THE TANGANYIKA PROBLEM. 
333 
appearance, engendered by some unspecified conditions, 
what would become of palaeontological determinations of 
shells in general ? # 
We could understand, and accept it as a fact, that one 
form in the whole fauna of a lake had acquired the concho- 
logical characters of a form belonging to a fauna of a 
remote age, and yet be independent of it, as is probably the 
case with the Paramelania of Tanganyika and the Pyrgu- 
liferct of the upper chalk. But the chance of this sort of 
thing happening among a large number of forms in two 
faunas which have no connection is improbable in the 
extreme. 
However, waiving for the present this whole comparison, 
and waiving as well the early marine characters of the 
halolimnic gastropods altogether, the advocates of con- 
vergence are still not by any means out of the wood. 
There are other marine organisms in Tanganyika besides 
gastropods ; there are prawns, and sponges, and jelly-fish, 
and polyzoa, which are not found elsewhere in the lakes of 
Central Africa. Where did these things come from, and 
out of what fresh-water form could they possibly converge ? 
In order to account for the presence of the halolimnic 
fauna in Tanganyika through convergence, we must, in 
fact, either resuscitate the old doctrine of special creation or 
adopt Bastien’s theory of heterogenesis, whereby a puppy 
could breed pigs. To the present writer the view that the 
* In the comparison referred to, and with which I shall have to deal subsequently, I 
may point out that I took great trouble to consult all the experienced paleontologists 
who were available, in order that I might not be accused of making a comparison which 
experienced paleontologists themselves would not. And, as I pointed out in the memoir 
dealing with this matter, the paleontologists thus consulted were unanimous in affirming 
that, so far as conchological identifications are ever justifiable, the above comparisons 
were so. In this matter I used, as I shall explain, the methods adopted and accepted 
by paleontologists, and, if these are not to be trusted, it is simply so much the worse 
for paleontology. And it does not prove convergence either. 
