THE TANGANYIKA PROBLEM. 
145 
is everywhere found to people the greater and the lesser 
Central African lakes. 
In the preceding examination of the components of the in- 
dividual faunas of the different African lakes, it will have 
been observed that if we exclude the obviously migrating 
vertebrates, the fishes, the amphibia, and the like, the remain- 
ing invertebrate constituents of these faunas are almost always 
in a specific sense, different in each of the lakes. There are, 
in fact, only one or two specific forms, such as Melania 
tuberculata , which occur generally in all the lakes — that is 
to say, we have in each depression a fauna which, in the 
species composing it, differs from the fauna in any of the 
other depressions. It is obvious from this fact that even 
when lakes are within twenty miles of each other, as in the 
case of Tanganyika and Kela, the invertebrate faunas of 
two such depressions do not readily, at any rate, communi- 
cate with one another. There is, in fact, very little indica- 
tion, if any, that the invertebrate faunas of the lakes 
intercommunicate or inter-colonise at all. This is parti- 
cularly well seen in Lake Kivu. Kivu is a great lake and 
must have been in much the same condition that it is now, 
at any rate, for centuries, yet there is not a Vivipara to be 
found in it, although they swarm in the lakes a hundred 
miles to the north and to the south. Kivu is, in fact, in 
direct water connection with Tanganyika, but so far as is at 
present known, only one small fish, Tilapia burtoni , has 
ever migrated from the one lake to the other. From these 
facts and similar ones which we encountered again and 
again, and which have been further verified by the observa- 
tions of numbers of other travellers who have been among 
the 'Central African lakes, it would seem that the inverte- 
brate components of the fauna of the different African fresh 
waters do not tend to migrate at all, while the fishes of 
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