I 



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 Modifica- 

 tion — Summary of the Last and Present Chapters. 



FRESH-WATER PRODUCTIONS 



As lakes and river systems are separated from each other by bar- 

 riers of land, it might have been thought that fresh-water produc- 

 tions 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 be- 

 longing 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, 

 etc., 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 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 won- 

 derful case, and probably indicates dispersal from an antarctic 

 centre during a former warm period. This case, however, is ren- 

 dered in some degree less surprising by the species of this genus 



351 



