12 THE TANGANYIKA PROBLEM. 



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," 1881, p. 149. 

 tSollas, W. J., Trans. Roy. Soc. Dublin, vol. iii., series ii., p. 87. 



