CLASSIFICATION. gg 



some of them, m\\ hereafter be adopted. Two primary 

 divisions have been, for half a century, generally recog- 

 nized by naturahsts, viz. 



1. Tliat including the Vertebrate Animals, or those 

 endowed with an internal bony skeleton, the con- 

 stant feature of wliich, is the vertebral column ; and 

 possessing a brain, spinal marrow, and a system of 

 nerves connected with them. 



2. That including the Invertebrate Animals, or 

 those destitute of the structure belonging to the pre- 

 ceding ; the nervous system being only cjmiglionary. 



These great primary divisions are founded, essen- 

 tially, upon differences in the conformation of the ner- 

 vous system, and separate the animals possessing a com- 

 mon sensorium or brain, and cerebral nerves, together 

 with a ganglionary system, from those wliich possess a 

 ganglionary system only. The first is characterized 

 in the most definite manner, and the animals included 

 in it, are constituted on the same general plan of organ- 

 ization. It has usually been subdivided into four 

 great classes. Mammals, Birds, Reptiles, and Fishes. 

 But there are indications from the recent investigations 

 into the structure of the Marsupial animals, which may 

 lead to their establishment as a class between the Mam- 

 mals and Birds ; and there are not wanting reasons 

 for the division of the Reptiles mto two independent 

 classes. But whatever changes may take place in the 

 grouping of the component parts of the division, the 

 division itself must be retained with nearly its present 



