332 THE TANGANYIKA PROBLEM. 



forms in the same sense that we might legitimately regard 

 the miratesta found by the cousins Sarrasin in the deep 

 water of Celebes as a modified form of the modern fresh- 

 water Opisthobranchs. The crabs are not the deriva- 

 tives of the African thelphusoids, but structurally antecede 

 them. The prawns cannot be viewed as being derived 

 from any fresh-water Crustacea which exist in the fresh 

 waters of the interior of the African continent ; for 

 prawns have not been found elsewhere in the great 

 African lakes. We cannot suppose that the Medusa 

 has originated from any fresh-water form any more than 

 it has originated de novo in Tanganyika, nor can we 

 view the gymnolaematous polyzoa as having originated 

 from any of the phylactolemetus fresh-water forms. 



From these considerations, based on the facts which are 

 now available, it is quite clear that the halolimnic group 

 can neither be regarded as an atavistic nor an anamorphic 

 modification of the normal African fresh-water fauna. In 

 the same way it will be found quite impossible to regard 

 the marine aspect of the halolimnic fauna as the result of 

 convergence, produced by the influence of some undefined 

 conditions which may exist there. And I mention this 

 specially, because such a view has recently been brought 

 forward as a possibility in an essay on my incomplete 

 researches by Professor Stromer. 



The primary difficulty of any idea of convergence in 

 such a case as this is, that its logical developments neces- 

 sarily lead its advocates very much further than they pro- 

 bably ever intended to go. 



If, for example, and waiving anatomical considerations 

 altogether, we were to assume that the apparent specific 

 identity which exists between the shells of the halolimnic 

 gastropods and certain marine, Jurassic forms was a mere 



