FRESH-WATER SHELLS. 3 



mind, more especially in connection with discontinuous 

 ranges, that many types are of immense antiquity, 

 having survived numerous changes of climate and great 

 oscillations of level, so that the distant regions in which 

 they are now found may, in some cases, have formed 

 parts of former continuous and very wide ranges. It 

 ought also to be remarked, perhaps, that some eminent 

 biologists, as, for instance. Professor Semper, think it 

 possible that similar faunas in distant parts of the world 

 may sometimes have been brought about by the con- 

 vergence of formerly distinct types,^ and this, of course, 

 cuts the Gordian knot much in the same way as did a 

 formerly prevalent belief in multiple centres of special 

 creation. Fresh-water forms are said to have been 

 derived originally from the sea, and even now certain 

 marine animals in all probability are gradually adapting 

 themselves to fresh water, a process which may have been 

 going on all through the ages, but I do not see that this 

 much affects our present inquiry ; it should certainly be 

 taken into account, however, whenever the occurrence at 

 distant places of allied fresh-water forms having near 

 relatives in the sea is considered. 



Fresh-water shells, it seems, if left to their own 

 endeavours, would generally find great difficulty in 

 extending their range beyond the limits of their own 

 river-basin, for the dry land obviously presents a for- 

 midable barrier to these animals, and, as Mr. Darwin 

 remarks, each river-system with all the pools and lakes 



^ Semper, "Animal Life,"' Eng. transl, ed. 4, 1S90, pp. 294 and 



461. 



B 3 



