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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- 
22* 
