THE EVOLUTIONARY CAREER 



315 



ix. The Echinoderms. A group of persistent and specialized 

 animals that have displayed episodes of considerable interest. 

 X. The Arthropods. Perhaps the oldest known group of 



Tertiary 



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Fig. 44. — Periods of Main Groups of Fishes. 



The dotted lines suggest the affinities. 



multicellular higher animals. After their early episodial 

 developments they have been a type of continually increasing 

 dominance. 



xi. The Chordates. Here we provisionally include the extinct 

 Ostracoderms. On the whole the idea of " progress " in 

 evolution is best illustrated by the Chordates. Fig. 43 suggests 

 this in the w^ay in which the various sub-groups of vertebrate 

 animals are made to show convergence. This figure and also 

 Fig. 44 are also attempts to make phylogenies. But it must be 

 understood that there is hardly any warrant in the paleontological 

 evidence for the linking together of the sub-groups, and the 

 consequent derivations, that are suggested by the dotted lines. 



