35 2 
THE TANGANYIKA PROBLEM. 
valid geological, zoological, or palaeontological objection to 
such a state of things having occurred, and I think the 
geological position in regard to the matter is correctly 
summarised in the following paragraph, which appears in 
Huxley’s discourse upon persistent geological types : — 
“ For anything that geology or palaeontology can show to 
the contrary, a Devonian fauna and flora in the British 
islands may have been contemporaneous with the Silurian 
life in North America, and with a carboniferous fauna in 
Africa ; ” and so also, for anything which geology or 
palaeontology can show to the contrary, there may still be 
in Tanganyika a Jurassic fauna existing contemporaneously 
with the general modern fauna of to-daiy. If, then, the 
comparison between the Oolitic and the Tanganyika 
gastropods is as good as the comparisons upon which 
palaeontologists are accustomed to regard the fauna of 
different deposits as of the same age, we have precisely as 
much reason for regarding the halolimnic and the marine 
Jurassic faunae as similarly identical. 
I have already shown in what precedes that, so far 
as conchological determinations of generic and specific 
identity are possible under any circumstances, the com- 
parisons between the Jurassic and halolimnic shells are 
quite as good, if not better, than most of the palaeonto- 
logical determinations upon which palaeontology is content 
to rest assured. 
It is, however, not so much in the nature of the com- 
parison as in the number of corresponding points between 
the two faunae, that the importance of the above comparison 
will in reality be found to lie. We could, of course, regard 
it as not unlikely that one shell in a great lake like Tan- 
ganyika might have become modified so as to re-peat the 
shell form of a gastropod belonging to the Jurassic seas, and 
