XL THE HUNS AND THE END OF THE WESTERN EMPIRE

of the nomadic Mongolians. One was the consolidation of the great empire of China, its extension northward and the increase of its population during the prosperous period of the Han dynasty. The other was some process of climatic change; a lesser rainfall that abolished swamps and forests perhaps, or a greater rainfall that extended grazing over desert steppes, or even perhaps both these processes going on in different regions but which anyhow facilitated a westward migration. A third contributary cause was the economic wretchedness, internal decay and falling population of the Roman Empire. The rich men of the later Roman Republic, and then the tax-gatherers of the military emperors had utterly consumed its vitality. So we have the factors of thrust, means and opportunity. There was pressure from the east, rot in the west and an open road.