We come now to the highest branch of the Animal Kingdom, that to which
we ourselves belong,--the Vertebrates. This type is usually divided into
four classes, Fishes, Reptiles, Birds, and Mammalia; and though many
naturalists believe 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 bony arch above and a bony arch below, forming two
cavities that contain all the systems of organs, the whole being
surrounded 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 Reptile,--or be balanced on two legs, while the front limbs become
wings, as in Birds,--or be raised upon four strong limbs terminating in
paws or feet, as in Quadrupeds,--or stand upright with head erect, while
the limbs consist of a pair of arms and a pair of legs, as in Man,--does
not in the least affect that structural conception under which they are
all included. Every Vertebrate has a backbone; every Vertebrate has a
bony arch above that backbone and a bony arch below it, forming two
cavities,--no matter whether these arches be of hard bone, or of
cartilage, or even of a softer substance; every Vertebrate has the
brain, the spinal marrow or spinal cord, and the organs of the senses in
the upper cavity, and the organs of digestion, respiration, circulation,
and reproduction in the lower one; every Vertebrate has four locomotive
appendages built of the same bones and bearing the same relation to the
rest of the organization, whether they be called pectoral and ventral
fins, or legs, or wings and legs, or arms and legs.
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