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.  Notwithstanding the rudimentary condition of these limbs in some Vertebrates and their difference of external appearance in the different groups, they are all built of the same structural elements.  These are the typical characters of the whole branch, and exist in all its representatives.

What now are the different modes of expressing this structural plan that lead us to associate certain Vertebrates together in distinct classes?  Beginning with the lowest class,—­the Fishes are cold-blooded, they breathe through gills, and they are egg-laying; in other words, though they have the same general structure as the other Vertebrates, they have a special mode of circulation, respiration, and reproduction.  The Reptiles are also cold-blooded, though their system of circulation is somewhat more complicated than that of the Fishes; they breathe through lungs, though part of them retain their gills through life; and they lay eggs, but larger and fewer ones than the Fishes, diminishing in number in proportion to their own higher or lower position in their class.  They also bestow greater care upon their offspring than most of the Fishes.  The Birds are warm-blooded and air-breathing, having a double circulation; they are egg-laying like the two other classes, but their eggs are comparatively few in number, and the young are hatched by the mother and fed by the parent birds till they can provide for themselves.