230 OUTLINES OF EVOLUTIONARY BIOLOGY 



Existing Groups 



as we have seen, represents the entire organic world ; in the 

 latter it represents the remote Protistan ancestors from which 

 both plants and animals have sprung. The first great branching 

 represents in the one case the systematist's main division of 

 organisms into vegetable and animal kingdoms, and in the other 

 the gradual differentiation of the ancestral Protista (which at one 



time constituted the 



, . 



entire organic world) 

 into the primitive 

 unicellular plants or 

 Protophyta on the 

 one hand and the 

 primitive unicellular 

 animals or Protozoa 

 on the other ; and so 

 on with subsequent 

 ramifications. 



The parallelism 

 between the two is 

 sufficiently striking 

 to justify us in believ- 

 ing that it would be 

 complete if only our 

 knowledge of classi- 

 fication and phylo- 

 geny were so ; we 

 should then doubtless 

 see at once that the 

 taxonomic tree and 

 the phylogenetic tree 

 are, after all, one and 

 the same thing, for we should arrange all organisms strictly in 

 accordance with the course of their evolution. 



One point remains to be noticed in connection with the phylo- 

 genetic tree. The branching has been monopodial rather than 

 dichotomous or polychotomous, each branch, in addition to giving 

 off lateral branches, being itself continued on, so to speak, at 

 each forking. The descendants of any given ancestral group 

 did not all undergo modification to the same extent, but some 

 adhered more or less closely to the ancestral condition and have 

 continued to do so to the present day. Even the primitive 



Ancestral Cocltniernes 



Ancestral Protista 



FIG. 87. Diagram illustrating the Relation 

 between Classification and Phytogeny . 



