THE TANGANYIKA PROBLEM. 143 



have had a different origin from the general African fresh- 

 water constituents, it is highly probable that some of the 

 fishes which we now, from their dispersal, regard as belong- 

 ing to the normal African fresh-water series, are in reality 

 scattered members of this same halolimnic group. At all 

 events it is quite clear that it is not correct to suppose 

 that the fish-fauna of Tanganyika is at all similar to that 

 of the other African lakes, and this strikingly distinctive 

 character of the Tanganyika fishes constitutes the prima 

 facie evidence for supposing that at least some of the fishes 

 which characterise the fauna of Lake Tanganyika are of 

 the same stock as the halolimnic invertebrates themselves. 



The halolimnic fauna consists then of a group of animals, 

 the invertebrate section of which is rigidly confined to the 

 lake, and which have no obvious relation to the normal 

 African fresh-water invertebrates, but it is also indicated that 

 some of the African fishes which we have hitherto regarded 

 as normal fresh-water fishes of the African continent may 

 belong in reality to the now scattered vertebrate section of 

 the halolimnic group. 



It remains for us now then to examine in much greater 

 detail the characters of the individual components of the 

 halolimnic group, and thereby to attempt to acquire the 

 information necessary in order to ascertain the actual 

 affinities of these forms ; while finally we shall have to 

 consider the question of the nature and the origin of the 

 whole of the halolimnic group in Tanganyika, and this 

 question itself forms the Tanganyika problem as it now 

 exists. But before proceeding to discuss in detail the 

 structure of the halolimnic forms, it is desirable to con- 

 sider certain widespread phenomena relating to the normal 

 fresh-water fauna of the African lakes in general. 



