602 DR. W. A. CUNNINGTON ON THE 



geological aspect of the problem, be states tbtvt very exteDsive 

 lacustrine deposits are actually known in Central Africa, reaching 

 from the Congo basin to Tanganyika and from that lake to 

 Yictoi'ia Nyanza on the one hand and Nyasa on the other. From 

 such considerations he supposes that tliere formerly existed in 

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

 and Upper Congo with Nyasa and Tanganyika and probably 

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

 water fauna specially rich in Prosobranchs. After the formation 

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

 basins, where modifications of the original fauna resulted fiom 

 the new environment. Lake Tanganyika, possessing most nearly 

 the characteristics of the ocean, became inhabited by molluscs 

 which have assumed (by a phenomenom of convei-gence) a maiine 

 aspect in the highest degree*. 



With this conception 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 seiious 

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

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

 the deposits in this area being pi-obably 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 series 

 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 affords of development free from competition with 

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

 duction of new chai'acters. Where isolation arid a gradual 

 increase in salinity have coexisted, it is difficult to recoguise 

 which factor is i-esponsible 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 Algae of Tanganyika which exhibit marine affinities may 

 well have been produced by a gradual increase in the ,$;a.linity of 

 the lake dui-ing 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'Afriqne Orientale." 

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

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



