CHAPTER XIII 

 Geographical Distribution — contimied 



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 present 

 chapter. 



FRESH-WATER PRODUCTIONS 



AS lakes and river-systems are separated from each 

 l\ 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 formidable barrier, that they would never have 

 extended to distant countries. But the case is exactly the re- 

 verse. Not only have many fresh-water species, belonging 

 to different classes, an enormous range, but allied species 

 prevail in a remarkable manner throughout the world. When 

 first collecting in the fresh waters of Brazil, I well remember 

 feeling much surprise at the similarity of the fresh-water 

 insects, shells, &c., and at the dissimilarity of the surroimd- 

 ing terrestrial beings, compared with those of Britain. 



But the wide ranging power of fresh-water productions 

 can, I think, in most cases be explained by their having be- 

 come fitted, in a manner highly useful to them, for short 

 and frequent migrations from pond to pond, or from stream 

 to stream, within their own countries; and liability to wide 

 dispersal would follow from this capacity as an almost neces- 

 sary consequence. We can here consider only a few cases; 

 of these, some of the most difficult to explain are presented 

 by fish. It was formerly believed that the same fresh-water 

 species never existed on two continents distant from each 

 other. But Dr. Giinther has lately shown that the Galaxias 



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