THE FAUNA OF THE GREAT AFRICAN LAKES. 639 



probable by the geology of the district in which Tanganyika 

 lies. There is, however, no such objection forthcoming ; 

 what is known of the geology of this district simply shows 

 that the lake region has been enormously elevated and 

 depressed in by no means very ancient times, and this is in 

 exact accordance with the facts of morphology, as we shall 

 see. 



We have then, I hope, cleared the way to a direct ap- 

 preciation of the facts of morphology themselves. We have 

 seen that a greater number of the hypotheses which have 

 been made to account for the peculiarities of the African 

 lake faunas are inapplicable to the questions they were 

 intended to solve, when the facts of environment and dis- 

 tribution have been fully weighed, and, lastly, that there is 

 no evidence either geological or palaeontological which in 

 any way militates against the conclusions drawn purely 

 from a study of the morphology of these forms being any 

 way unsound. Whatever result can be got out of the 

 study of the pure morphology and affinities of these animals 

 is likely to be the best explanation of their existence in 

 Tanganyika that can be obtained. 



It would, however, be premature as well as beyond the 

 limits of this article to enter into any detailed account of 

 the anatomy of the individual members of the unique 

 Tanganyika group ; all I shall do is to indicate the general 

 tendency of the investigation, as far as it has been at pre- 

 sent carried. 



The incapacity which certain members of the Tanga- 

 nyika fauna present, of being directly associated with any 

 other living species, is a most remarkable fact ; it shows 

 among other things that these aberrant lake forms must 

 have had time to remain as they were until similar forms 

 had become elsewhere extinct, and thus these animals still 

 living in Tanganyika must be comparatively old. 



The Medusa which was first sent home by Mr. Swann 

 and examined by Gunther has never been satisfactorily 

 associated with any living form. The deep-water gastro- 

 pods which I obtained in the lake are in the same predica- 

 ment, but these forms offer many structural points, which 



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