There are certain anatomical differences between the existing negroes of Asia and Oceania on the one hand and the negroes of modern Africa on the other[2]. Whether the African negro was the first human colonizer of Africa, or was preceded by more brutish or more generalized types, such as the Galley-Hill man, is not yet known to us. But from the little we possess in the way of fossil human remains and other evidence it seems probable that every region of Africa, even Algeria and Egypt, once possessed a negro population. In Mauretania (Morocco to Tripoli) these ancient negroes were partly driven out by prehistoric Caucasian invaders and partly absorbed by intermarriage, the mixture resulting in the darkened complexions of so many of the North African peoples. In Egypt a dwarfish type of negro seems to have inhabited the Nile delta some 10,000 years ago; and big black negroes formed the population of upper Nubia and Dongola so late as about 4000 years ago. Yet there are reasons for thinking that not all parts of Tropical Africa were colonized by negroes, or rather by the typical big black negro, until 2000 or 3000 years ago. Although the fringe of the Congo basin, for example, has been inhabited for a very considerable period (as is testified by the presence of stone implements somewhat deeply buried in the soil), the central part of that area would seem to have been invaded quite recently by man; while in South Africa beyond the Zambezi there have been periods in which the only human type was the Bushman, rather than the big black negro. The comparatively recent human colonization of the forests of the southern Cameroons and the inner Congo basin may have been due to the density of tree growth and the opposition of the gorilla; and (in Congoland) to the swamps and the presence of large shallow lakes now dried up into river-courses. Several French and German pioneers have described to the writer of this book the way in which, when attempting to explore the forests of South Cameroons, far back from the coast, their caravans of negro porters were attacked by the gorillas; and the utterly uninhabited character of considerable areas along the Congo-Cameroons water-parting is said to be due to the terror inspired in the native mind by these enormous, fierce, and resolute creatures. The same fact may have hindered at one time the populating of similar forest countries between the Mubangi and the main Congo. South of the main Congo there are no gorillas; but a good deal of this central Congo region has been under water until quite recent times, and even now its inhabitants are often compelled to live in pile dwellings raised above flood level.