122 



NATURAL SCIENCE. 



of violent changes. No extensive freshwater lakes are to be found 

 within the warmer regions of the globe, except in Central Africa, and 

 to these we must turn if we wish to study the effects of long- 

 continued and slow change on a lacustrine fauna. 



So little is yet known as to the natural history of the equatorial 

 lakes of Central Africa, or of the Tertiary geology of the district, 

 that one cannot speak confidently as to the process of their formation. 



Fig. 3.— The Great Lakes of Central Africa. 



Still, however, we have reason to believe that their history has been 

 like that of the Mediterranean lakes before alluded to, except — and 

 the exception is important — that the longer continuance, or greater 

 energy of the uplift in Central Africa, has raised the sea-bottom high 

 above its ancient level and has gradually transformed portions of its 

 area into freshwater lakes. 



