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 of the 

 molluscan section of the fresh-water fauna of the island of 

 Celebes, it will be seen that the fresh-water mollusca of these 

 two widely separated districts present, in the first place, a 



