g THE EVOLUTION OF MAST. 



modified into hard bone, are the earliest Vertebrates capable 

 of leaving petrified records of their existence and structure. 

 Fortunately, this want is more than counterbalanced 

 by the much more important testimony of Comparative 

 Anatomy and Ontogeny, which henceforth form our 

 safest guides within the Vertebrate pedigree. Thanks to 

 the classic researches of Cuvier, Johannes Miiller, Huxley, 

 and especially of Gegenbaur, we are in possession of such 

 extensive and instructive records of creation in this most 

 important branch of tribal history, that we can prove at 

 least the more significant features in the development of our 

 Vertebrate ancestors, with the most gratifying certainty. 



The characteristic peculiarities by which Vertebrates 

 in general are distinguished from all Invertebrates, engaged 

 our attention some time ago, when we examined the structure 

 of the ideal Primitive Vertebrate (Figs. 52-56, p. 256). The 

 most prominent characters were as follows: (1) the formation 

 of the notochord between the medullary and intestinal tubes; 

 (2) the differentiation of the intestinal tube into an anterior 

 gill-intestine and a posterior stomach-intestine ; (3) the 

 inner articulation, or formation of metamera. The Verte- 

 brates share the first two qualities with the larval Ascidians 

 and with the Chorda-animals ; the third quality is entirely 

 peculiar to them. Accordingly, the most important struc- 

 tural advance, by which the earliest vertebrate forms origin- 

 ated from the most nearly allied Chorda- Animals, consisted 

 in an internal metameric structure. This showed itself 

 first most distinctly in the articulation of the muscular 

 system, which broke up on the right and left into a series 

 of consecutive muscular plates. At a later period the 

 articulation declared itself prominently in the skeleton, and 



