Before touching upon the question of the common nature and origin of the human race, a necessary preliminary to the question of their diffusion, it may be requisite to state the sense of certain terms of common occurrence in natural history, as _species_, _genus_, and _varieties_. A race of animals, or plants, which constantly transmit from one generation to another the same peculiar organization, constitute what is technically called a _species_; and two races are held to be specifically distinct, where a marked difference or organization exists, which is unvaryingly transmitted. A species, therefore, includes those animals and plants which may be presumed to have sprung from the same parent stock. “We unite,” says De Candolle, “under the designation of a species, all those individuals who mutually bear to each other so close a resemblance as to allow of our supposing that they may have proceeded originally from a single being, or a single pair.” The term _genus_ has a more comprehensive signification. It is applied to a group of animals or plants, the several tribes of which seem constructed after a common general model, each being distinguished from the rest by a peculiarity of organization, for which we cannot account but by supposing them to have proceeded from originally different individuals. Animals of the horse kind, which includes the ass and the zebra, furnish an example of genus. They display the phenomena of general resemblance, but with such marked differences, which are regularly transmitted, that we cannot suppose them the common offspring of the same individuals, but to have descended from originally different pairs. Animals of the feline race, as the cat and the tiger, and of the bovine kind, as the ox, buffalo, and bison, are similar instances of genera. A genus, therefore, embraces several species. But within the limits of a species varieties occur, or deviations from the type exhibited by the parent stock, which are due to external causes, climate, soil, food, and other agencies, which have an obvious and marked effect upon animal and vegetable forms, however little their operation is understood. Some of these varieties are transient, but others become fixed and permanent in the race, and are so optically striking, as in several cases to suggest the idea of a specific difference, where the species is identical. Now, the question to be considered in relation to man is, whether the diversities which he exhibits in different parts of the globe are compatible with his race coming under the denomination of a species, having a common ancestry; or whether it forms a genus including several tribes, having a general resemblance, but so characteristically different as to lead the philosophical investigator to the verdict, that the diverging streams of humanity have originated independent of each other, and have not proceeded from the same fountain head.