The obvious gradation amongst the families of both the vegetable and animal kingdoms, from the simple lichen and animalcule respectively up to the highest order of dicotyledonous trees and mammalia, has already been intimated. Confining our attention, on this occasion, to the animal kingdom—it is to be observed that the gradation is much less simple and direct than is generally supposed. Even in its larger masses, it certainly does not proceed, at all parts of its course at least, upon one line; for the two subkingdoms of middle rank, mollusca and articulata, form unquestionably distinct approaches to the highest, the vertebrata. It even appears that there are intimations of more than two such great lines at various parts of the animal scale, as will be afterwards more particularly explained. Nevertheless, no doubt is entertained of a general advance of organization from the radiate into both the molluscous and articulate forms, and from the latter of these again (if not the former also) into the vertebrate; as also along the classes of (for example) the vertebrata, in this sequence—fishes, reptiles, birds, mammals.

While the external forms of all these various animals are so different, it is very remarkable that the whole are, after all, variations of a fundamental plan, which can be traced as a basis throughout the whole, the variations being merely modifications of that plan to suit the particular conditions in which each particular animal has been designed to live.