298 ESSAY ON CLASSIFICATION. 



There is not a dissenting voice among anatomists 

 respecting the natural limits of the Vertebrata as a 

 branch of the animal kingdom. Their character, how- 

 ever, does not so much consist in the structure of their 

 backbone or the presence of a dorsal cord, as in the 

 general plan of that structure, which exhibits a cavity 

 above and a cavity below a solid axis. These two cavities 

 are circumscribed by complicated arches, arising from the 

 axis, which are made up of different systems of organs, 

 the skeleton, the muscles, vessels, and nerves, and include, 

 the upper one the centres of the nervous system, the 

 lower one the different systems of organs by which assi- 

 milation and reproduction are carried on. 



The number and limits of the classes of this branch 

 are not yet satisfactorily ascertained. At least, natural- 

 ists do not agree about them. For my part, I believe 

 that the Marsupialia cannot be separated from the Placental 

 Mammalia as a distinct class, since we observe, within 

 the limits of another type of Vertebrata, the Selachians, 

 which cannot be subdivided into classes, similar differ- 

 ences in the mode of development to those winch exist 

 between the Marsupials and the other Mammalia. But I 

 hold at the same time with other naturalists, that the 

 Batrachia must be separated, as a class, from the true 

 Eeptiles, as the characters which distinguish them are of 

 the kind upon which classes are founded. I am also 

 satisfied that the differences which exist between the 

 Selachians (the Skates, Sharks, and Chimserse) and the 

 Fishes, are of the same kind as those which distinguish 

 the Amphibians from the Eeptiles proper, and justify, 

 therefore, their separation, as a class, from the Fishes 

 proper. I consider also the Cyclostomes as a distinct 

 class, for similar reasons ; but I am still doubtful whether 



