Note 87 — Chapter XI · RACIAL DIFFERENCES ARE PERMANENT

Cuvier, op. cit. Compare also, on this point, the opinion of Alexander von Humboldt: “In the epochs preceding the existence of the human race the action of the forces in the interior of the globe must, as the earth’s crust increased in thickness, have modified the temperature of the air and made the whole earth habitable by the products which we now regard as exclusively tropical. Afterwards the spatial relation of our planet to the central body (the sun) began, by means of radiation and cooling down, to be almost the sole agent in determining the climate at different latitudes. It was also in these primitive times that the elastic fluids, or volcanic forces, inside the earth, more powerful than they are to-day, made their way through the oxidized and imperfectly solidified crust of our planet” (Asie centrale, vol. i, p. 47).