612 DR. W. A. GUNNIlSrGTOiSf OTf THE 



the conditions in Tanganyika produced a move sti'iking series of 

 such forms tlian elsewhere. This view is not regarded as accept- 

 able. — It has been suggested that the salinity of tiie Tanganyika 

 water ha,s produced marine-like forms, but further evidence is 

 needed. — The view that Ta,nganyika owes its remarkable organ- 

 isms to a prolonged period of isolation is i-egarded as the most 

 likely suggestion. It does not run counter to geological 

 conceptions. 



This theory does not account for the medusa. Boulenger 

 suggests that it may have sui-vived from an Eocene sea in 

 Northern Africa. Gravier considers, on the contrary, that it 

 may be a recent migrant from the ocean. 



The faunas of the remaining lakes are of less interest. Victoria 

 Nya,nza is 3iext in importance to Tanganyika. The most con- 

 spicuous group is the fishes, with a large proportion of endemic 

 species. The MoUusca also are noteworthy. The fauna of Nyasa 

 is similar in character, but with fewer peculiar types. There are, 

 however, 5 endemic genera of fishes. The smaller lakes contain 

 representatives of fewer animal gioups as well as fewer species. 

 Kivu is the extreme case, with only 23 species, of which 4 are 

 endemic. The poverty of its fauna may be associated with its 

 exceptional salinity. Albert Nyanza displays the smallest pro- 

 portion of endemic types, viz. 13 per cent. It is suggested that 

 periods of isolation would account for the peculiarities of Yictoiia 

 Nyanza and Nyasa. 



Certain animal types are unexpectedly absent fi-om the African 

 lakes. Such a,re the fresh-water Crustacea ^se^/zts and Gammartis 

 a,nd the fish parasite Echinorhynchus. No explanation of this is 

 forthcoming. 



The number of specific forms in these lakes appears to be 

 proportional to the size. This would seem to hold good also for 

 the number of genera, and families and even for the number of 

 groups represented. It is possible that this pi'inciple is of 

 general application. It is thought to be a,nalogous to the phe- 

 nomenon exhibited by the flora of ocea,nic islands. 



The affinity between Afi-ican and Indian fresh-water types is 

 recognisa,ble in several instances among the inhabitants of the 

 lakes. This affinity is explained by the former existence of a 

 continent which embraced these countries in the Carboniferous 

 and subsequent periods. 



Little is known of the deeper regions of any of the lakes. It 

 remains to be discovered whether associations of abyssal animals 

 exist in them. Neither the vertical distribution of plankton 

 forms, nor their seasonal variations have yet been studied. 

 Further information is needed on the salts dissolved in the water, 

 the depth and nature of the lake ba,sins, water temperatures, etc. 

 It is nevertheless possible fi-om esta.blished facts to foi'm a true 

 conception of the nature of the lakes and their organisms. 



