THE TANGANYIKA PROBLEM. 21 
the world the similar morphological characteristics which 
they actually possess. This similarity would not, how- 
ever, have been produced as a result of intercommunication 
between fresh-water centres, but as a result of their all 
having arisen from the general sea fauna of a particular 
geological age. The question before us, then, is whether 
there is to be found in the nature of things any cause 
which can be regarded as sufficient to produce such an 
effect 
It is a fact that increase or decrease of the amount of 
saline matter in water does act prejudicially, on many, if not 
on all the organisms living in it, and this can be shown in a 
variety of ways, perhaps best by the study of a lake which, 
like Shirwa, was originally fresh, but which through losing 
its outlet has now become intensely salt. Shirwa is a large, 
oval body of water about 50 miles long, and enclosed on 
all sides, first by lacustrine plains and then by lofty granitoid 
hills. At the north there is an old channel which repre- 
sents a former connection with the Lujenda River, but this, 
probably through rising of the ground, has now become 
closed, and the waters of the lake have not flowed out at 
any rate for many centuries. The lake is fed by numerous 
rivers which flow into it from the great mountains of Mlanj i 
and Zomba, and the slight amount of salt which the rain 
washes off the land has accumulated in the evaporating-pan 
formed by the lake, until it is now a thick strong brine. 
One of the principal rivers which flows into Shirwa is the 
Mulmgoosi, and where it enters the lake, there is still an 
area of almost fresh water, bounded without by the brine 
wastes of the lake. In this fresh-water oasis, but not in the 
upper course of the stream, I found some Viviparas living, 
of the same species unicolor which inhabits Lake Nyassa. 
There was also the Nyassa Limnaea and Melania tuber - 
