alley of the
Nile and Euphrates.[1066] The Arabs of the northern Sahara, followed by
small flocks of sheep and goats, vibrate between the summer pastures on
the slopes of the Atlas Mountains and the scant, wiry grass tufts found
in winter on the borders of the desert.[1067] When the equatorial rains
begin in June, the Arabs of the Atbara River follow them north-westward
into the Nubian desert, and let their camel herds graze on the delicate
grass which the moisture has conjured up from the sandy soil. The
country about Cassala, which is flooded during the monsoon rains by the
rivers from the Abyssinian Mountains, is reserved for the dry
season.[1068] In the same way the Tartar tribes of the Dnieper, Don, Volga
and Ural Rivers in the thirteenth century moved down these rivers in
winter to the sea coast, and in summer up-stream to the hills and
mountains.[1069] So for the past hundred years the Boers of the South
African grasslands have migrated in their tent wagons from the higher to
the lower pastures, according to the season of the year, invading even
the Karroo Desert after the short summer rains.[1070]

[Sidenote: Marauding expeditions.]

This systematic movement of nomads within their accepted boundaries
leads, on slight provocation, to excursions beyond their own frontiers
into neighboring territories. The growing herd alone necessitates the
absorption of more land, more water-holes, because the grazed pastures
renew their grass slowly under the prevailing conditions of drought. An
area sufficient for the support of the tribe is inadequate for the
sustenance of the herd, whose increase is a perennial expansive force.
Soon the pastures become filled with the feeding flocks, and then
herdsmen and herds spill over into other fields. Often a season of
unusual drought, reducing the existing herbage which is scarcely
adequate at best, gives rise to those irregular, temporary expansions
which enlarge the geographical horizon of the horde, and eventuate in
widespread conquest. Such incursions, like the seasonal movements of
nomads, result from the helpless dependence of shepherd tribes upon
variations of rainfall.

The nomad's basis of life is at best precarious. He and want are
familiar friends. A pest among his herds, diminished pasturage, failing
wells, all bring him face to face with famine, and drive him to robbery
and pillage.[1071] Marauding tendencies are ingrained in all dwellers of
the deserts and steppes.[107