The abundant fertility of river flood-plains inevitably attracts population and necessitates some kind of artificial protection against inundation.  The most primitive form of this protection is obvious and widespread, restricted in neither locality nor race.  When the flood season converts the flat plain of the White Nile below Gondokora (7 deg.  N. Lat.) into an extensive marsh, countless hills of the white ant emerge over the waters.  During the dry season, the ants build up their hills to about ten feet, and then live in safety in the upper section during the flood.  They greatly surpass in intelligence and constructive ability the human occupants of the valley, the low and wretched Kytch tribe of the Dinka Negroes, who like the ants are attracted by the natural vegetation of the flood-plain, and who use the ant-hills as refuge stations for themselves and their cattle during the flood.[602] Elsewhere in Africa the natives are more intelligent, for flood-plain villages built on artificial mounds have existed from the earliest times.  Diodorus Siculus tells us that those of ancient Egypt, when the Nile was high, looked like the Cyclades Islands.[603] Similar ones are constructed by the Barotse tribe on the upper Zambesi.[604] The Niger River, rising in the Foota Jallon and Kong Mountains which form a region of heavy rainfall from February to July, inundates a plain of several thousand square miles for a distance of 250 miles above Timbuctoo.  Here again the villages of the agricultural Song-hoi duplicate those of Egypt, built on the same clay mounds, wreathed in the same feathery palms, and communicating with one another only by small boats.[605] The same picture is presented by the Yangtze Kiang plain during the summer overflow—­low artificial hills rising from the expanse of muddy water and topped with trees and villages, while sampans moored to their base show the means of communication.[606] In the broad flood-plain of the lower Mississippi River, the chronicles of the De Soto expedition state that the Indian villages visited stood “on mounds made by art.”  The Yazoo River Indians, at the commencement of the eighteenth century, had their cabins dispersed over the low deltaic land on earthen mounds made by their own hands.  There is also strong evidence that some of the works of the Mound-builders in the “bottoms” of the middle and lower Mississippi served as protected sites for the dwellings of their chiefs.[607]

[Sidenote:  Diking of rivers.]

Such meager provisions against inundation suffice for the sparse population characterizing the lower stages of civilization, but they must be supplemented for the increasing density of higher stages by the embankment of the stream, to protect also the adjacent fields.  Hence the process of confining rivers within dikes goes back into gray antiquity.  Those of the Po and its tributaries were begun before the political history of the Lombardy plains commenced.  Strabo mentions the canals and dikes of Venetia, whereby