All these political delimitations were naturally accompanied by explorations, partly scientific, but mainly political. Major Serpa Pinto twice crossed Africa in an attempt to connect the Portuguese settlements on the two coasts. Similarly, Lieutenant Wissmann also crossed Africa twice, between 1881 and 1887, in the interests of the Congo State, though he ultimately became an official of his native country, Germany. Captain Lugard had investigated the region between the three Lakes Nyanza, and secured it for Great Britain. In South Africa British claims were successfully and successively advanced to Bechuana-land, Mashona-land, and Matabele-land, and, under the leadership of Mr. Cecil Rhodes, a railway and telegraph were rapidly pushed forward towards the north. Owing to the enterprise of Mr. (now Sir H. H.) Johnstone, the British possessions were in 1891 pushed up as far as Nyassa-land. By that date, as we have seen, various treaties with Germany and Portugal had definitely fixed the contour lines of the different possessions of the three countries in South Africa. By 1891 the interior of Africa, which had up to 1880 been practically a blank, could be mapped out almost with as much accuracy as, at any rate, South America. Europe had taken possession of Africa. One of the chief results of this, and formally one of its main motives, was the abolition of the slave trade. North Africa has been Mohammedan since the eighth century, and Islam has always recognised slavery, consequently the Arabs of the north have continued to make raids upon the negroes of Central Africa, to supply the Mohammedan countries of West Asia and North Africa with slaves. The Mahdist rebellion was in part at least a reaction against the abolition of slavery by Egypt, and the interest of the next few years will consist in the last stand of the slave merchants in the Soudan, in Darfur, and in Wadai, east of Lake Chad, where the only powerful independent Mohammedan Sultanate still exists. England is closely pressing upon the revolted provinces, along the upper course of the Nile; while France is attempting, by expeditions from the French Congo and through Abyssinia, to take possession of the Upper Nile before England conquers it. The race for the Upper Nile is at present one of the sources of danger of European war.