CHAPTER XII 

 geographical distbibution — continued 



Distribution of fresh - water productions — On the inhabitants oil 

 oceanic islands — Absence of Batrachians and of terrestrial 

 Mammals — 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 

 present 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 id 

 apparently a still more impassable barrier, that they 

 never would have extended to distant countries. But 

 the case is exactly the reverse. Not only have manyl 

 fresh-water species, belonging to quite different classes! 

 an enormous range, but allied species prevail in a 

 remarkable 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 

 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 fre- 

 quent migrations from pond to pond, or from stream 

 to stream ; and liability to wide dispersal would follow 



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