352 THE ORIGIN OF SPECIES 



having the power of crossing by some unknown means consid- 

 erable 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 re- 

 moval 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 differences of the fish on the opposite 

 sides of most mountain-ranges, which are continuous and conse- 

 quently 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 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 belonging to a fresh-water group 

 might travel far along the shores of the sea, and could, it is prob- 

 able, 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 naturalized species 

 have spread rapidly throughout the same country. But two facts, 

 which I have observed — and many others no doubt will be discov- 

 ered — 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 



