THE ANIMALS OF LAKES AND RIVERS 203 
The relatively unfavourable conditions which exist in 
rivers made it necessary to develop accessory breathing- 
ergans, 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 
still, 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 peculiar 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 those 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 
