$M THE NEW EVOLUTION T"^^ 



The figure formed by this recombination of ele- 

 mental structural features into the various phyla repre- 

 sents the basic picture of animal life — a flat picture 

 with all its details of simultaneous appearance — in 

 which all of the evolutionary trees are rooted and 

 from which they rose, one from each phylum, upwards 

 through successive ages. 



The interpretation of the origin of the major groups 

 or phyla as the result of recombinations — or varying 

 combinations — of characters inherent in animals as a 

 whole which took place in early embryonic stages 

 supplies the key to the very sharp distinctions usually 

 to be seen between the different classes in each phylum. 



It has been mentioned that among the major 

 groups the cephalochordates, balanoglossids, cephalo- 

 discids, tunicates and vertebrates are rather more 

 closely related to each other than are the remaining 

 major groups. 



These five phyla form an assemblage capable of 

 definition as a unit — though a very heterogeneous 

 unit — which to a certain extent is intermediate in 

 character between a collection of five major groups 

 and a single major group alone. 



As a typical major group let us take the echino- 

 derms. The echinoderms agree among themselves, 

 and differ very markedly from all other animal types, 

 in being coelomate creatures with a fairly perfect 

 radial and usually pentamerous symmetry, with a 

 body wall containing calcareous plates and generally 

 armed with spines, with the coelome divided into two 

 well marked portions, the perivisceral cavity and the 



[zoo] 



