22 THE TANGANYIKA PROBLEM. 



cidata, as well as a few Cichlid fishes, among which there 

 were several Nyassa species. Beyond this oasis, in the 

 open body of the lake, and among its numerous islands, 

 none of these animals occurred, and the only forms of life 

 present were some curious algae, some catfish, and a small 

 Planorbis, which lived upon the reeds and not in the lake 

 at all. All round the marshy shores of Lake Shirwa, how- 

 ever, there are extensive plains which were at one time 

 unquestionably portions of the floor of the lake, and em- 

 bedded in these there are found countless millions of Vivi- 

 paras and Limneas ; in fact, the remains of nearly all the 

 molluscs found now living in Nyassa. From these facts 

 three things are obvious : ( i ) that Shirwa at one time 

 flowed out and was fresh ; (2) that it was once peopled by 

 the Nyassa fauna ; and (3) that it is now uninhabited by 

 that fauna, except in the oases of fresh-water such as that 

 which I have just described. At some time the lake 

 ceased to have an outlet, and its somewhat profuse fauna 

 died out as a result of the stagnation of its waters and the 

 acquisition of their peculiar briny characters. The increase 

 of the salinity of the water of Lake Shirwa must, however, 

 have taken place with extreme slowness, much more slowly, 

 in fact, than we could ever bring about a similar change 

 of environment experimentally. Nevertheless, we find that 

 the salt has eventually killed out the whole of the typical 

 fresh-water molluscan fauna of Nyassa, and most of the fish. 

 The consideration of the history of Lake Shirwa shows us, 

 then, that a typical fresh-water fauna like that of Nyassa 

 does not readily acclimatise itself to an increase of salt, such 

 as that which has taken place in the lake, and this is so 

 notwithstanding that the increase may have taken place 

 with extreme slowness. I do not know the percentage or 

 the exact composition of the saline materials which the 



