146 THE TANGANYIKA PROBLEM. 



these same fresh-water areas do not migrate with anything 

 like the vigour that one might have been naturally led to 

 suppose. 



There is, however, another characteristic of the faunas of 

 the different great and small African lakes which is certainly 

 interesting, and may very possibly be of wide theoretical 

 importance. If we examine the lists of species consti- 

 tuting the faunas of these lakes, we find that for some 

 reason or another the number of different species present in 

 any particular lake is directly proportional to its size. The 

 fauna of Nyassa consists of about 75 species. The ordinary 

 fresh-water fauna of Tanganyika of 80 species. The fauna 

 of the Victoria Nyanza, although not completely known, 

 contains about 60 species. The fauna of the Albert Nyanza, 

 which is much less than either of the preceding lakes, 

 contains only 20 forms. The Albert Edward, about the 

 same number ; Kivu only 10 species ; while the really small 

 lakes, Kela, Elimantita, Beringo, Nivasha, Nakaroo, etc., 

 contain only about half-a-dozen species each. It is thus 

 obvious that from some cause or other the number of 

 specific forms in an African lake is roughly proportional to 

 the size of the lake itself. 



The physiographical features of the African lakes are 

 unique ; nowhere else in the world have we numerous sheets 

 of fresh-water of all sizes scattered over an area bigger than 

 Australia, and yet all of which are under practically the 

 same climatic conditions. In this way the faunistic pheno- 

 mena presented by the African lakes are distinctly simpli- 

 fied ; we have not to reckon with the wide climatic differ- 

 ences that are forced upon the faunas of the lakes which 

 occur in the latitudes of North America, Europe and Asia, 

 and consequently we cannot legitimately invoke difference of 

 conditions to account for the specific richness of Nyassa as 



