470 Evolution and Distribution of Fishes 



a region from Senegal to the Upper Nile at least, and south 

 to southern Congo, Nyasa, and the Zambesi. But even 

 more arresting are the varied elevations at which one and 

 the same species may be found. This has caused all writers 

 on African biochorology to accept — what the rocks abund- 

 antly demonstrate — that great changes have occurred in 

 surface configuration during Tertiary time. These changes 

 might be due to denudation alone, or to this along with 

 elevation, or conversely depression of land, or to all of these 

 combined. For the finding of what are, and long have been, 

 freshwater fishes of the same species, from near sea level 

 up to almost 5000 feet as in Lake Kivu, requires appropri- 

 ate explanation. To this we can return later. 



Another important feature is that the fishes which in- 

 habit the above area, are practically without exception 

 freshwater, alike in their past history as already traced, in 

 their present affinities, and in their relation to fishes of 

 other regions of the world. Thus the dipnoan, the mormy- 

 rid, the cyprinid, the characinid, the silurid, the cyprino- 

 dont, the cichlid, and the mastacembelid families, are all 

 continental in origin and distribution. 



Still another point of importance, often emphasized 

 by past writers, is that while a relatively small number of 

 common or nearly related species, inhabits the entire area, 

 each special lake or drainage region is characterized by a 

 group of endemic species. With entire show of reason, 

 these endemic species have been regarded as types that 

 have slowly evolved, after the localized regions had be- 

 come separated from wider areas with which they were 

 formerly connected. Of such localized regions Lake Tan- 

 ganyika is undoubtedly the most striking example. For of 

 the Cichlidae alone this lake has 75 species, or almost one- 

 third of the 215 found in Africa. Lakes Victoria, Rudolf, 

 and Nyasa show like conditions. 



One must be cautious however in concluding that ab- 

 sence of any species from a definite locality, so far as pre- 

 sent knowledge goes, is proof that such never existed there. 

 Lake Tanganyika is a case in point. For while it was once 

 supposed that only one species was common to it and lakes 

 beyond it, we now know that at least 7 species are common. 



