68 CLASSIFICATION AND CREATION. 



but in Centipedes, Spiders, and Winged Insects 

 the breathing-apparatus is aerial, consisting of 

 air-holes on the sides of the body, connected 

 with a system of tubes and vessels extending into 

 the body and admitting air to all parts of it. 

 In the winged Insects this system is very elabo- 

 rate, filling the body with air to such a degree as 

 to render it exceedingly light and adapted to easy 

 and rapid flight. The general arrangement of 

 parts is the same in this class as in the two oth- 

 ers, the typical character being alike in all. 



We come now to the highest branch of the An- 

 imal Kingdom, that to which we ourselves be- 

 long, — the Vertebrates. This type is usually di- 

 vided into four classes, Fishes, Reptiles,. JBiuds, 

 and Mammalia ; and though many nat uralists be- 

 lieve that it includes more, and I am myself of 

 that opinion, I shall allude here only to the four 

 generally admitted classes, as they are sufficient 

 for my present purpose, and will serve to show 

 the characters upon which classes are based. 

 In a former paper I have explained in general 

 terms the plan of structure of this type, — a 

 backbone, with a solid arch above and a solid 

 arch below, forming two cavities that contain all 

 the systems of organs, the whole being surround 

 gd by the flesh and skin, Xow whether a body 

 so constructed lie prone in the water, like a 

 Fish, — or be lifted on imperfect legs, like a 



