IV. 



Great Lakes. 



MUCH has been written on the origin of lakes, and on the 

 character of their inhabitants ; but the Hterature of the 

 subject is buried in so many monographs and scattered papers 

 that it may be useful to linger for a moment to review the present 

 state of our knowledge. Especially is this advisable when we bear 

 in mind the rapid advance in geographical and geological exploration 

 which has taken place in the past decade. It should also be remem- 

 bered that no one branch of Natural Science can stand alone ; each 

 in turn can aid and advance the others. If we wish to understand 

 the fauna and flora which inhabit a lake, it is necessary to learn how 

 that lake originated, and what was the date of its origin. The 

 inhabitants of each existing lake do not include merely such plants 

 and animals as, reasoning from analogy, we should say were most 

 fitted for it. They are always mingled with certain forms which 

 bring before one's mind the bygone history of the lake. The peculiar 

 species may be allied to forms usually confined to salt water ; or they 

 may be species incapable of migrating across barriers of land, though 

 now found in several distinct basins. We may find lingering in the 

 depths of certain lakes species which point to extensive climatic 

 changes since the lakes were first inhabited. This evidence of 

 climatic change may also help us to understand why certain species 

 are present, and others, seemingly equally fitted to inhabit the lake, 

 are now absent. 



Even a small lake must be studied from many points of view, if 

 we wish to obtain anything beyond the merest catalogue of its 

 inhabitants. Still more is this necessary when we deal with the vast 

 sheets of fresh water which are found in the interior of our continents; 

 for they, like the large oceanic islands, may contain lingerers of 

 antique types, now lost in regions less isolated. We will attempt, 

 therefore, in the following pages to show how lakes originate ; and we 

 will give, also, a short description of the great lakes now existing. 

 It must be pointed out, however, that we know little as yet about the 

 origin of great lakes, and the natural history of the most important 

 group is still practically unknown. These notes were partly written 

 to point out this deficiency in our knowledge, and if they help to stir 

 up naturalists to a more thorough study of the Great Lakes of 

 Central Africa, one of their principal ends will be attained. 



The regions occupied at the present day by great freshwater lakes 



