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CHAPTER XVII. 



In what precedes we have seen that the structural 

 characters of the organisms constituting the halolimnic 

 group, and the distribution of that group, show that these 

 animals have originated in the first place independently 

 from the general fresh- water fauna of Africa. And in the 

 second that they have arisen from some sea which normally 

 contained some such types as these. It remains for us 

 now, therefore, to ascertain, further, if there is evidence 

 to show in what direction and from what part of the 

 ocean Tanganyika was originally stocked with marine 

 life. While finally we shall have to see if there is also 

 evidence which will indicate the age of the sea-fauna, 

 to which the halolimnic group originally belonged. In 

 proceeding with these inquiries, it is, however, necessary 

 to revert temporarily to certain considerations respecting 

 the distribution of the fish-fauna in the lakes and rivers 

 of Africa ; for, up to the present time, this matter has 

 never been treated adequately, yet the facts with respect to 

 it throw a most important sidelight on the two particular 

 points we are about to discuss. It will be remembered 

 that in Chapter VIII. I pointed out that it is a mistake 

 to view the fish-fauna of Tanganyika as having nothing 

 strange about it, and I showed that there is some reason 

 for regarding the Polypterus and Protopterus of Tan- 



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