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 
