It is worth remarking in the first place that the Ariaspian inhabitants of the Helmund valley had received from Cyrus the name of Euergetai, or benefactors, because they had assisted him at a time when he had been in great difficulties. This is enough to satisfy us that the district was known and had been traversed by a military force long before Alexander entered it, and that he was making no venturesome advance in ignorance of what lay before him. The valley of the Helmund (or Etymander) could not have differed greatly in its geographical features 300 years before Christ from its present characteristics. The Helmund of the Seistan basin then occupied a different channel to its present outlets into the Seistan swamps. How different it is difficult to tell, for it has frequently changed its course within historic times, silting up its bed and striking out a new channel for itself, splitting into a number of streams and wandering uncontrolled in loops or curves over the face of the flat alluvial plains to which it brought fertility and wealth. It has been a perpetual source of political discussion as a boundary between Afghanistan and Persia, and it has altered the face of the land so extensively and so often that there is nothing in ancient history referring to the vast extent of agricultural wealth and the immensity of its population which can be proved to be impossible, although it seems likely enough that false inferences have been drawn from the widespread area of ruined and deserted towns and villages which are still to be seen and may almost be counted. It is not only that the water-supply and facilities for irrigation, by shifting their geographical position, have carried with them the potentialities for cultivation. Other forces of Nature which seem to be set loose on Seistan with peculiar virulence and activity have also been at work. The sweeping blasts of the north-west wind, which rage through this part of Asia with a strength and persistence unknown in regions more protected by topographical features, carrying with them vast volumes of sand and surface detritus, piling up smooth slopes to the windward side of every obstruction, smoothing off the rough angles of the gaunt bones of departed buildings, and sometimes positively wearing them away by the force of attrition, play an important part in the kaleidoscopic changes of Seistan landscape. Villages that are flourishing one year may be sand-buried the next. Channels that now run free with crop-raising water may be choked in a month, and all the while the great Helmund, curving northward in its course, pours down its steady volume of silt from the highlands, carrying tons of detritus into open plains where it is spread out, sun-baked, dried, wind-blown, and swirled back again to the southward in everlasting movement. Thus it is that the evidence of hundreds of square miles of ruins is no direct evidence of an immense population at any one period.