Chap. XIIL] FRESH- WATER PRODUCTIONS 171 



CHAPTER XIIL 



Geographical Distribution — continued. 



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 

 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 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 remember 

 feeling much surprise at the similarity of the fresh- 

 water insects, shells, &c, 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 



