180 THE ORIGIN OF SPECIES 



CHAPTER XIII 



GEOGRAPHICAL DISTRIBUTION — continued 



Distribution of fresh-water productions — On the inhabitants of oceanic 

 islands — Absence of Batrachians and of terrestrial Manamals — On the 

 relation of the inhabitants of islands to those of the nearest mainland — 

 On colonization from the nearest source with subsequent moditication 

 — Summary of the last and present chapters 



Fresh- luater Productions 



AS lakes and river-s3^stems are separated from each 

 other by barriers of land, it might liave 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 coun- 

 tries. But the case is exactly the reverse. 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 re- 

 member feeling much surprise at the similarity of the 

 fresh-water insects, shells, etc., and at the dissimilarity 

 of the surrounding terrestrial beings, compared with thoa 

 of Britain. 



But the wide-ranging power of fresh-water productio 

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

 become fitted, in a manner highly useful to them, for 

 short and frequent migrations from pond to pond, or 



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