86 VERTEBRATE ZOOLOGY 



there is little evidence of its having descended from any form at all 

 like Balanoglossus. Rather would we beheve that Balanoglossus is 

 an early lateral offshoot from the main Une of chordate stock that 

 leads more directly to Amphioxus and the vertebrates. 



Summary of the Theories of Vertebrate Ancestry 



After a review of these various and contradictory theories as to the 

 origin of the vertebrates, are we any nearer a solution of this per- 

 plexing problem than when we started? Certainly it cannot be 

 claimed that the problem is solved, but at least we have examined the 

 question and have considered the various possibilities. Of all the 

 alternatives, the one that makes Amphioxus a central main-line an- 

 cestral form, perhaps slightly degenerate, but only a little speciaUzed, 

 seems best supported by evidence. Amphioxus, on the one hand, is so 

 much hke the vertebrates that it has been classed as an acraniate 

 vertebrate by so able a writer as Osborn. It is also, on the other hand, 

 unquestionably related to the tunicates. The assumption of an Am- 

 phioxus-like creature as a common ancestor of vertebrate and tunicate, 

 and the diagnosis of the Hemichordata as an early lateral offshoot of 

 a still more primitive chordate trunk, seems much more logical than 

 alternative assumptions. It also rids us of the necessity of deriving 

 one phylum from the highly differentiated members of another phy- 

 lum, and is therefore more acceptable than annehd, arthropod, or 

 nemertean theories. 



