the cultural influences due to the earlier inhabitants of the lands they entered; the Arab stock itself remained high and dry, the stranded relic of more primitive conditions, though itself not absolutely free from a reacting influence. The only thing that ever has restrained the incursions of these nomadic tribes into such neighbouring lands as offer hope of plunder is the military power of those who endeavour to place a barrier for the protection of the settled community of the cultivated area, and every Arab outspread has been due, not to the pressure of hunger resulting from the desiccation of Arabia, nor to religious enthusiasm, but simply to the weakness of the power which tried to maintain a dam against them.
In the 7th century A.D. the two powers bordering on the Arab area were the Byzantine and Persian empires. Both of these were, to all appearance, flourishing and stable, but both alike were in reality greatly weakened by external and internal causes which were closely parallel in the two. Externally, both had been severely shaken by some centuries of warfare in which they had disputed the supremacy of Western Asia, and both had suffered from rear attacks by more barbarous foes. Internally, both alike had a thoroughly unsatisfactory social structure, though the details differ: in the Byzantine Empire almost the whole burden of a very heavy taxation fell upon the middle classes, the curiales, and the armies were mainly composed of foreign mercenaries, whilst in the Persian Empire a rigid caste system
