But of more importance for the future development of geographical knowledge were the great caravan routes of Asia, to which we must now turn our attention.  Asia is the continent of plateaux which culminate in the Steppes of the Pamirs, appropriately called by their inhabitants “the Roof of the World.”  To the east of these, four great mountain ranges run, roughly, along the parallels of latitude—­the Himalayas to the south, the Kuen-Iun, Thian Shan, and Altai to the 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 mediaeval 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 realise how largely Europe after the Crusades was dependent on the East for most of the luxuries of life.  Nothing produced by the looms of Europe could equal the silk of China, the calico of India, the muslin of Mussul.  The chief gems which decorated the crowns of kings and nobles, the emerald, the topaz, the ruby, the diamond, all came from the East—­mainly from India.  The whole of mediaeval medical science was derived from the Arabs, who sought most of their drugs from Arabia or India.  Even for the incense which burned upon the innumerable altars of Roman Catholic Europe, merchants had to seek the materials in the Levant.  For many of the more refined handicrafts, artists had to seek their best material from Eastern traders:  such as shellac for varnish, or mastic for artists’ colours (gamboge from Cambodia, ultramarine from lapis lazuli); while it was often