Chap. XIII.] FEESH- WATER PRODUCTIONS. 173 



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 geographical 

 changes, and consequently time and means for much 

 migration. Moreover Dr. Gunther has recently been 

 led by several considerations to infer that with fishes 

 the same forms have a loDg 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 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 

 descended 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 

 observed — and many others no doubt will be discovered 

 — throw some light on this subject. When ducks 

 suddenly 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 



