted, 

 and if it be further admitted that Metazoa are 

 descended from Protozoa, then we see that there is 

 really a constraining force of a most powerful nature 

 compelling every animal to commence its life-history 

 in the unicellular condition, the only condition in 



