182 THE ORIGIN OF SPECIES 



mountain-ranges, which are continuous, and which con- 

 sequently must from an early period have completely 

 prevented the inosculation of the river-systems on the 

 two sides, leads to the same conclusion. Some fresh- 

 water fish belong to very ancient forms, and in such 

 cases there will have been ample time for great geo- 

 graphical changes, and consequently time and means for 

 much migration. Moreover Dr. Giinther has recently 

 been led by several considerations to infer that with ' 

 fishes the same forms have a long endurance. Salt 

 water fish can with care be slowly accustomed to live ; 

 in fresh water; and, according to Valenciennes, there 

 is hardly a single group of which all the members are 

 confined to fresh water, so that a marine species belong- 

 ing to a fresh -water group might travel far along the 

 shores of the sea, and could, it is probable, become 

 adapted without much difiiculty to the fresh waters of 

 a distant land. 



Some species of fresh-water shells have very wide 

 ranges, and allied species which, on our theory, are de- 

 scended from a common parent, and must have proceeded 

 from a single source, prevail throughout the world. 

 Their distribution at first perplexed me much, as thei 

 ova are not likely to be transported by birds; and th 

 ova, as well as the adults, are immediately killed b 

 sea-water. I could not even understand how some nat 

 uralized species have spread rapidly throughout the sam( 

 country. But two facts, which I have observed — an 

 many others no doubt will be discovered — throw somi 

 light on this subject. When ducks suddenly emergi 

 from a pond covered with duck-weed, I have twid 

 seen these little plants adhering to their backs; and 



m 



