The Tanganyika Problem Reviewed 477 



single species having made its way up the Russisi river into 

 Lake Kivu." Of the remaining four species he records 

 T. nilotica from both lakes, and it has a much wider range 

 as before noted. The remaining three are now also known 

 from Tanganyika. 



So as some succeeding authors have indicated the history 

 evidently is that Kivu and Tanganyika once formed parts 

 of a continuous lake — and this at no very remote past date, 

 geologically speaking. By subsequent upheaval however of 

 the north end, and possibly when the Mfumbiro mountains 

 were ejected as volcanic masses, many of the 120 species 

 of fish found in Tanganyika were destroyed. But two at 

 least survived the upheaval strains, and remain identical 

 with those of the larger and lower lake. So instead of 

 "having made its way up the Russisi river into Lake Kivu" 

 against the wild cataract torrents that Moore graphically 

 describes, we would accept it that two species of the group 

 survived elevation with the lake and the surrounding land, 

 till fully 2000 feet of altitudinal difference was established 

 between. The invertebrate organisms confirm the above 

 conclusions. 



To account then for the rich and varied freshwater 

 fauna of Tanganyika, it can be said that so far as the 

 invertebrates are a criterion the lake may have existed as 

 such from Jurassic time onward. For all of the inverte- 

 brates — even the crustaceans — had by then evolved to the 

 dignity and structural complexity of the present lake in- 

 habitants. If then Tanganyika had, by crustal earth- 

 movements, become of considerable depth as a "rift" lake 

 by late Cretaceous time, it would the more securely shelter 

 the organisms that drifted or floated into it. 



But the fish fauna raises very important considerations. 

 And its exceptional richness, beyond that of the invertebrate 

 groups, gives added importance to the question. The two 

 species of Tanganyika, Polypterus congicus and Protopter- 

 lis aethiopicus — may well represent the oldest and earliest 

 surviving fishes in these lakes. Boulenger's view also 

 (227: 23) that the former is intermediate in structure be- 

 tween the Senegambian P. lapradii and P. endlicheri of the 

 Upper Nile is suggestive; while its presence along the 



