These conditions have affected the inhabitants of the plain in five ways.
(i) There is some stimulus to advance owing to the rhythmic swing of the seasons : as a hot summer alter nates with a cold winter, men, to live at all, must not be savages of a type which may exist on equatorial plains. They must be hardy and brave, and must have a certain amount of physical endurance.
(ii) As the land is flat over vast areas there is neither such a natural defence as the desert gave to the Egyptian or the marsh to the Babylonian, nor is there a natural position for defence such as the citizens of Rome pos sessed. These people must defend themselves. The climatic conditions are hard, so that individuals or families would almost certainly perish if left to them selves. For defence alike against enemies and climatic conditions there must be organization of a sort. Thus these folk lived—and live—in tribes.
(iii) As grass is the staple, in most regions the only, vegetable production, it is obvious that these tribes cannot live directly on what is grown on the land ; they must at any rate be able to use the energy in a more concentrated form. They must live on animals and on what animals produce. Thus these folk were, as they mostly still are, sheep-herds, cattle-herds, goat-herds and horse-riders, living on butter, milk and flesh.
(iv) Now, as the scanty pasture in one place is ex hausted or destroyed by sand driven by powerful winds, they are forced to pass on to another. The natural difficulty of movement on land—a difficulty largely due to friction—is overcome by a greater force still : the de sire for life. Further, as there is no means of defence at one spot more than in another, there is no inducement to stay in one place and very strong reasons why tribes should continually move on, so that the spirit of nomad ism, of wandering, becomes part of their very being.
(v)- Again, as they had in early times no protection except themselves, they were compelled to destroy all adversaries whom they overcame, being assured that otherwise they themselves would be destroyed if ever the fortunes changed. They were thus a cruel race of men.
Whether driven out by increasing dryness or moved only by their own natural restlessness, these dwellers in the central plain during all the times of which history tells have been a disturbing influence on the more or less settled peoples on the margins, and again and always again have emerged from beyond the mountain rim to overthrow rather than to set up, to destroy rather than to create. We have seen how the Assyrian Empire was
so weakened by incursions of northern tribes that it fell soon afterwards. Perhaps even earlier still we have traces of the advent on eastern lands of these nomads from beyond the mountains, while the earlier Greek civilization also was for a time overwhelmed by incur sions from the north. However that may be, it is certain that as we come to know more surely of the actual events in the world, the influence on history of these migrating tribes becomes more and more clear.
Now Europe, as we have seen, was to a considerable extent separated from Asia in 'prehistoric times. Thus the plain was not one but two. It is doubtful whether such people as inhabited each section of the plain in those distant times were exposed to the conditions that induce habits characteristic of steppe-dwellers, but at any rate they were different races. Thus the invaders from the plain were of two kinds—the bearded inhabitants of the north of Europe, the Teutons, and the beardless dwellers on the plains of Asia, the Tatars and Mongols. This difference of race corresponded to other differences due to geographical causes. As it happened, the peoples of the European half of the plain were more in touch with civilizing influences than were those of the eastern half. We have seen that as the result of natural conditions the south of Europe was civilized : for reasons which we shall discuss later the south of Asia was not. In anv case, the belt of highland is so much broader in Asia than in Europe that a greater barrier is interposed to the movement either of men or of ideas, so that the Asiatic half of the plain is more shut off from other centres of civilization. The climatic conditions, too, are more rigorous in the east than the west. As the mass of Asia is so much greater than Europe and so much more shut off from the sea, the temperatures of the centre are more extreme. The breadth and height of the Asiatic high lands, too, prevent rain reaching the centre in anything but small amount, and the distance from the ocean, and especially the western ocean, also tends to make the rainfall smaller than that of Europe.