596 DR. W. A. CUNNINGTON ON THE 



represented, there is often a great richness of species within the 

 groups. The fishes are far fewer than tliose of Tanganyika and 

 only lialf of thera are endemic, while the Mollusca agree pretty 

 closely in the number of endemic types. On the other hand, there 

 are most extensive series of Oligochfetes and Turbellarians, and an 

 extraordinary wealth of Gammarids, the species being in each case 

 nearly all endemic. In the case of the Caspian, I am able to 

 quote figures, according to which some 64 per cent, of the animal 

 forms are found nowhere else in the world (155, p. 34), If this 

 statement can be relied upon, the Caspian Sea, while sufficiently 

 remarkable, is less so than Tanganyika, which has nearly 73 per 

 cent, of endemic types. 



Returning to a consideration of the fauna of Tanganyika, 

 other features revealed in the Systematic Account may be 

 summarised in a few sentences. Those groups which are most 

 conspicuous in possessing endemic genera and species are the 

 Pisces, Mollusca (especially Gnsteropoda), and Macrura, with the 

 Brachyura following closely. The Copepoda and Osti-acoda are 

 well represented by endemic species (but not genera), with the 

 Porifera and Polyzoa showing smaller numbers. While othei- 

 groups with few endemic species, appear, by contrast, devoid of 

 significance, there are only five of all those represented in the 

 lake — Mammalia, Crocodilia, Chelonia, Bati'achia, and Cffilen- 

 terata — which do not contain endemic types. It may be added 

 that certain endemic forms are held to exhibit a marine aspect 

 and have been termed thalassoid (lialolimnic according to Moore). 

 Such are many of the Gasteropod molluscs and perhaps a Polyzoon. 

 In the same category comes the medusa, which, of course, is not 

 confined to Tanganyika. 



The exceptional character of the Tanganyika fauna having 

 been sufficiently emphasiserl, an explanation of this marked 

 peculiarity must be sought. In other words, a general con- 

 sideration of what Moore called the Tanganyika "problem" must 

 be undertaken. In oixler to appreciate the actual value of the 

 purely biological evidence, it is necessary clearly to understand 

 the relations which exist between marine and fresh-water 

 organisms. The essential points may therefore be stated as 

 briefly as possible *. 



In the first place, while certain organisms are characteiistic of 

 the sea and others of fresh water, the distinctions which exist 

 between marine and fresh- water forms are neither very great nor 

 very definite. Secondly, it may be emphasised that the barriers 

 which tend to prevent a change of meflium are not wholly insur- 

 mountable. At the same time fresh-water types are usually 

 recognised in consequence of certain structural peculiarities 

 directly due to their mode of life. Such features should be 

 excluded a,s far as possible when deciding the systematic position 

 of an organism, for it is only thus that a ti'ue idea of its inter- 



* These matters are discussed at greater length iu several recent papers. Consult 

 SoUas (173), Cunniugton (71), Gurney (97). 



