Chap. xiii. Fresh-water Productions. 343 



CHAPTEE XIII. 



Geographical Distbibution— 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 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 maimer throughout the world. When first collecting 

 in the fresh waters of Brazil, I well remember feeling much sur- 

 prise 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 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. Glinther 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. This case, however, is rendered in some degree less 



