What greatly contributed towards the rapid development of Bagdad and towards making it the great emporium of the East, was the admirable system of canals which intersected the rich alluvial plains of lower Babylonia and which enabled the inhabitants of this region to utilize the surplus waters of the Euphrates for irrigating the fertile lands which lay between this river and the Tigris. Contrary to what is so often thought, the Arabs, under the Caliphs, gave as much attention to the canalization of Mesopotamia as had their predecessors, the Persians and the Babylonians. But these canals, besides being used for purposes of irrigation, likewise served for the transportation of merchandise from distant regions. Thus, to give a single instance of their use for this purpose, we are informed that “great boats and barges were loaded at Rakkah, ‘the port,’ as it was called, of the Syrian desert on the Upper Euphrates, there taking over from the land-caravans the corn of Egypt and the merchandise from Damascus; and these boats, coming down the great river, and then along the Isa Canal, discharged their cargoes at the wharves on the Tigris banks at the lower harbor in Karkh.”[427] To give some idea of the fertility of the irrigated region of Mesopotamia during the reign of the Caliphs it suffices to quote the words of an Arabic writer regarding certain cornlands in the vicinity of Bagdad, where, we are told, “the crops never failed, neither in winter nor in summer,” and of the report given of a certain mill in which there were no fewer than a hundred millstones which produced the extraordinary annual rental of a hundred million dirhams, a sum that was equivalent to several million dollars of our money. Marco Polo, that king of mediæval travelers, who, by the vast compass of his journeys, was better qualified than any man of his time to express an opinion on the relative importance of the cities of Asia, declares that Bagdad--which he calls Baudas--is “the noblest and greatest city of these regions” and that it “used to be the seat of the Caliph of all the Saracens in the world, just as Rome is the seat of the Pope of all the Christians.”[428] But the illustrious Venetian voyager did not visit Bagdad until several decades after the destruction of the city by the Mongol hordes under Hulagu Khan.