406 FBESII- WA TER PROD XICTI0N8. 



distant from each other. But Dr. Giinther has lately 

 shown that the Galaxias attenuatus inhabits Tasmania, 

 New Zealand, the Falkland Islands and the mainland of 

 South America. This is a wonderful case, and probably 

 indicates dispersal from an Antarctic center during a 

 former warm period. This case, however, is rendered in 

 some degree less surprising by the species of this genus 

 having the power of crossing by some unknown means con- 

 siderable 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 attribu- 

 ted 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 consequently must, from an early 

 period, have completely prevented the inosculation of the 

 river, systems on the two sides, leads to the same conclu- 

 sion. 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 con- 

 fined 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. 



