Chap. XIIL] FRESH-WATER PRODUCTIONS. 171 



CHAPTEE XIII. 



GEOGRAPHICAL DiSTKiBUTiON — Continued. 



Distribution of fresh-water productions — On the inhabitants of 

 oceanic islands — Absence of Batraehians 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 

 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 formidable barrier, that they would 

 never have extended to distant countries. But the 

 case is exactly the reverse. Not only have many fresh- 

 water species, belonging to different classes, an enor- 

 mous 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 in- 

 sects, shells,' &c., and at the dissimilarity of the sur- 

 rounding terrestrial beings, compared with those of 

 Britain. 



But the wide ranging power of fresh-water produc- 

 tions can, I think, in most cases be explained by their 

 having become fitted, in a manner highly useful to 



