ary Arabs drawn into the movement by religious enthusiasm.
The environment of Spain tended to conserve the knowledge of
agriculture, industry, architecture, and science which they brought in
and which might have cemented Spaniard and Moor, had it not been for the
intense religious antagonism existing between the two races.

The history of nomad conquerors shows that they become weakened by the
enervating climate and the effeminating luxury of the moist and fertile
lowlands. They lose eventually their warlike spirit, like the Fellatah
or Fulbe founders of the Sudanese states,[1100] and are either displaced
from their insecure thrones by other conquerors sprung from the same
nomad-breeding steppe, as the Aryan princes of India by the Mongol
Emperor, and the Saracen invaders of Mesopotamia by the victorious
Turkomans; or they are expelled in time by their conquered subjects, as
the Tartars were from Russia, the Moors from Spain, and the Turks from
the Danube Valley.

[Sidenote: Centralization versus decentralization in nomadism.]

Nomad hordes unite for concerted action to resist encroachment upon
their pastures, or for marauding expeditions, or for widespread
conquest; but such unions are from their nature temporary, though a
career of conquest may be sustained for decades. The geographically
determined mobility which facilitates such concentration favors also
dispersal, decentralization. This is the paradox in nomadism. Geographic
conditions in arid lands necessitate sparse distribution of population
and of herds. Pastoral life requires large spaces and small social
groups. When Abraham and Lot went to Canaan from Egypt, "the land was
not able to bear them that they might dwell together, for their
substance was great." Strife for the pasturage ensued between their
respective herdsmen, so the two sheiks separated, Lot taking the plains
of Jordan and Abraham the hill pastures of Hebron. Jacob and Esau
separated for the same reason. The encampment of the Kirghis shepherds
rarely averages over five or six tents, except on the best grazing
grounds at the best season of the year. The flow of spring, well or
stream also helps to regulate their size. The groups of Mongol yurts or
felt tents along the piedmont margin of the Gobi vary from four tents to
a large encampment, according to water and grass.[1101] Prevalsky mentions
a population of 70 families or 300 souls in the Lob Nor district
distributed in 11 villages, or less th