Arabia, alone among the countries accessible to the ambition of the powerful sovereigns of antiquity, escaped the humiliation of conquest. The genius of Alexander had planned its subjugation, but death prevented the realization of his vast, perhaps impracticable, design. The legions of Augustus, trained under the discipline of the greatest of the Cæsars, proved unequal to the task of triumphing over a region where the soil, the elements, and the valor of its defenders formed a combination invincible by human prowess. The Persians, for a period of insignificant duration, occupied the western and southern coasts, having previously expelled the Abyssinians, who had invaded and retained a portion of Yemen during the sixth century. No nation, however, was ever able to claim supremacy over any considerable portion of the Arabian Peninsula. For this immunity it was indebted not only to the natural obstacles which defied the advance and the maintenance of an invading army, but also to the superstitious fears with which cunning and credulity had surrounded its name. It was a land of mysterious portents and prodigies, whose borders were guarded by malignant demons; whose deserts, all but impenetrable to the boldest adventurer, were inhabited by cannibal giants and monstrous birds of prey that watched over treasures placed by evil spirits under the spell of enchantment. Every caravan that left Phœnicia for Central Arabia carried quantities of storax, which the Tyrian merchants declared was burnt in the neighborhood of the frankincense shrubs, that its offensive fumes might drive away the winged serpents which were their custodians. The climate was said to be so pestilential that slaves and criminals alone were employed to gather the precious gum, their liberty being conditional upon their success. These politic inventions, implicitly believed by the ignorant, while they insured to the shrewd traders of Phœnicia a monopoly of the valuable products of the Peninsula, exercised no inconsiderable influence over the popular mind of the ancients, and clothed the Desert with terrors which even the reputation and allurements of its prodigious wealth were unable entirely to overcome.