CHAP. XIII CHANGES IN AFRICAN RIVER SYSTEMS 249 



forms peculiar to them, such as the Llanberris Charr {Sahno 

 perisi, Gthr.) found in the two Llyns at Llanberris, the Salmo 

 colli, Gthr., of Lough Eske, and the Salmo gray i, Gthr., of Lough 

 Melvin.^ The fact, then, that the same species occur both in 

 the Upper Nile and in the Jordan, suggests that there must 

 once have been some connection between them. If the fish 

 which occurred in both rivers were eels, able to wriggle their 

 way for some distance across land, or catfish {Clarias), able to 

 live for months embedded in the dried mud on the floor of a 

 desiccated pool, the facts would have little weight. But the 

 fish in question have no such remarkable powers of vitality, 

 and are killed by a few minutes' exposure to the air, or to 

 water of a different composition from that to which they are 

 accustomed. 



Dr. Giinther suggested that this remarkable fish fauna 

 must have originated in the lakes of the central plateau of 

 Africa, and thence spread in every direction. 



That such changes in the river systems of Africa, as are 

 necessary for Dr. Gunther's theory, actually occur, is shown by 

 the controversy as to the relations of the Congo and Lake Tangan- 

 yika. When Stanley in i 87 i proved that the river Ruzizi at the 

 northern end of Tanganyika was an inlet, instead of an outlet, 

 there seemed no point left for the escape of its surplus waters. 

 Cameron accordingly in 1877 circumnavigated the lake, and 

 then found that the Lukuga had a slow current flowing west- 

 ward from the lake. Stanley returned in 1876, and found 

 no outlet at this point. Hore early in 1879, and Thomson at 

 its close, and other explorers later on, have found a powerful 

 stream flowing from the lake toward the Congo. Tanganyika, 

 therefore, is sometimes connected with the Congo, and at other 

 times it is not. 



If Tanganyika had not been visited till twenty years hence, 

 it is quite possible that its connection with the Congo might 

 never have been seen, and the presence in it of so many Congo 

 species would have been difficult to explain. 



The dispersal of the fish fauna of the rivers and lakes in 

 the Victoria Nyanza region may have been brought about by 



^ F. Day has denied the validity of these species (Fishes of Great Britain and 

 Ireland, vol. ii. (1884), pp. 112-114) ; but Gunther's faith in them is unshaken, and Day 

 admits them as distinct varieties. They are therefore certainly distinct forms. 



