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, Birds, | 
and Mammalia; and though many naturalists 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 
ed by the flesh and skin. Now whether a body 
so constructed lie prone in the water, like a 
Fish,—or be lifted on imperfect legs, like a 
