,8,, GREAT LAKES. 123 



It will, perhaps, be asked how it is that, notwithstanding our 

 confessed ignorance of the African lakes and their inhabitants, we 

 venture to express any opinion as to their former connection with 

 the sea. Though not much is yet known as to the fauna of these 

 lakes, we are not entirely without information. From the slightly 

 brackish Lake Tanganyika, Mr. Edgar A. Smith has recorded thirty- 

 two species of moUusca, nineteen of which are peculiar to that lake. 

 Among them are two species of a remarkable genus closely allied to 

 the marine Troclms^ and one or two other shells seem also to suggest 

 marine rather than freshwater ancestors. Lake Tanganyika is now 

 2,700 feet above the sea-level, but the occurrence in its waters of 

 modified marine mollusca points to the probable elevation of this 

 part of Africa to at least that extent. Till their fauna is better 

 known, we cannot speak confidently as to the origin of the Central 

 African lakes. 



The decision of the question How did the African lakes originate ? 

 ought to help us to understand various peculiar problems in geogra- 

 phical distribution presented by that continent. Lake Tanganyika, 

 though nearer to the east than to the west coast, overflows into the 

 Congo ; the Victoria Nyanza and the associated lakes are connected 

 with the Nile ; Lake Nyassa's outlet is towards the Indian Ocean. 

 Thus the African lakes belong to three distinct basins, each probably 

 possessing a peculiar fauna. Most of the great lakes are so near to 

 the watershed that if the higher ones were connected with the sea 

 there might exist an equatorial Mediterranean entirely cutting off 

 South Africa and Madagascar from the rest of the continent. Much 

 has still to be learnt about Africa, and the African lakes evidently 

 contain the answer to many riddles. 



REFERENCES. 



1. SoIIas, "W. J.^On the Origin of Freshwater Faunas. Trans. R. Dublin Soc, 



ser. iii., vol. iii., pp. 87-118, 1884. 



2. Credner, Rudolf. — Die Reliktenseen. Peteriuann's Mittheilungen, vol. xix., Nos. 



86 and 89. 1887-1888. 



3. Gilbert, G. K. — Lake Bonneville. Monographs of the United States Geological 



Survey, vol. i., 1890. 



4. Russell, I. C. — Geological History of Lake Lahontan. Ibid., vol. xi., 1885. 



5. Call, R. EI. — On the Quaternary and Recent Mollusca of the Great Basin. 



Bulletins of the U.S. Geological Survey, No. 11, 1884. 



6. Thomson, J. — To the Central African Lakes and back. 2 vols. 8vo. 1881. 



Clement Reid. 



