602 



DR. W, A. CUNNINGTON ON THE 



geological aspect ot" the pioblem, he states that very extensive 

 lacustrine deposits are actually known in Central Africa, reaching 

 from the Congo basin to I'angaiiyika and from that lake to 

 Victoria Nyanza on the one hand and Nyasa on the other. l>om 

 such considerations he supposes that there formerly existed in 

 east-central Africa a vast lake basin which nnited the Middle 

 and Upper Congo with Nyasa and Tanganyika and probably 

 Victoria JSTyanza. Living in this region was a uniform fresh- 

 water fauna specially rich in Prosobra.nchs. After the formation 

 of the Great Rift Valley the lakes were isolated in their present 

 basins, where modifications of the original fauna, resulted from 

 the new environment. Lake Tanganyika, possessing most nearly 

 the characteristics of the ocean, became inhabited by molluscs 

 which have assumed (by a phenomenom of conveigence) a marine 

 aspect in the highest degree*. 



With this concejjtion I do not find myself wholly in accord. 

 Without expressing an opinion on the nature of the Gasteropods 

 of Nyasa and the Upper Congo region, there are two serious 

 objections to its acceptance. In the first place, there is in- 

 sufiicient geological evidence for a lake basin so large in extent — 

 the deposits in this area being probably terrestrial and fluviatile. 

 In the second place, there is no reason why the conditions in 

 Nyasa and Victoria Nyanza, which closely resemble those of 

 Tanganyika, should not have produced an equally striking seiies 

 of thalassoid Gasteropods in those lakes, and yet this is not the 

 case. At the same time I am quite prepared to agree that 

 the marine aspect of the molluscs is probably due to convergence. 



Another obvious suggestion is that the salinity of the water 

 has been a determining factor in producing ma,rine-like forms. 

 Here it is much less easy to come to a decision, for the question 

 of increased salinity is of course directly associated with that of 

 a period of isolation. But prolonged isolation itself, with the 

 opportunity it aifords of development free from competition with 

 the outside world, is obviously a cause predisposing to the pro- 

 duction of new characters. Where isolation and a gradual 

 increase in salinity have coexisted, it is difificult to recognise 

 which factor is responsible for a particular result. Experimental 

 evidence is not wanting to show^ that certain salts, even in 

 minute quantities, exert a profound influence on aquatic organ- 

 isms, but it does not follow that a marine aspect would be 

 produced as a result. West, indeed, goes so far as to assert 

 that the Alg?e of Tanganyika which exhibit marine affinities may 

 well have been produced by a gradual increase in the salinity of 

 the lake during an extended period of time (200, p. 191). Here, 

 it is true, the two factors are inextricably associated, but if the 

 suggestion is not unreasonable for the Algse, the surmise may be 



* Since this account has been in the press, a still more recent paper by Germain 

 has reached my hands — " Histoire Oceanographique des Lacs de I'Afrique Orientale." 

 Bull. Inst. Oceanogr. Monaco, No. 369, 1920. In it, he adds little which is new, 

 merely re-affirming the opinions expressed in his earlier article. 



