There is therefore nothing to show that humanity had several distinct nuclei. It is clear that man started from one point alone, and that through his power of adapting himself to the most different climates, he has, little by little, covered the whole face of the inhabitable earth. There is another problem. Did the white, the yellow, and the black man exist from the first moment of the appearance of our species upon the globe, or have we to explain the formation of these three fundamental races by the action of climate, by any special form of nourishment, the result of local resources; in other words, by the action of the soil? Innumerable dissertations have been written with a view of explaining the origin of these three races, and of connecting them with the climate or the soil. But it must be admitted that the problem is hardly capable of solution. The influence which a warm climate exercises upon the color of the skin is a well-known fact, and it is a matter of common observation that the white European, if transported into the heart of Africa, or carried to the coast of Guinea, transmits to his descendants the brown color which the skin of the Negro possesses, and that in their turn the offspring of Negroes, who have been brought into northern countries, become as they descend paler and paler and end by being white. But the color of the skin is not the only characteristic of a race; the Negro differs from the white, less by the color of his skin than by the structure of the face and cranium, as also by the proportion of his members to one another. Is it not, moreover, a fact that the hottest countries are inhabited by people with white skins? Such, for instance, are the Touaricks of the African Sahara, and the Fellahs of Egypt. On the other hand, men with black faces are found in countries enjoying a mean temperature, as, for instance, the inhabitants of California on the coast of the Pacific Ocean. We have now another question to consider. Should these white, yellow, or black men, to whom we must add those who are brown and red, all of whom differ one from another in the color of their skin, in height, in their physiognomy, and in their outward appearance, be grouped into different species, or are we to regard them merely as varieties of species--that is to say, _races_?