Unprotected from the perpendicular rays of the sun, the African is black, muscular, and cheerful; under the shadow of primeval forest, man assumes a coppery hue, lacking the endurance of the negro, and becomes in disposition cold and melancholy. And again, if we look for the natural causes which tend to promote or retard association, we find in climates and continental configurations the chief agencies. The continent of the two Americas, in its greatest length, lies north and south, the eastern continental group extends east and west. Primitive people naturally would spread out in those directions which offered the least change of climate from that of the primitive centre. Obviously, variations of climate are greater in following a meridian than along a parallel of latitude. Thus, the tropical man passing along a meridian is driven back by unendurable cold, while a continent may be traversed on any parallel, elevations excepted, with but little variation in temperature. A savage, exposed and inexperienced, not knowing how to protect himself against severe changes of climate, could not travel far in a northerly or southerly direction without suffering severely from the cold or heat; hence, other things being equal, the inhabitants of a country whose greatest length lay east and west, would intermingle more readily than those whose territory extended north and south. [Sidenote: CLIMATE AND MOUNTAIN RANGES.] That the eastern hemisphere attained a higher degree of civilization than the western, may be partly due to the fact, that the former presents wider spaces of uniform climate than the latter. The climatic zones of the New World, besides being shorter, are intersected by mountain barriers, which tend to retard the intercourse that would otherwise naturally follow. Thus the Mexican table-land, the seat of Aztec civilization, is a _tierra fria_ situated above the insalubrious _tierra caliente_ of either coast and the healthful _tierra templada_ of the slopes, but below the mountain ranges which rise from this table-land, forming a _tierra frígida_, a region of perpetual snow. To this day, the natives of the Mexican plateau cannot live on the sea-coast, though less than a day's journey distant.