Chap.XIII.] FRESH-WATER PRODUCTIONS. 1^3 



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 con- 

 fined to fresh water, so that a marine species belonging 

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

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

 without much difficulty 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 pro- 

 ceeded from a single source, prevail throughout the 

 world. Their distribution at first perplexed me much, 

 as their ova are not likely to be transported by birds; 

 and the ova, as well as the adults, are immediately 

 killed by sea-water. I could not even understand how 

 some naturalised species have spread rapidly through- 

 out the same country. But two facts, which I have ob- 

 served — and many others no doubt will be discovered — 

 throw some light on this subject. When ducks sudden- 

 ly emerge from a pond covered with duck-weed, I have 

 twice seen these little plants adhering to their backs; 

 and it has happened to me, in removing a little duck- 



