CHAPTER IX 

 EVOLUTION OF THE VERTEBRATES 



In no phylum is the course of evolution more clearly or com- 

 pletely indicated than in the phylum Chordata, particularly the 

 subphylum Craniata, commonly called vertebrates. As has 

 already been shown, the details of comparative anatomy and em- 

 bryology in this group are remarkably clear evidences of relation- 

 ship of the various classes in a succession of stages which point 

 clearly to a chronological as well as merely comparative series. 

 In addition the geological record is excellent. The bony endo- 

 skeleton and many hard integumentary structures, such as teeth, 

 scales and protective armor are well fitted for preservation as 

 fossils, and conditions have often been favorable for the fossiliza- 

 tion of many individuals. 



The Origin of the Vertebrates. In spite of these conditions 

 within the phylum, we are faced at the outset by a break in the 

 paleontological record second to no other. The vertebrates appear 

 in the Ordovician as well developed fishes without any indication 

 of their invertebrate ancestors. In the contemporary invertebrate 

 fossils and those of the Cambrian are found many forms that may 

 have been ancestral or derived from the same source, but the gulf 

 is great. We still have only theories to explain the origin of the 

 vertebrates. While each theory finds facts to support it both in 

 the study of living forms and in the geological record, as might be 

 expected of organisms which have some degree of common relation- 

 ship, none establishes that completeness and certainty of phylog- 

 eny which is to be desired. 



The three leading theories of vertebrate descent are the Am- 

 phioxus theory, the annelid theory, and the arthropod theory. 

 Several minor theories have been formulated which are of interest 

 only to the special student. 



Since Amphioxus is a chordate, the probability that it is similar 

 to the ancestor of the vertebrates is merely one step in establishing 

 their origin and does not indicate a connection with the inverte- 

 brate phyla. The annelid theory brings out facts which, with the 



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