THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 175 



purinas, and the like, that it is pardonable if zoologists re- 

 quire something more than merely conchological characters to 

 establish an identity among these forms. ^ But this supposed 

 homology between the shells of a living and extinct species of 

 Gasteropod (about the anatomy of neither of which up to the 

 present anything whatever has been known) is the one 

 fragment of positive evidence which can be produced in favour 

 of the relation of the Halolimnic fauna to an extinct fresh-water 

 stock. 



The hypothesis, moreover, is combated by the same objec- 

 tions emanating from the facts of distribution of the Halolimnic 

 animals that were fatal to the first hypothesis, and they have 

 here equal force. If the Halolimnic fauna of Tanganyika is 

 the remnant of an old African fresh-water stock, it must have 

 been present at one time in all the lakes which are as old as 

 Tanganyika ; but we have seen that with respect to Lake 

 Nyassa this does not appear to have been the case. It is very 

 improbable that many of the remaining so-called rift-valley 

 lakes are not as old as Tanganyika, yet we have seen that they 

 do not contain the Halolimnic forms. Therefore, in order to 

 support this second hypothesis, we shall be obliged to have 

 recourse to hypothetical catastrophes which must be supposed 

 to have destroyed the Halolimnic fauna in every lake but one. 

 Hypotheses of this sort spring, however, from the carcass of a 

 theory only after it is dead, and our second hypothesis is there- 

 fore opposed to the facts of distribution as they at present stand; 

 its acceptance would, moreover, be revolutionary to many 

 zoological conceptions of the present time. It would neces- 

 sarily lead us to believe that deep-water crabs may be indige- 

 nous fresh-water forms ; that deep-water Gastropods and 

 sponges were common in Cretaceous times ; that jelly-fish 

 were once fresh-water organisms, and so on through a number 

 of consequences, which, when the nature of the evidence sup- 

 porting the original hypothesis is weighed, must seem little 

 better than grotesque. On the other hand, all the facts of 



' I am quite aware that this statement cuts at the roots of many geological 

 determinations ; but I am prepared to maintain that the criticism is sound. 



