A Short History of the World by H.G. Wells

This appearance of a conquering Mongolian people in Europe may be taken to mark a new stage in human history. Until the last century or so before the Christian era, the Mongol and the Nordic peoples had not been in close touch. Far away in the frozen lands beyond the northern forests the Lapps, a Mongolian people, had drifted westward as far as Lapland, but they played no part in the main current of history. For thousands of years the western world carried on the dramatic interplay of the Aryan, Semitic and fundamental brunette peoples with very little interference (except for an Ethiopian invasion of Egypt or so) either from the black peoples to the south or from the Mongolian world in the far East.

It is probable that there were two chief causes for the new westward drift 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.