152 LIFE AND DEATH. 



must have been almost exactly like its predecessor, and must 

 always have consisted of individuals which possessed the characters 

 of the species. 



This does not exclude the possibility that in spite of an assumed 

 sedentary mode of life, the need for locomotion and for obtaining 

 food in fresh places may have arisen at some period of life. But 

 whenever formation of swarm-spores takes place instead of simple 

 fission, this does not depend upon the persistence 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 Monoplastid 

 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 important 

 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 undoubtedly absent, it is equally certain that it 

 was present at an earlier period (Limnadia Hermanni). \Ve 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 



