322 -FRESH-WATER PRODUCTIONS. [CHAP. X T TL 



equator. The various beings thus left stranded may be compared 

 with savage races of man, driven up and surviving in the moun- 

 tain fastnesses of almost every land, which serves as a record, 

 full of interest to us, of the former inhabitants of the surrounding 

 lowlands. 



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 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 surround- 

 ing terrestrial beings, compared with those of Britain. 



But the wide ranging power of fresh-water productions can, 1 

 think, in most cases be explained by their having become fitted, 

 in a manner highly useful to them, for short and frequent migra- 

 tions 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 Falk- 

 land Islands, and the mainland of South America. This is a 

 wonderful case, and probably indicates dispersal from an Ant- 

 arctic centre during a former warm period. This case, however, 

 is rendered in some degree less surprising by the species of this 

 genus having the power of crossing by some unknown means 



