FRESH-WATER PRODUCTIONS. S8l 



This case, however, is rendered in some degree less sur- 

 prising 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 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, with- 

 out any change of level. The wide differences of the fish 

 on the opposite sides of most mountain-ranges, which are 

 continuous and 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 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 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 rap- 



