352 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-day. 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 repeat the 

 shell form of a gastropod belonging to the Jurassic seas, and 



