"This," Herodotus adds[54], "is the best demonstration I can give of the wealth of the Babylonians: All the lands ruled by the King of Persia are assessed, in addition to their taxes in money, for the maintenance of the King's household and army in kind. Under this assessment the King is maintained for four months out of the twelve by Babylonia, and for the remaining eight by the rest of Asia together, so that in wealth the Assyrian province is equivalent to a third of all Asia." The "Asia" over which the Achaemenids ruled included Russian Central Asia and Egypt as well as modern Turkey and Persia, and Egypt, under the same assessment, merely maintained the local Persian garrison[55]. Its money contribution was inferior too--700 talents as compared with Assyria's 1,000; and though these figures may not be conclusive, because the Persian "province of Assyria" probably extended over the northern steppes as well as the _Sawâd_, it is certain that under the Arab Caliphate, when Irak and Egypt were provinces of one empire for the second time in history, Irak by itself paid 135 million _dirhems_ (francs) annually into Harun-al-Rashid's treasury and Egypt no more than 65 million, so that a thousand years ago the productiveness of the _Sawâd_ was more than double that of the Nile. Another measure of the land's capacity is the greatness of its cities. Herodotus gives statistics[56] of Babylon in the fifth century B.C.--walls 300 feet high, 75 feet broad, and 58 miles in circuit; three- and four-storied houses laid out in blocks; broad straight streets intersecting one another at regular intervals, at right angles or parallel to the Euphrates. Any one who reads Herodotus' description of Babylon or Ibn Serapion's of Bagdad, and considers that these vast urban masses were merely centres of collection and distribution for the open country, can infer the density of population and intensity of cultivation over the face of the _Sawâd_. When the Caliph Omar conquered Irak from the Persians in the middle of the seventh century A.D., and took an inventory of what he had acquired, he found that there were 5,000,000 hectares[57] of land under cultivation, and that the poll-tax was paid by 550,000 householders, which implies a total population, in town and country, of more than 5,000,000 souls, where a bare million and a half maintains itself to-day in city alleys and nomads' tents.