CLASSIFICATION. 85 
some of them, will hereafter be adopted. Two primary 
divisions have been, for half a century, generally recog- 
nized by naturalists, viz. 
' 1. That including the Vertebrate Animals, or those 
endowed with an internal bony skeleton, the con- 
stant feature of which, 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 ganglionary. 
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 which 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 into 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 
vol. i. 23 
