1 8 THE TANGANYIKA PROBLEM. 



which are now found all over the world, it would, from 

 these considerations, appear possible that we shall have 

 to reckon with, and to define, some factor which has not 

 hitherto been recognised, and that this is so will, I think, 

 become quite clear if we examine the conception of the fresh- 

 water fauna problem which is detailed in the luminous 

 paper by Professor Sollas to which I have already referred. 

 We find that, after pointing out the unquestionable obstacles 

 which stand in the way of any attempt on the part of many 

 marine organisms to colonise the fresh- waters of the land, 

 the author inclines to the conclusion that the existing fresh- 

 water faunas of the earth must have originated in pans, 

 which were at one time part of the sea, and were subsequently 

 cut off from it, the present fresh- water forms being survivors 

 of this process, just as, if the Baltic were to be cut off 

 now and become fresh, some of its marine organisms 

 might persist as fresh- water types. The present curious 

 universal distribution of the primary fresh-water fauna 

 over the land's surface being assumed by Sollas to have 

 been brought about afterwards, and to be due mainly to 

 a capacity which fresh-water organisms possess for 

 migration. At the time Sollas wrote, the extraordinary 

 dispersion of many fresh-water animals throughout the 

 world, such as Planorbis, Limnaea, Alelania, Bithynia, and 

 Vivipara, was not fully apparent, and still less were the 

 facts relating to the actual capacity for migration which 

 these and other fresh-water animals really possess. I shall 

 refer to this matter again subsequently, but for present 

 purposes, it may be stated, that the more recent observations 

 of numerous investigators do not show that there is really 

 any evidence whatever for a capacity on the part of fresh- 

 water organisms, such as the above molluscs, to spread them- 

 selves, as they are spread, into the different isolated sheets 



