280 FRESH-WATER PRODUCTIONS. 



CHAPTER XIII. 

 geographical distribution — continued. 



Distribution of Fresh-water Productions — On the Inhabitants of 

 Oceanic Islands — Absence of Batrachians and of Terrestrial 

 Mammals — On the Relation of the Inhabitants of Islands to 

 those of the Nearest Mainland — On Colonization from the Nearest 

 Source with Subsequent Modification — Summary of the Last and 

 Present Chapters. 



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 dif- 

 ferent 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 surround- 

 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 

 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 distant 

 from each other. But Dr. Giinther has lately shown that 

 the Galaxias attenuatus inhabits Tasmania, New Zealand, 

 the Falkland Islands, and the mainland of South America. 

 This is a wonderful case, and probably indicates dispersal 

 from an antarctic centre during a former warm period. 



