THE ANIMALS OF LAKES AND RIVERS 203 



The relatively unfavourable conditions which exist in 

 rivers made it necessary to develop accessory breathing- 

 organs, and these apparently led the way to the acquisi- 

 tion of purely terrestrial characters, in the allies of the 

 ancestral Polypterus. 



Another peculiarity of the fish fauna of Tanganyika 

 is the enormous number of fish belonging to the family 

 Cichlidae. The lake is characterized by its very large 

 number of fish, nearly a hundred different species 

 having been described. Of these, more than half 

 belong to the family Cichlidae, and, more remarkable 

 stiU, of the fifty-eight species of this family described 

 in the lake only one is known outside the lake. We have 

 already noted, in speaking of the Scottish lakes, that 

 it is not unusual to find pecuUar varieties or species of 

 fish in lakes, but the fact that many peculiar genera 

 occur in Tanganyika speaks to long isolation. 



The other fish of the lake are for the most part 

 similar to thosfe which occur in the remaining African 

 lakes. For instance, there are a considerable number 

 of cat-fish, very usual inhabitants of equatorial lakes. 

 As we shall see directly the invertebrates show the same 

 general peculiarities as the fish — that is, we find a com- 

 bination of forms found in other African lakes, and of 

 quite peculiar forms of primitive type, the result being 

 to give the lake an unusually large fauna. Moore 

 believes that this shows that the lake had once a direct 

 connexion with the sea to the west, through what is 

 now the Congo basin. He places the rupture of this 

 connexion so long ago as the period called by geologists 

 the Jurassic, and believes that at the time of the 

 rupture the lake was peopled by Jurassic forms. These 

 in course of time evolved into the peculiar types 

 now found in the lake, which retain many archaic 



