Africa's smooth outline, its plateau surface rimmed with mountains which enclose but fail to divide, and its monotonous configuration have produced a racial and cultural uniformity as striking as Europe's heterogeneity. Constant movements and commixture, migration and conquest, have been the history of the black races, varied by victorious incursions of the Hamitic and Semitic whites from the north, which, however, have resulted in the amalgamation of the two races after conquest.[225] Constant fusion has leveled also the social and political relations of the people to one type; it has eliminated primordial groups, except where the dwarf hunters have taken refuge in the equatorial forests and the Bushmen in the southwestern deserts, just as it has thwarted the development of higher social groups by failure to segregate and protect. It has sown the Bantu speech broadcast over the immense area of Central Africa, and is disseminating the Hausa language through the agency of a highly mixed commercial folk over a wide tract of the western Sudan. The long east-and-west stretch of the Sudan grasslands presents an unobstructed zone between the thousand-mile belt of desert to the north and the dense equatorial forests to the south, between hunger and thirst on one side, heat and fever and impenetrable forests on the other. Hence the Sudan in all history has been the crowded Broadway of Africa. Here pass commercial caravans, hybrid merchant tribes like the Hausa, throngs of pilgrims, streams of peoples, herds of cattle moving to busy markets, rude incursive shoppers or looters from the desert, coming to buy or rob or rule in this highway belt. [See map page 105.] [Sidenote: Differentiation versus assimilation.] Historical development advances by means of differentiation and assimilation. A change of environment stimulates variation. Primitive culture is loath to change; its inertia is deep-seated. Only a sharp prod will start it moving or accelerate its speed; such a prod is found in new geographic conditions or new social contacts. Divergence in a segregated spot may be overdone. Progress crawls among a people too long isolated, though incipient civilization thrives for a time in seclusion. But in general, accessibility, exposure to some measure of ethnic amalgamation and social contact is essential to sustained progress.[226] As the world has become more closely populated and means of communication have improved, geographical segregation is increasingly rare. The earth has lost its "corners." All parts are being drawn into the circle of intercourse. Therefore differentiation, the first effect of the historical movement, abates; the second effect, assimilation, takes the lead. [Sidenote: Elimination by historical movement.]