172 FEESH-WATEU PEODTJCTIONS. [Chap. XUI. 



short and frequent migrations from pond to pond, or 

 from stream to stream, within their own countries ; 

 and liability to wide dispersal would follow from this 

 capacity as an almost necessary consequence. We can 

 here consider only a few cases ; of these, some of the 

 most difficult to explain are presented by fish. It was 

 formerly believed that the same fresh-water species 

 never existed on two continents distant from each other. 

 But Dr. Giinther has lately shown that the Galaxias 

 attenuates inhabits Tasmania, jSTew Zealand, the Falk- 

 land Islands, and the mainland of South America. This 

 is a wonderful case, and probably indicates dispersal 

 from an Antarctic centre 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 considerable 

 spaces of open ocean : thus there is one species common 

 to Xew 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 opposite sides of most 



