THE TANGANYIKA PROBLEM. 
3 2 7 
which is presented by the mixed fauna of Lake Tanganyika, 
of the present day. 
Whatever opinion may be held respecting the view that 
the salinity of the ocean has been a prime factor in pro- 
ducing some of the characteristics which the fresh-water and 
marine faunas of the present day actually possess, is a 
matter which is wholly independent of the particular 
Tanganyika problem with which this work is primarily 
concerned. 
These more general subjects were merely introduced in 
order that some foreseen confusions might possibly be 
avoided, and the ground made clearer, for the consideration 
of the Tanganyika problem itself. But before finally 
approaching this particular problem by a systematic ex- 
amination of the different components of the fauna of 
Tanganyika, and the other African lakes, I dealt generally 
with what is known of the geology of the districts of 
Central Africa. This course was necessary, in the first 
place, because our conceptions of the country have been 
somewhat altered by the observations acquired during the 
Tanganyika expeditions. No support has been lent by 
these to the view that the marine origin of the halolimnic 
fauna of Lake Tanganyika is opposed to our knowledge of 
the geological nature of the African land-mass. On the 
contrary, it was shown that there was no evidence what- 
ever against such a view, and that there is a good deal 
which, more or less, distinctly favours it. 
These preliminary matters have been considered, the 
fauna of the great African lakes was described, and 
subsequently the halolimnic fauna was defined as something 
distinct from the general fresh-water fauna of the African 
continent. In the first place, it was shown to be a group 
of organisms of diverse nature, but a group all the 
