CHAPTER III 

 THE ORIGIN AND EVOLUTION OF THE VERTEBRATES 



The problem of vertebrate ancestry is an old one and one that 

 evades a direct solution. Only through circumstantial evidence are 

 we at present likely to reach even a reasonably satisfactory answer. 



Certain postulates must be made, however, upon which may be 

 built up a theory. In the first place it may be assumed that the first 

 vertebrates were aquatic. Almost equally warranted is the assump- 

 tion that they were free-swimming, active creatures. They were 

 also evidently axiate animals with fairly advanced cephalization, i. e., 

 with well- differentiated heads and sense organs. They were meta- 

 meric, with well-developed segmental musculature and with a large 

 open coelom. They also had a dorsal tubular central nervous system, 

 a notochord and pharyngeal clefts three fundamental chordate 

 characters. 



An animal with these characters could not be very different from a 

 fish. Let us assume, then, with Osborn, that the first true vertebrate 

 was a " free-swimming, quickly darting type, with double pointed, 

 fusiform body in which the segmented propelling muscles are external 

 and a stiffening notochord is central." We have found no fish either 

 past or present that appears to be primitive enough to be considered 

 as the first fish. The earliest fossil fishes are armored types that have 

 evidently become specialized secondarily for bottom feeding habits. 

 Some of the most primitive sharks, especially the Devonian shark 

 Cladoselache (Fig. 65), almost meet our expectations of what a pri- 

 mordial fish may have been, but even these fishes bear evidences of 

 having evolved from much simpler fish-like ancestors. Where then 

 can we turn for a true fish prototype? The existing lancelets (Branchi- 

 ostoma or Amphioxus) with their fusiform bodies, notochord, and 

 segmental musculature, appear to be the closest approximation we can 

 find of what the ancestral fish creature may have been, and one of the 

 prevailing theories of vertebrate ancestry is the so-called " Amphioxus 

 theory," a theory that appears to the writer to be more satisfactory 

 than any other and which we shall herewith present in a new form. 



71 



