154 LIFE AND DEATH. [III. 



instead of simple fission, this does not depend upon the per- 

 sistence of an ancestral form in the ontogenetic cycle, but is due 

 to the intercalation of an entirely new ontogenetic stage, which 

 happens to resemble an ancestral form, in the possession of 

 cilia, etc. 



I imagine that I have now sufficiently explained the above 

 proposition, that the repetition of the phylogeny in the ontogeny 

 does not and cannot occur among unicellular organisms. 



With the Polyplastides the opposite is the case. There is no 

 species, as far as we know, which does not — either in each in- 

 dividual, or after long cycles which comprise many individuals 

 (alternation of generations) — invariably revert to the Mono- 

 plastid state. This applies from the lowest forms, such as 

 Magosphaera and the Orthonectides, up to the very highest. 

 In the latter a great number of intermediate phyletic stages 

 always occur, although some have been omitted as the result 

 of concentration in the ontogeny, while others have sometimes 

 been intercalated. 



Sexual reproduction is the obvious cause of this very im- 

 portant arrangement. Even if this is an hypothesis rather than 

 a fact we must nevertheless accept it unconditionally, because 

 it is a method of reproduction found everywhere. It is the 

 rule in every group of the animal kingdom, and is only absent 

 in a few species in which it is replaced by parthenogenesis. 

 In these latter instances sexual reproduction may be local, and 

 entirely absent in certain districts only {Apus), or it may be 

 only apparently wanting ; in some cases where it is undoubt- 

 edly absent, it is equally certain that it was present at an earlier 

 period {Limnadia Hermanni). We cannot as yet determine 

 whether its loss will not involve the degeneration and ultimate 

 extinction of the species in question. 



If the essential nature of sexual reproduction depends upon 

 the conjugation of two equivalent but dissimilar morphological 

 elements, then we can understand that a multicellular being 

 can only attain sexual reproduction when a unicellular stage 

 is present in its development ; for the coalescence of entire 

 multicellular organisms in such a manner that fusion would 

 only take place between equivalent cells w^ould seem to be 

 impracticable. In the necessity for sexual reproduction, there 

 is therefore also impHed the necessity for reverting to the 



