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 _Sawad_. 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.

And in Omar's time the _Sawad_ was no longer at its best, for, a few
years before the Arab conquest, abnormally high floods had burst the
dykes; from below Hilla to above Basra the Euphrates broadened into a
swamp, and the Tigris deserted its former (and present) bed for the
Shatt-el-Hai, leaving the Amara district a desert. The Persian
Government, locked in a suicidal struggle with Rome, was powerless to
make good the damage, and the shock of the Arab invasion made it
irreparable[58]. Under the Abbasid Caliphs of Bagdad the rest of the
country preserved its prosperity, but in the thirteenth century Hulaku
the Mongol finished the work of the floods, and under Ottoman dominion
the _Sawad_ has not recovered.

Can it still be reclaimed? Surveys have been taken by Sir William
Willcocks, as Adviser to the Ottoman Ministry of Public Works, and his
final conclusions and proposals are embodied in a report drawn up at
Bagdad in 1911[59].

"The Tigris-Euphrates delta," he writes, "may be classed as an arid
region of some 5,000,000 hectares.... All this land is capable of easy
levelling and reclamation. The presence of 15 per cent. lime in the soil
renders reclamation very easy compared with similar work in the dense
clays of Egypt. One is never far away from the giant banks of old canals
and the ruins of ancient towns."

But he does