638 SCIENCE PROGRESS. 



Tanganyika fauna is then in a most complicated and un- 

 certain condition at the present time, but it is quite hkely 

 that more Hght will be forthcoming in this direction later on. 



There is yet one more possibility concerning the origin 

 of these forms which it is necessary to take into account. 

 It is quite conceivable that the prolonged isolation which 

 these animals living in Tanganyika have undergone may 

 have allowed them to sport into forms simulating oceanic 

 types, and this is in itself a most attractive hypothesis, but 

 it is again met by the same fatal objection of the complete 

 restriction of the marine fauna to the confines of Tan- 

 oanyika itself. If these organisms were the result of sport- 

 ing, why have not the faunas of the other great lakes 

 exhibited the same sportive characters ; why, for example, 

 has the fauna of Nyassa retained its original fresh-water 

 characters although there is more geological evidence for the 

 antiquity of the fauna of this lake than in the case of that of 

 Lake Tanganyika. 



From all this it is apparent that the alternate hypotheses 

 to marine contamination which may be made to account for 

 the presence of certain animal forms in Lake Tanganyika 

 seem one after another either to break down entirely or to 

 become so uncertain as to be of little value, except as incen- 

 tives to the production of evidence the combined effect of 

 which has been to leave the theory of marine contamination 

 as the only road towards the solution of the problems which 

 the African lakes present. 



Thus without appealing to the facts of morphology 

 themselves, and quite apart from the affinities which the 

 Tanganyika group really exhibit, it seems quite impossible 

 to account in any way except through the hypothesis of 

 direct marine contamination for the presence of certain 

 animals in the lake. These objections are, moreover, some- 

 thing more than negative ; in every case they afford positive 

 evidence against the possibility of adopting any of the alter- 

 nate hypotheses detailed above. The only remaining 

 objection which can now be raised apart from the direct 

 morphological evidence which may bear upon the theory of 

 marine contamination is that such a view is rendered im- 



