Chat. XII. FRESH-WATER PRODUCTIONS. 851 



CHAPTER XII. 



GEOGnAPiiicAL DisTKiBUTiox — Continued. 



Distribution of Prcsh-wnfcr Productions— On the Inhabitants of Ocranic IrJlands— 

 Absence of 15atrucl)ian8 and of Terrestrial Mammals— On the Relation of tlie In- 

 tuibitants of I^landx to those of the nearest Main-land— On Colouizaticm from tho 

 nearest Source with subsequent Modification— Summary of the last and present 

 Chapter. 



Fresh-water Productions. 



As lakes and river-sj'stenis arc separated from each other 

 by barriers of land, it might have been thought that fresh- 

 water productions would not have ranged widely within the 

 same country, and, as the sea is apparently a still more for- 

 midable barrier, that they never would have extended to dis- 

 tant countries. But the case is exactly the reverse. Not only 

 have many fresh-water species, belonging to quite different 

 classes, an enormous range, but allied species prevail in a re- 

 markable manner throughout the world. I well remember, 

 when first collecting in the fresh waters of Brazil, feeling much 

 surprise at the similarity of the fresh-water insects, shells, etc., 

 and at the dissimilarity of the surrounding terrestrial beings, 

 compared with those of Britain. 



But this power in fresh- water productions of ranging wide- 

 ly, though so imcxpected, can, I think, in most cases be ex- 

 plaiiiod by their having become fitted, in a manner highly 

 useful to them, for short and frequent migrations from pond to 

 pond, or from stream to stream ; and liability to wide disper- 

 sal would follow from this capacity as an almost necessary con- 

 seipience. We can here consider only a few cases. In regard 

 to fish, I believe that the same species never occur in the 

 fresh waters of distant continents. But on the same continent 

 the species often range widely and almost capriciously ; for 

 two river-systems will have some fish in common and some 

 different. A few facts seem to favor the possibility of their 

 occasi(mal transport by accidental means ; like that of the live 



