VIRUSES, RICKETTSIAE AND BACTERIA 



Autotrophic bacteria ->- Protophyta ->Metaphyta 



(i) i _ i 



Heterotrophic bacteria Protozoa — > Metazoa 



25 



(2) 



Autotrophic bacteria -> Protophyta -> Metaphyta 

 Heterotrophic bacteria -> Protozoa -> Metazoa 



Autotrophic bacteria Protophyta ->- Metaphyta 



(3) I t 



Heterotrophic bacteria — > Protozoa ->■ Metazoa 



Autotrophic bacteria - 



(4) } 



Heterotrophic bacteria — > Protozoa 



Protophyta ->■ Metaphyta 

 — > Metazoa 



Grasse suggests that the first system is the most probable but that 

 all the others have something to be said in their favour. On the 

 other hand Oparin (1957) thinks that the heterotrophic bacteria 

 are the most primitive and that they gave rise to the autotrophic 

 forms and to the Protozoa and Protophyta, a view that is not 

 mentioned in Grasse's scheme. It can be seen that nothing is 

 definite ; all is hypothesis and opinion. 



At present the following schemes all seem equally likely: 



(1) The Bacteria and the Protozoa had an independent 

 phylogenetic origin. 



(2) The Bacteria are more primitive than the Protozoa and gave 

 rise to the Protozoa. 



(3) The Bacteria are secondarily simplified and were derived 

 from the Protozoa. 



(4) The Bacteria are a polyphyletic grade. Some are more 

 primitive than the Protozoa, others are derived from the 

 Protozoa. 



We have at present insufficient evidence to enable us to choose 

 between these hypotheses. 



