THE TANGANYIKA PROBLEM. 
13 
colonising the fresh-waters of the land through the medium 
of the rivers which flow from them into the ocean. Semper 
has dwelt upon the hardness of the conditions of varying 
temperature, etc., which unquestionably obtain in rivers 
and lakes ; while Sollas has pointed out the equally unques- 
tionable fact that a very large number of marine forms 
are precluded from making any attempt to colonise the rivers 
by the fact that they begin their existence as free-swimming 
larvae, and that it is physically impossible for such larvae 
either to force themselves up a stream, or to maintain them- 
selves under the conditions which would surround them in 
a river. As a matter of fact, when the old question of the 
nature and origin of fresh-water faunas is re-examined in 
the newer light which these and still more recent investiga- 
tions can be made to throw upon the subject, it becomes 
more and more apparent that none of the existing concep- 
tions which have been entertained are fully capable of 
explaining the origin of such faunas, since it can be shown 
that no existing explanation of the peculiar composition of 
these faunas can be brought into accordance with our 
knowledge of the facts. The reasons for this statement will 
begin to appear if we consider the composition of the 
invertebrate section of the fauna in any two widely sepa- 
rated land masses. The fauna of a Central African lake, 
for example, compared with, let us say, the fauna found 
in the fresh-waters of the island of Celebes. To make 
matters still more simple we will consider only the 
mollusca of these two areas we have selected. If then 
we arrange the molluscan constituents of the Victoria 
Nyanza in a tabular form as on page 14 beside those ot the 
molluscan section of the fresh-water fauna of the island ol 
Celebes, it will be seen that the fresh-water mollusca of these 
two widely separated districts present, in the first place, a 
