In this enquiry, we have before us what in the strictest sense of the word is a system. All the individuals of the extensive division of the animal kingdom which we have to review, possess a cranium for the protection of the brain,—a heart, implying a peculiar circulation,—five distinguishable organs of sense ; but the grand peculiarity, whence the term vertebrata is derived, is to be found in the spine; that chain of bones which connects the head and body, and, like a keel, serves as a foundation for the ribs; or as the basis of that fabric which is for respiration.

I have said, that we are to confine ourselves to a portion only of this combined structure; to separate and examine the anterior extremity, and to observe the adaptation of its parts, through the whole range of these animals. We shall view it as it exists in man, and in the higher division of animals which give suck, the mammalia—in those which propagate by eggs, the oviparous animals,—birds, reptiles, amphibia, and fishes; and we shall find the bones which are identified by certain common features, adjusted to various purposes, in all the series from the arm to the fin. We shall recognise them in the mole, formed into a powerful apparatus for digging, by which the animal soon covers itself, and burrows its way under ground.