ON CLASSIFICATION. 11 



which form the rest of the central chain of ganglia, but as we 

 advance from fishes towards the higher forms of the vertebrata, we 

 observe them to preponderate more and more in bulk, until at 

 last in man they assume that extraordinary developement adapted 

 to the exalted position which he is destined to occupy. It is in 

 the cerebral ganglia, therefore, that we have the representative of 

 the supracesophageal masses of the articulated and molluscous 

 classes, which, as we have already seen, preside especially over the 

 senses, and correspond in their proportions with the capabilities of 

 the tribes of animals included in those divisions. The spinal cord, 

 as the rest of the central axis of the nervous system of vertebrata 

 is denominated, is made up of a succession of ganglia, in communi- 

 cation with symmetrical pairs of nerves connected with them, and 

 which preside over the generally diffused sense of touch, and the 

 voluntary motions of the body. But besides the cerebro-spinal sys- 

 tem, we find in the vertebrated classes another set of nervous centres, 

 to which nothing corresponding has been satisfactorily identified in 

 the lower divisions ; namely, the sympathetic system, which mainly 

 controls the involuntary movements of the body connected with the 

 vital functions. 



The vertebrata are further distinguished by the possession of 

 an internal organized skeleton, either composed of cartilage or 

 bone, which is made up of several pieces, and serves as the general 

 support of the frame, forming a series of levers upon which the 

 muscles act. 



This last division of the animal world embraces the following 

 classes : 



1. Fishes. 4. Birds. 



2. Amphibia. 5. Mammalia. 



3. Reptiles. 



Such will be the classification which we shall adopt in the 

 following pages ; and although, perhaps, the definitions of the five 

 great groups may be considered by the scientific reader as some- 

 what scanty, enough, we trust, has been said to render intelli- 

 gible the terms which we shall hereafter have frequent occasion to 

 employ. 



(14.) A question naturally presents itself in this place which re- 

 quires consideration : May we expect, as we advance from the lower 

 types of organization to such as are more perfect, to be led on 

 through an unbroken and continuous series of creatures, gradually 

 rising in importance and complexity of structure, each succeeding 



