Human progress is stimulated by the tendency exhibited by population to outstrip the means of subsistence. No sooner has a community attained a certain degree of physical well-being than this great natural law comes into play. The numbers begin to press too heavily on the land, and the younger and more vigorous are driven to seek new spheres for their energies. They colonise distant lands, subdue their weaker neighbours, and the mother state becomes a centre of dominion, of luxury and its attendant arts. It is the process which gave the world the priceless boon of Greek civilisation, and made Rome a storehouse whence we moderns have drawn our principles of law and government. In the earlier centuries of our era the regions lying between the Gobi Desert and Lake Baikal were the habitat of a congeries of Mongolian tribes belonging to the Ural-Altaic family.[527] They were a pastoral race, living in tents of felt and skins which they moved when the surrounding pastures had been exhausted by their flocks and herds.
The nomad instinct thus became with them a second nature, and as they were tireless horsemen and inured to hardships, it led them to carry bloodshed and rapine over neighbouring territories. In their case the tendency to spread over the face of the earth was keener far than in that of communities engaged in settled avocations. But much of their strength was expended in inter-tribal war, until a man of genius arose who knew how to reconcile discordant interests and to forge a weapon of aggression which no living force could withstand. This Napoleon of Asia was known to his contemporaries as Temuchin, and to posterity as Chingiz Khān. He was born in 1162, the son of a chieftain whose authority was supreme in the tract between the Amur and the Great Wall of China. His youth was spent in struggles for supremacy with rival chieftains, but he at length welded together the whole Mongolian race by sheer personal ascendency, and dangling before his followers the bait of plunder. Then began a career of conquest which finds no parallel save in that of his greater successor Tamerlane. He entered Khwārazm[528] in 1218 at the head of three hordes,[529] overran Khojend, Samarkand, Bokhārā, and devastated Northern Persia. Merv, Nīshāpūr, Herāt, and other great and wealthy cities were overwhelmed in the avalanche. After penetrating far into India he returned to his darling steppes in 1225, gorged with booty. The impetus thus given to the teeming forces of disorder continued. Two lieutenants of Chingiz Khān skirted the southern shore of the Caspian and carried ruin through Georgia and the Crimea, returning by way of Bulgaria, while a third subdued nearly the whole of China. The death of Chingiz in 1227 brought no cessation to the movement. The greed of his followers was inflamed by rumours of the wealth and luxury of the Russian republics; and in 1238 his grandson, Bātū Khān, headed an invading host which ravaged the central and eastern plains, and ruined Riazan, Rostov, Yaroslav, and Tver. In the following year the cities of South-Western Russia shared their fate; and then the Khān retired to his camp at Serai on the Lower Volga, where he rested awhile from rapine and slaughter. His headquarters became a centre for intrigue among the Russian princes, who were permitted to retain a certain degree of authority by their conquerors.