^1^ THE NEW EVOLUTION ^|^ 



arising in the early embryonic stages of the animal 

 types concerned more or less immediately after the 

 fixation of the structural complex characteristic of 

 the phyla. 



Within the classes the same phenomenon is again 

 repeated in a less extreme form in the different orders, 

 as is especially well seen in the insects, crustaceans, 

 gastropods and brittle-stars. 



Abrupt discontinuities, becoming progressively less 

 and less pronounced, may be followed further into 

 suborders, families, genera and species, as we saw 

 (p. 133) among the butterflies. 



Discontinuities of every sort, though often obvious 

 and striking, are very much less conspicuous and 

 marked within the phylum Vertebrata than they are 

 in the large invertebrate phyla. In the Vertebrata 

 even the classes are not always sharply distinguished 

 from each other. Thus many fossils cannot with cer- 

 tainty be determined as reptiles or as amphibians. 

 More or less intermediate between the birds and rep- 

 tiles are two curious creatures from the Jurassic which 

 figure together in text-books under the common name 

 of Archaopteryx. Similarly intermediate between the 

 mammals on the one hand and the reptiles and am- 

 phibians on the other are the strange egg-laying mam- 

 mals or monotremes of Australia and New Guinea. 



The relative homogeneity of the phylum Vertebrata 

 is easily understood when it is realized that the 

 vertebrates are the most highly specialized of all 

 forms of animal life, and that the entire structural 

 range in all the vertebrates taken together is scarcely 



[2.2.1] 



