336 THE TANGANYIKA PROBLEM. 



series which bore the slightest resemblance to any of the 

 halolimnic forms. 



Finally, Tausch, by the extensive comparisons which he 

 made, has unconsciously shown that the single original com- 

 parison between Pyrgulifera and Paramelania is itself radi- 

 cally untenable. He found that some forms of the so-called 

 Pyrgulifera were more or less indistinguishable from the 

 genus Melanopsis, and, as he expressly states, he could 

 arrange an insensibly graduated series of shells stretching 

 from the typical Pyrgulifera, on the one hand, to the 

 typical Melanopsis on the other. The shell varieties of 

 Pyrgulifera are thus indistinguishable from the shell 

 varieties of Melanopsis. But the animal Melanopsis bears 

 no sort of relationship whatever to the animal Paramelania, 

 and this is a fact of which Tausch was not aware. 



The idea that the halolimnic fauna is the remnant of a 

 cretaceous fresh-water stock is thus seen to be based on a 

 single and also on an erroneous comparison, and it is con- 

 sequently absolutely incapable of further development in 

 any way. 



We find, then, that all the hypotheses which we have as 

 yet considered, and which from time to time have been 

 advanced to account for the presence and character of the 

 halolimnic group — namely, the direct metamorphosis of a 

 normal fresh-water stock, convergence of certain members of 

 a fresh-water stock, special creation and hetrogenesis, 

 operating among a fresh-water stock, and finally the per- 

 sistence of an extinct fresh-water stock, although they may 

 be all quite intelligible, and even possible, in other cases, 

 are each quite inapplicable to the actual facts of the special 

 problem presented by the presence of the halolimnic group 

 in Tanganyika as it now exists. Without invoking special 

 creation and hetrogenesis, we cannot explain the exist- 



