250 EAST AFRICAN FLORA AND FAUNA part in 



a change, similar to that which is even now taking place in the 

 case of Tanganyika. Probably at one period a wide plateau 

 of gneiss and schists extended over the Victoria Nyanza basin. 

 As the surface of the plateau was at a great elevation, the 

 rainfall must have been heavy, and the hollows occupied 

 by numerous lakes, with rivers flowing from them in every 

 direction. It is well known that the direction of some of the 

 head streams of the Mississippi is occasionally reversed, and 

 they then flow northward into the Red River, and thus to 

 Hudson Bay. The same thing must have happened on this 

 central African plateau. The different rivers must have had 

 their highest sources in the same swamps ; dams would have 

 been formed across the streams by the growth of vegetation, 

 the accumulation of shoals, and the falling in of the banks. 

 Occasionally a slight earth-movement would block the outlet 

 of a lake, and cause it to open at a spot whence its waters 

 passed to another river system. Thus floods, dams, and earth- 

 movements would continually give the fish in one stream an 

 opportunity to work their way into a stream of another river 

 system. Hence all the rivers and lakes of such a plateau 

 would be tenanted by a common fauna. 



But at length came a change ; the centre of this plateau 

 subsiding, formed a basin without an outlet. The rivers 

 were therefore " beheaded " ; instead of rising in the centre of 

 the plateau, their sources were now on the outer slope. The 

 streams, which originally flowed outward from the highest part 

 of the plateau, reversed their direction and flowed inward, 

 so as to help to form a lake in the centre of the depression. 

 Later on the separation between the waters of the central area 

 and the rivers that originally rose on it was rendered more 

 effective by the formation of the two Rift Valleys, when 

 strips of land subsided on either side of the central Nyanza 

 depression. There is therefore no difficulty in understanding 

 why rivers which flow into the Atlantic and into the Indian 

 Oceans, and are now widely separated by an area of internal 

 drainage, are inhabited by the same forms of fish. 



So far Dr. Gunther's suggestion accounts for all the facts. 

 But it does not remove the real difficulty, which is, that some 

 members of this fish fauna are absent from the Lower Nile 

 and yet are present in the rivers and lakes of Palestine. 



