428 ORIGIN OF SPECIES 



attenuatus inhabits Tasmania, New Zealand, the Falkland 

 Islands, and the mainland of South America. This is a won- 

 derful case, and probably indicates dispersal from an Ant- 

 arctic centre during a former warm period. This case, how- 

 ever, is rendered in some degree less surprising by the spe- 

 cies 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 con- 

 siderable 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 conse- 

 quently must from an early period have completely prevented 

 the inosculation of the river-system 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. 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 probable, become adapted without much difficulty 

 to the fresh waters of a distant land. 



Some species of fresh-water shells have very wide ranges, 



