^^^1 ZOOGENESIS W&l 



Specialization, leads us directly to the original simul- 

 taneous appearance of the major groups through eogen- 

 esis and the subsequent development and refinement 

 of each through evolution. 



The flat picture of animal life presented as the result 

 of eogenesis — v^^hich may be regarded as mutational 

 development from the primitive single cell — shows 

 many wholly distinct and separate major groups from 

 each of which a phylogenetic, developmental or evo- 

 lutionary tree rises upward through geologic time. 



The larger phyla or major groups are divided into 

 classes, and as a rule the classes within each major 

 group are entirely distinct from each other and do 

 not inter grade. Thus in the mollusks we find pelecy- 

 pods or bivalves, scaphopods, solenogasters, gastro- 

 pods or snail-like creatures, and cephalopods. In the 

 echinoderms there are starfishes, brittle-stars, sea- 

 urchins, sea-cucumbers, crinoids, cystids and Mas- 

 toids. In the arthropods there are crustaceans, 

 arachnids, myriopods and insects. In the vertebrates 

 or backboned animals there are mammals, birds, rep- 

 tiles, amphibians and fishes. 



The distinctness of these classes each from the other 

 is probably of the same nature as the much broader 

 distinctions between the various phyla. That is, 

 each class should be interpreted as a selective recom- 

 bination through broad mutations in every eco- 

 nomically possible form of the features inherent in 

 and distinctive of the phylum. In other words, it 

 is a fair assumption that the differences between the 

 several classes within each phylum are differences 



[2.2.1] 



