This region is made up of those spaces lying north of the Great Wall of China, that largest fence ever reared by man to ward off an enemy, and farther west by the greatest barrier raised upon earth through creation, and also used by man as a line of defense, a fortress of refuge, that unique mountain system extending from Eastern China to Persia, and then, with a break, to the Caspian. From the Caspian westward the immense space is bounded by the Caucasus and the Black Sea, till it reaches the Danube and the mountains just north of that river. This vast region, or Mongol careering ground as we may call it, began on the east at waters which are really the Pacific, and on the west touched the Danube, which finds its source very near the Rhone and the Rhine, both flowing into the Atlantic, since the North Sea, with its waters, is merely a part of that ocean. The width of this region extends from the southern boundary just given to the Arctic, or Frozen Ocean. The entire southern part, somewhat less than half of this entire area, was an open, treeless country, grass-growing land and sand plains. All along on the northern side of this southern division were great stretches of grass land, with small groves of trees, from one acre to one hundred in area. Lands of this kind are seen in Siberia to our day. In the center were fruitful spots, deserts and oases. In the east, next to the center, were boundless plains, with a greater proportion of forest toward the distant east and toward the north, but with clear spaces everywhere. On the south, from the Danube to the Chinese Sea, the country was open at all points. Such was the Mongol careering ground, and after they had overrun Europe to the Adriatic and north of it they retired to the western part of this great open country of Eastern Europe, and made their capital at Sarai, just east of the Volga, and perhaps two hundred miles north of the Caspian. But before writing of the Mongol invasion of Russia, it will be necessary to give a somewhat detailed history of Russia previous to that event.