608 DR. W. A. CUNNINGTON ON THE 



as Gondwanaland. In the ensuing period — the Jurassic — Gond- 

 wanaland began to break up, but there is some evidence that in 

 kite Cretaceous or even early Tertiary times a land-bridge still 

 existed between East Africa and the Indo-Malayan region, by 

 way of the Seychelles and Maldives. The geological record 

 is thus quite in keeping with the facts of distribution to which 

 reference has been made. 



In bringing this study to a conclusion, it must be admitted 

 that in mai^y directions information is very limited. There is no 

 doubt that the discovery of additional species is to be expected 

 whenever a re-examination of any of the lakes occurs, but there 

 are other points of considerable interest on which knowledge is 

 much to be desired. 



Despite the investigations of Moore in Nyasa and Tanganyika, 

 very little is really known concerning the deeper regions of any 

 of the lakes, and the existence of a distinct abyssal fauna is 

 uncertain. In a paper dealing with the distribution of the 

 molluscs, Moore speaks of obtaining certain thalassoid forms in 

 Tanganyika at a depth of 800-1200 feet (244-366 metres) 

 (129, p. 171). He I'egarded these particular Gasteropods as a, 

 deep-water assemblage, but a more thorougli examination may 

 well ]-eveal other animals which jDermanently inhabit the bottom 

 muds. Thei-e is little doubt from an analogy with other deep 

 lakes that the deeper waters of these African lakes aie almost, if 

 not quite, devoid of life. At the same time, further investigation 

 may indicate a definite association of abyssal forms not only in 

 Tanganyika, but in the other lakes under review. — Associated 

 likewise with the distribution of organisms within the limits of 

 the lakes are questions such as the vertical distribution of 

 plankton forms, as ascertained by tow-nettings. During the 

 Third Tanganyika Expedition my opei^ations were practically 

 confined to surface tow-nettings, but special tow-nets worked 

 from suitable craft would afford the necessary information. — 

 Again, the seasonal valuations of plankton organisms are almost 

 unknown, although I was able to detect a marked decrease in the 

 quantity of material during tJie rainy season. Detailed know- 

 ledge of this kind can only be gained by the aid of large collections 

 extending over many months. — The distribution of local forms or 

 varieties within the limits of a single lake was discussed by 

 Moore in his book. He considered that certain well-marked 

 varieties or even species were confined to particular areas in the 

 greater African lakes (137, p. 149). My own observations lead 

 to a different conclusion, and the matter undoubtedly merits 

 further investigation. 



Lastly, there are some outstanding physical matters which are 

 of importance on account of their I'elation to biological pheno- 

 mena. Very little is known concerning the chemical impurities 

 of the wa,ter in these lakes, and even the depth and general 

 nature of the basin is imperfectly known save for Tanganyika, 

 Victoria and N'yasa. Further knowledge is likewise desii-able as 



