THE Chinese Empire under the Manchu dynasty consisted of a large, relatively homogeneous central area—the Eighteen Provinces, 1 often referred to as China Proper—in which was contained the greater part of the population of the Empire, and a number of outlying territories: Manchuria, Mongolia, Sinkiang and Tibet. The central area was isolated by natural barriers: impenetrable mountain ranges on the west, deserts to the north-west, tropical swamp to the south and the Pacific Ocean on the east. Only in the semi-arid region to the north (which was unsuited to the characteristically Chinese style of cultivation) were the obstacles to movement and penetration, though still real, less effective. Here, where the sedentary Chinese had to face their barbarian pastoral-nomadic neighbours, they had long ago supplemented nature by building the Great Wall.