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 P/anorbis, Limnaea , Melania , 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 
