THE TANGANYIKA PROBLEM. 19 



of water which occur on a great continental land mass, 

 and throughout all the great continental land masses of 

 the world. If we examine the great fresh-water basins 



o 



and river systems of Africa, for example, the first fact 

 which confronts us is the localisation of groups of varieties 

 of the universal types. Thus, confining our attention, 

 merely for the sake of simplicity, to the molluscs of Lake 

 Nyassa, we find only one species — namely, Melania tuber- 

 culata — existing in this lake which occurs in any of the 

 other great lake basins.* In Tanganyika, excluding 

 its peculiar halolimnic group, all the species of molluscs 

 in it are peculiar to the district : they do not even occur 

 in the adjacent lakes of Mwero, Rukwa, or Bangweolo, 

 such types being either unrepresented in these lakes or 

 represented by different species. So again in Kivu there 

 is not even a Vivipara, and the only series of great 

 African lakes that possesses a large number of identical 

 species is that constituted by Albert Edward, the Albert, 

 and the Victoria Nyanzas, which communicate with the 

 Nile now, and were almost unquestionably once more or less 

 connected together. These facts — and there are other 

 similar ones connected with the distribution of species in 

 the lakes and river systems of North America — seem to 

 show in an unmistakable manner that many typical fresh- 

 water forms have little if any capacity to migrate from 

 their original centres of distribution, and, consequently, 

 that the peculiar specific varieties which they present in 

 these centres are due either to their original character or 

 to natural selection, having operated since their origin, in a 

 generally similar manner, in all these particular spots. In 

 like manner, the distribution of the Characinid fishes in the 

 American and African fresh waters is quite inexplicable 



* Shirwa is a lake in the same basin as Nyassa, and was at one time connected with it. 



2* 



