482 Evolution and Distribution of Fishes 



genera again in Victoria, are largely different from those 

 of the White Nile below. The Siluridae have specially 

 multiplied and varied in the White Nile. The Characin- 

 idae have passed down into the main Nile river, and on to 

 near its mouth. The Congo shows a more recently derived 

 and immigrant group of species, whose ancestral types en- 

 tered it from several sources. 



From the eastern or Asiatic side, and probably along 

 the Mesopotamia-Syria line, migrant species of the Cyprin- 

 idae entered, after working across Asia from their primitive 

 N. American home. But that the western and the eastern 

 streams of migrants only slightly if at all intermixed, is 

 graphically illustrated by the fish-fauna of Lake Tsana in 

 Abyssinia. High though it now is — about 6000 feet — it 

 received such a stream of incoming cyprinids from the east, 

 that 17 species are now known from it, while the affluent 

 Blue Nile has 12. In marked contrast not a single western 

 Characinid has reached its waters. And though 29 species 

 of Silurids occur in the White Nile, only two now reach 

 eastward to Tsana. 



Lake Tanganyika therefore, and probably also Nyasa, 

 are evidently old freshwater lakes, that in their earliest 

 origin may date back beyond the Cretaceous to the Jurassic 

 or even a more remote epoch, though a late Cretaceous date 

 seems ample. But the fishes that so distinguish the former 

 now, seem clearly to be — except for such puzzling groups 

 as Mormyridae and Mastacembelidae — S. American fresh- 

 water derivatives, that usually show common generic an- 

 cestral types with others in the lakes, swamps, and rivers 

 of that continent. But owing to isolation — probably from 

 the Pliocene onward — and to action of varied and com- 

 plexly interacting environal factors not as yet sufficiently 

 estimated, proenvironal variations have proceeded apace, 

 and have resulted in the evolution of numerous species, 

 that are largely peculiar to the two lakes. Lake Kivu 

 we have regarded as originally the upper end of Tangan- 

 yika, and so it had a fish-fauna that was largely the same. 

 But during upheaval of it and the surrounding country, 

 most of the species were obliterated, though at least two 

 species are now known to be common to the two lakes. 



