But if the highlands of the earth were early the dwelling-place of the shepherd-warrior, it was within the adjoining lowland plains and fertile river basins that the arts of civilization were first called into being, that towns were built, that population became numerous, and that systems of social polity were developed. The lowland plains of Asia and Europe constitute, in the present day, the most populous regions of the globe, and include by far the more numerous portion of the human race. The like regions in the New World are fast filling with inhabitants, as the redundant population of older lands is directed, in an ever-flowing stream, across the waters of the Atlantic. The most important and extensive among the lowland plains of the Old World are the following: IN ASIA.--Plain of the Euphrates and Tigris (the ancient Mesopotamia and Babylonia); Plain of Hindustan, or Northern India; Plain of China, embracing the northeast part of that country; Plain of Siberia; Plain of Turkestan. Among lowland regions of less importance are the plains of Pegu, Siam, and Tonquin, all within the Indo-Chinese peninsula, or India beyond the Ganges. IN EUROPE.--The Great Eastern Plain, embracing nearly the whole of Russia; Plain of Hungary, embracing the middle portion of the valley of the Danube; Plain of Wallachia and Bulgaria, or the Lower Danube; Plain of Lombardy, or Northern Italy; Plain of Languedoc, in the south of France; Plain of Andalusia, in the south of Spain; Plain of Bohemia, or basin of the Upper Elbe. The limits and direction of these regions may be traced upon any ordinary map, by means of their coincidence with the great river basins of the Eastern Hemisphere. They include the longer slopes of the land, which are directed toward the north and northwest, as well as the less extensive low grounds which border the Indian and Pacific Oceans. The Siberian plain alone comprehends an area equal to that of Europe, and the rivers by which it is watered are among the most considerable in the Old World. So vast an area, under other conditions of climate, might have become the home of populous nations, the seat of civilization and empire. But its high latitudes, which involve the rigor of an Arctic sky, condemn a large portion of Siberia to the condition of a sterile wilderness, and must prevent even its more favored districts from being other than thinly inhabited. The dreary swamps and morasses of the _tundras_, which replace, during the brief summer of those latitudes, the plain of ice and snow, stretch along the shores of the Arctic Sea through a vast extent of this widespread region.