north. Between the Himalayas and the Kuen-lun is the great Plateau of Tibet, which runs into a sort of cul-de-sac at its western end in Kashmir. Between the Kuen-lun and the Thian Shan we have the Gobi Steppe of Mongolia, running west of Kashgar and Yarkand; while between the Thian Shan and the Altai we have the great Kirghiz Steppe. It is clear that only two routes are possible between Eastern and Western Asia: that between the Kuen-lun and the Thian-Shan via Kashgar and Bokhara, and that south of the Altai, skirting the north of the great lakes Balkash, Aral, and Caspian, to the south of Russia. The former would lead to Bassorah or Ormuz, and thence by sea, or overland, round Arabia to Alexandria; the latter and longer route would reach Europe via Constantinople. Communication between Southern Asia and Europe would mainly be by sea, along the coast of the Indies, taking advantage of the monsoons from Ceylon to Aden, and then by the Red Sea. Alexandria, Bassorah, and Ormuz would thus naturally be the chief centres of Eastern trade, while communication with the Mongols or with China would go along the two routes above mentioned, which appear to have existed during all historic time. It was by these latter routes that the Polos and the other mediæval travellers to Cathay reached that far-distant country. But, as we know from Marco Polo's travels, China could also be reached by the sea voyage; and for all practical purposes, in the late Middle Ages, when the Mongol empire broke up, and traffic through mid Asia was not secure, communication with the East was via Alexandria.
Now it is important for our present inquiry to
