The second and first centuries B.C. mark a new phase in the history of mankind. Mesopotamia and the eastern Mediterranean are no longer the centre of interest. Both Mesopotamia and Egypt were still fertile, populous and fairly prosperous, but they were no longer the dominant regions of the world. Power had drifted to the west and to the east. Two great empires now dominated the world, this new Roman Empire and the renascent Empire of China. Rome extended its power to the Euphrates, but it was never able to get beyond that boundary. It was too remote. Beyond the Euphrates the former Persian and Indian dominions of the Seleucids fell under a number of new masters. China, now under the Han dynasty, which had replaced the Ts’in dynasty at the death of Shi-Hwang-ti, had extended its power across Tibet and over the high mountain passes of the Pamirs into western Turkestan. But there, too, it reached its extremes. Beyond was too far. China at this time was the greatest, best organized and most civilized political system in the world. It was superior in area and population to the Roman Empire at its zenith. It was possible then for these two vast systems to flourish in the same world at the same time in almost complete ignorance of each other. The means of communication both by sea and land was not yet sufficiently developed and organized for them to come to a direct clash. Yet they reacted upon each other in a very remarkable way, and their influence upon the fate of the regions that lay between them, upon central Asia and India, was profound. A certain amount of trade trickled through, by camel caravans across Persia, for example, and by coasting ships by way of India and the Red Sea. In 66 B.C. Roman troops under Pompey followed in the footsteps of Alexander the Great, and marched up the eastern shores of the Caspian Sea. In 102 A.D. a Chinese expeditionary force under Pan Chau reached the Caspian, and sent emissaries to report upon the power of Rome. But many centuries were still to pass before definite knowledge and direct intercourse were to link the great parallel worlds of Europe and Eastern Asia. To the north of both these great empires were barbaric wildernesses. What is now Germany was largely forest lands; the forests extended far into Russia and made a home for the gigantic aurochs, a bull of almost elephantine size. Then to the north of the great mountain masses of Asia stretched a band of deserts, steppes and then forests and frozen lands. In the eastward lap of the elevated part of Asia was the great triangle of Manchuria. Large parts of these regions, stretching between South Russia and Turkestan into Manchuria, were and are regions of exceptional climatic insecurity. Their rainfall has varied greatly in the course of a few centuries They are lands treacherous to man. For years they will carry pasture and sustain cultivation, and then will come an age of decline in humidity and a cycle of killing droughts.