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 Mlanji 

 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- 



