Chap. XII. FRESH- WATER PRODUCTIONS. 383 



CHAPTEE XII. 



Geographical Distribution — continued. 



Distribution of fresh-water productions — On the inhabitants of 

 oceanic islands — Absence of Batrachians and of terrestrial Mam- 

 mals — On the relation of the inhabitants of islands to those of 

 the nearest mainland — On colonisation from the nearest source 

 with subsequent modification — Summary of the last and pre- 

 sent chapters. 



As lakes and river-systems are 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 ap- 

 parently a still more impassable barrier, that they 

 never would have extended to distant 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 

 remarkable manner throughout the world. I well re- 

 member, when first collecting in the fresh waters of 

 Brazil, feeling much surprise at the similarity of the 

 fresh-water insects, shells, &c, and at the dissimilarity 

 of the surrounding terrestrial beings, compared with 

 those of Britain. 



But tins power in fresh-water productions of ranging 

 widely, though so unexpected, can, I think, in most 

 cases be explained 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 dispersal would follow 

 from this capacity as an almost necessary consequence. 

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



