The Great Arabian Peninsula, which lay due south of the most central part of the Empire, and bordered it on this side for about thirteen degrees, or (if we follow the line of the boundary) for above a thousand miles, might seem to have been the most important of all the adjacent countries, since it contains an area of a million of square miles, and is a nursery of brave and hardy races. Politically, however, Arabia is weak, as has been shown in a former volume; while geographically she presents to the north her most arid and untraversable regions, so that it is rarely, and only under very exceptional circumstances, that she menaces seriously her northern neighbors. Persia seems never to have experienced any alarm of an Arab invasion; her relations with the tribes that came into closest contact with her were friendly; and she left the bulk of the nation in unmolested enjoyment of their independence. Another country adjoining the Persian Empire on the south, and one which might have been expected to cause some trouble, was Ethiopia. To Egypt Ethiopia had always proved an unquiet, and sometimes even a dangerous, neighbor; she was fertile, rich, populous; her inhabitants were tall, strong, and brave; she had a ready means of marching into Egypt down the fertile valley of the Nile; and her hosts had frequently ravaged, and even held for considerable terms of years, that easily subjected country. It is remarkable that during the whole time of the Persian dominion Ethiopia seems to have abstained from any invasion of the Egyptian territory. Apparently, she feared to provoke the power which had seated itself on the throne of the Pharaohs, and preferred the quiet enjoyment of her own wealth and resources to the doubtful issues of a combat with the mistress of Asia.