THE TANGANYIKA PROBLEM. 
I 2 
gested that forms like some of the Trochoid molluscs, some 
jelly-fish, and many prawns which live in the brackish 
waters between inland lakes and the ocean constitute in 
themselves the modern instances of the transmutation of 
marine organisms into those which live in fresh-water. 
According to this view of the matter, fresh-water faunas 
are heterogeneous assemblages of organisms the forbears 
of which have at different times successfully colonised the 
fresh-waters of the earth from the ocean ; and the curiously 
universal distribution which is characteristic of some ot 
the constituents of these faunas, is regarded as having 
been brought about by the natural facilities which exist 
for the distribution and dissemination over the land of 
such organisms and their germs. Within the last few 
years, however, much definite information has been added 
to our knowledge of the nature of fresh-water faunas, and 
of the composition of those which occur in different parts 
of the earth. It has been shown, by the experimental 
researches of Beudant and others, that certain marine 
organisms can actually be acclimatised to fresh-water if the 
change is carried out with sufficient slowness, but at the same 
time it has become apparent that one of the most striking 
features which all fresh-water faunas present is the smallness 
of the number of the types composing them when compared 
to the fauna of the sea. From this it would appear that 
very many marine groups have never made any attempt 
to establish themselves in lakes and rivers, and this 
indication has led to the investigations of Semper* and 
Sollas,f who have shown that there are other obstacles 
besides the necessity of adaptation to new conditions of 
salinity which tend to prevent marine animals from 
* Semper, “Animal Life,” 1 88 1 , p. 149. 
t Sollas, W. J., Trans. Roy. Soc. Dublin , vol. iii., series ii., p. 87. 
