304 J. K. S. MOOEE. 



I^ have shown that one of these two faunas consists of the 

 normal and ubiquitous fresh-water stock, which is distributed 

 throuo-hout the whole African continent, and indeed through- 

 out the world. The second fauna is altogether diflferent from 

 this, and in the appearance of its widely divergent constituents 

 is utterly unlike any modification of the normal fresh-water 

 fauna that is known. It has long ago been recognised that 

 the superficial facies of the moUuscau shells belonging to this 

 series are those of a marine rather than a fresh-water stocky 

 and in recognition of the more complete marine affinities 

 which a closer scrutiny of the internal anatomy of these 

 animals has revealed, I^ have here, as elsewhere, spoken of the 

 whole series of forms in Tanganyika which exhibit these quasi- 

 marine characters as members of the halolimnic group.^ 



' " On the Zoological Evidence for the Connection of Lake Tanganyika with 

 the Sea," ' Proc. Roy. Soc.,' vol. Ixii, 1898, pp. 452—458. 



2 "The Molluscs of the Great African Lakes.— II. The Anatomy of the 

 Typhobias, &c.," 'Quart. Journ. Micr. Sci.,' vol. 41,1898, pp. 181—202. 



* If the practical distinction between fresh-water and marine faunas in general 

 ■were not a well-established and accepted fact, it would have been impossible 

 for geologists to separate, as they have done, fresh-water from marine deposits 

 by the characters of the animals they contain. It is generally assumed that 

 the modern fresh-water fauna has gradually originated far back in time by 

 organisms having one by one acquired characters which have enabled them to 

 successfully colonise fresh water in connection with the sea ; but the actual 

 phylogenetic descent of most of the true fresh-water organisms, except in a very- 

 broad sense, is lost in antiquity and hopelessly obscure. In some Crustacea, 

 in the Ganoids, and some other fishes we have enough palaeontological 

 evidence to demonstrate their actual migration from the sea, and such evidence 

 forms part of the ground whence it is argued from analogy that all fresh-water 

 organisms have originated in a similar way. Further evidence of this kind is 

 afforded by those cases, at once remarkable and few, where animals that are 

 generally marine exhibit a wonderful capacity to migrate inland, there being 

 every reason to believe that such organisms constitute the " modern instances " 

 of the origin of new fresh- water types. The true fresh water fauna of any period 

 is thus a heterogeneous assemblage of organisms, all of which have, so to speak, 

 voluntarily acquired the habit of living in fresh-water, and, excepting in this 

 peculiarity, they have no necessary relation with each other. The constant 

 facies which the fresh -water fauna presents all over the world are due primarily 

 to the universal distribution of its heterogeneous constituents, and secondarily 

 to the direct similar effect produced on organisms by a fresh-water life. 



