THE TANGANYIKA PROBLEM. 141 
duction of specific varieties, and this rather monotonous 
sameness in the nature and composition of the fresh-water 
faunas of the great lakes of Central Africa has now been 
definitely shown to hold good as a rule, characteristic of them 
all with one solitary and conspicuous exception. The fauna 
of Lake Tanganyika alone contains forms, and many of 
them, which have not the characteristics of any usual fresh- 
water types, but, although this fauna thus differs from 
that of all the other great African lakes, it is after all 
only a partial exception to the rule of uniformity in type 
which characterises the fauna of the great African lakes in 
general. It was seen that Tanganyika contains all the 
genera of fish and invertebrates which are found in Lake 
Nyassa, and the character of the normal fresh- water con- 
stituents of the fauna of Lake Tanganyika differs no more 
from the fresh- water types contained in the Victoria Nyanza 
or Nyassa than the constituents of the faunas of these two 
latter lakes differ from one another. Tanganyika is ren- 
dered peculiar, not by the general characters of its fresh- 
water fauna, but simply by the additional possession of a 
number of forms which are peculiar to that lake. The animals 
forming the invertebrate section of this peculiar group 
have an obviously marine aspect, and on that account I 
have spoken of them elsewhere* as forming a halolimnic 
series in Lake Tanganyika — that is to say, they form 
a group of animals which, although living in a fresh-water 
lake, have at the same time the characters of animals that 
are typical of the sea. The Tanganyika animals which 
possess par excellence these characteristics, are the endemic 
gastropods, the gymnolaematus polyzoa, and the jelly-fishes. 
But besides these forms there are other invertebrates 
which, although not so markedly marine in character, 
* Proceedings of the Royal Society (he. cit.). 
