This conclusion may be corroborated by other facts. In the middle ages the civilization of the Arabs had reached a stage which was undoubtedly superior to that of many European nations of that period. Both civilizations had sprung largely from the same sources, and must be considered branches of one tree. The Arabs who were the carriers of civilization were by no means members of the same race as the Europeans, but nobody will dispute their high merits. It is of interest to see in what manner they influenced the negro races of the Soudan. At an early time, principally between the second half of the eighth century and the eleventh century of our era, the Soudan was invaded by Hamitic tribes, and Mohammedanism was spreading rapidly through the Sahara and the western Soudan. We see that since that time large empires have been formed, and have disappeared again in struggles with neighboring states, and that a relatively high degree of culture has been attained. The invaders intermarried with the natives; and the mixed races, some of which are almost purely negro, have risen high above the level of other African negroes. The history of Bornu is perhaps one of the best examples of this kind. Barth and Nachtigal have made us acquainted with the history of this state, which has played a most important part in the history of North Africa. Why, then, have the Mohammedans been able to civilize these tribes, and to raise them to nearly the same standard which they had attained, while the whites have not been capable of influencing the negro in Africa to any considerable extent? Evidently on account of the different method of introduction of culture. While the Mohammedans influence the people in the same manner in which the ancients civilized the tribes of Europe, the whites send only the products of their manufactures and a few of their representatives into the negro country. A real amalgamation between the more highly educated whites and the negroes has never taken place. The amalgamation of the negroes by the Mohammedans is facilitated particularly by the institution of polygamy, the conquerors taking native wives, and raising their children as members of their own family. The spread of the Chinese civilization in eastern Asia may be likened to that of the ancient civilization in Europe. Colonization and amalgamation of kindred tribes, and in some cases extermination of rebellious subjects, with subsequent colonization, have led to a remarkable uniformity of culture over a large area.