CHAP XIII.] FRESH- WATER PRODUCTIONS. 323 



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 their 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 oppo- 

 site 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 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. Giinthor 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 *~o 

 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 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 dis- 

 covered 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 have unintentionally stocked the one with fresh- 

 water shells from the other. But another agency is perhaps more 

 effectual : I suspended the feet of a duck in an aquarium, where 



