LAND AND FRESH- WATER SHELLS. 341 



sent the stages of transition from the one species to the other. If 

 this be possible, then I would suggest that the fresh-water Nerite 

 {Neritina subsulcata) may have been transformed into the tree Nerite 

 {Neritina corned) in the following manner. 



I liave already referred to tlie circumstance that in tlie higher 

 portions of the St. Christoval streams, where the rocks are entireh" 

 volcanic, the fresh-water shells — and I may here add, especially 

 these of Neritina sjibsiilcata — suffer much more erosion than do 

 shells of the same species in the lower parts of the streams where 

 they flow through calcareous districts. Now, the geological structure 

 of this island being mainly ancient volcanic rocks incrusted near the 

 coast by recent calcareous formations, the time will come when these 

 calcareous envelopes will have been entirely stripped off by denuda- 

 tion. How this will influence the Nerites of the streams may be 

 thus explained. At present the normal characters of the species are 

 preserved in the calcareous portions of the streams ; but when all 

 the calcareous rocks have been stripped oft' by denudation, the Nerite 

 tiirough its whole lifetime will be subjected to that extensive process 

 of erosion, which now often denudes almost the entire surface of the 

 shells of those individuals that live in the volcanic portion of the 

 stream's course. Here, Natural Selection may step in to favour thp 

 survival of any slight variation that makes the Nerite more suited 

 to lead an entirely aiboreal existence. Such a geological agency 

 may in truth lead finally to the expulsion of the Nerite from the 

 stream's course. Varieties will survive only in proportion to tlieir 

 capability of adapting themselves to the new condition ; and they 

 alone will perpetuate their kind until a tree Nerite of distinct specific 



character is produced On this reasoning, tree Nerites ought 



to be more numerous in islands of volcanic formation ; but this is a 

 point on which I cannot pronounce from the lack of suflicient 

 evidence.^ 



According to Professor Semper, we have in Navicella " a modified 

 form of Neritina^' which genus it resembles in all essential anatomi- 

 cal characters, but " by long inurement to living in rushing mountain 

 streams, it has had its shell modified in the way most suited to those 

 conditions, while the operculum, in consequence of long disuse, has 

 become a peculiar degenerate or rudimentary organ." ^ 



^ Prof. Semper's observations in the Philippines bear on this matter. (" Natural Con- 

 ditions of Existence," &c., p. 188.) 

 Ibi.l, p. 212. 



