344 FRESH-WATER PRODUCTIONS. [Chap. XIII 



surprising by the species of this genus having the power of crossing 

 by some unknown means considerable spaces of open ocean : thus 

 there is one species common to New Zealand and to the Auckland 

 Islands, though separated by a distance of about 230 miles. On 

 the same continent fresh-water fish often range widely, and as if 

 capriciously ; for in two adjoining river-systems some of the species 

 may be the same, and some wholly different. It is probable that 

 they are occasionally transported by what may be called accidental 

 means. Thus fishes still alive are not very rarely dropped at distant 

 points by whirlwinds ; and it is known that the ova retain theii 

 vitality for a considerable time after removal from the water. 

 Their dispersal may, however, be mainly attributed to changes in 

 the level of the land within the recent period, causing rivers to flow 

 into each other. Instances, also, could be given of this having 

 occurred during floods, without any change of level. The wide 

 difference of the fish on the opposite sides of most mountain-ranges, 

 which are continuous, and which consequently 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 fonns, and in such cases 

 there will have been ample time for great geographical 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 ol 

 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 proceeded 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 throughout 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 duck-weed from one aquarium to another, that I 



