INTBODTJCTICOT. xix 



An examination of the above table will show the great uniformity in the fish-fauna 

 of the whole Nile from the Bahr-el-Gebel to the Delta. The fishes of the Lower Nile 

 are very much the same as those of the tropical parts, only two species (Mormyrops 

 anguilloides and Chrysichthys rueppelli) being apparently restricted to the north of the 

 First Cataract ; there is a decrease in the richness of the fauna from South to North, 

 but, with the exception of a few types confined to the White Nile and the Bahr-el-Gebel, 

 no difference in general character, and many species first discovered in the Lower Nile 

 have been found again in the White Nile, and vice versd. Very little is known of the 

 fishes of Lakes Albert and Albert Edward and the lower part of the Victoria Nile, the 

 waters of which are carried off by the Bahr-el-Gebel, but, so far as we can judge, they 

 do not differ very considerably from those of the Nile. Lake Victoria, on the other 

 hand, has a quite different fish-fauna, hardly any of the typically Nilotic species having 

 been found in its waters. In this respect, the Victoria Nyanza stands much in the 

 same relation to the Nile as the Tanganyika to the Congo, although the former lake 

 is incomparably poorer than the latter, especially as regards generic differentiation 

 of its Cichlid fishes. It is therefore highly probable that the fishes of Lake Victoria 

 have, for a long geological period, been cut off from the Nile, and that the Falls of the 

 Victoria Nile at the present day constitute a barrier to their dispersal. Lake Tsana 

 and the upper affluents of the Blue Nile also have a fish-fauna very different from 

 that of the Blue Nile from Rosaires northwards, being mainly characterized by the 

 great variety of Cyprinids, mostly of the genus Barbus, whilst Lake Victoria derives 

 its peculiar character from the multiplicity of species of Cichlids. 



This peculiar character of the fishes of Lake Victoria is all the more remarkable from 

 the long-known fact that the Nile fish-fauna extends to the great rivers of West Africa 

 (Senegal, Gambia, Niger), through the Chad Basin, as has recently been established, 

 and also to Lake Rudolf. As shown by the above table (column 15), as many as 

 51 species out of 104 found in the Nile below 1500 feet altitude are represented in the 

 western watershed. Of the 1G species known from Lake Rudolf, 13 are found also in 

 the Nile, and are mostly the same as extend to Lake Chad and the West African 

 Rivers. 



On the other hand, the Congo system, so closely interlocking with that of the Nile 

 in the region of the Bahr-el-Ghazal, has a quite distinct fish-fauna, possessing very few 

 species (15 out of 265) in common with the Nile. 



The points of affinity between the fishes of the Nile and those of the Jordan have 

 long ago been pointed out. But these are after all restricted to identity of a few species 

 of Silurids (Clarices) and Cichlids (Tilapia, Haplochromis), and their importance is 

 outweighed by the total absence from the latter river of such characteristic African 

 families as the Polypteridse, the Mormyridse, and the Characinidse. 



Most of the genera represented in the Nile system are common to the other rivers of 

 Tropical Africa. However, Cromeria (the monotype of a remarkable family), Ichthyo- 



c 2 



