Then came one of the great landmarks of the world’s history, a movement productive of a little good and some harm to civilisation. Christianity had first been organised as a socialistic religion, grafting on to the beautiful and indisputable precepts of its Founder the reaction of poor, ignorant, starved, and enslaved people against the unmoral philosophy, unequal wealth, and excessive materialism of the time. It then grew to be a somewhat dismal faith, taking no heed of the beauty of this world and of mundane opportunities for happiness; and above all it waged an active warfare with sexuality, not merely curbing immorality, but (wisely or unwisely) opposing polygamy and advocating celibacy. The Arab and the North African were not ripe for such a faith, and Judaism had already biassed them against the polytheistic tendencies of Greek Christianity. Muhammad, the prophet of western Arabia, founded on a basis of phallic worship and animistic belief the third great Semitic religion--Islam. His teaching was a direct challenge to Christianity, and soon became iconoclastic in every sense of the word. Though the Persian, Syrian, and Iberian elements of the Arab Empire for a time revived and perpetuated in somewhat grotesque aspect the science of Greece, the art of Persia, and the lore of India, the Muhammadan religion sealed most parts of Africa to European and Christian research. It is true, however, that the conquests of Islam enabled Arabs to penetrate further into the interior of tropical Africa than before, though from the dawn of civilisation they had been the most constant explorers of the eastern part of that continent. The Arabs began to mention names connected with the Niger and the western Sudan to the geographers of Italy and Sicily. Under the impulse of Islamic, Persian, and Arab colonies the east coast of Africa and the north coast of Madagascar came partially under Muhammadan rule in the tenth and eleventh centuries, the Arabs there carrying on almost continuously the commercial enterprise founded by their predecessors and brothers, the Phœnicians and Sabæans. Invasion after invasion crossed from Arabia, and passed over the lowlands surrounding Abyssinia to the central Sudan; or higher up, through Egypt, to Mauritania. But the Arabs who crossed the Nile in the latitudes of Khartum and Assuan made no attempts to follow the White Nile, the Blue Nile, or the Bahr-al-Ghazal to their sources,--left in fact all the Nile basin above the confluence of the Blue and White rivers absolutely untouched and unexplored. Egypt itself came under Arab rule in 640 A.D., and subsequently formed an independent principality under the Fatimite Khalifs.