FRESH-WATER PRODUCTIONS. 405 



CHAPTEE XIII. 

 GEOGRAPHICAL DISTRIBUTION — Continued. 



Distribution of fresh-water productions — On tlie 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 mainlaind — On colonization 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 

 other by barriers of land^ it might have been thought 

 that fresh-water productions vi^ould 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 

 reverse. JSTot only have many fresh-water species, belong- 

 ing 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, etc., and at the dissimilarity 

 of the surrounding 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 

 become 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 necessary 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 



