22 
THE TANGANYIKA TROT LEM. 
culata , 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 
