384 GEOGRAPHICAL DISTRIBUTION. Chap. XII. 



fish, I believe that the same species never occur in 

 the fresh waters of distant continents. But on the 

 same continent the species often range widely and 

 almost capriciously ; for two river-systems will have 

 some fish in common and some different. A few facts 

 seem to favour the possibility of their occasional trans- 

 port by accidental means ; like that of the live fish not 

 rarely dropped by whirlwinds in India, and the vitality 

 of their ova when removed from the water. But I am 

 inclined to attribute the dispersal of fresh-water fish 

 mainly to slight changes within the recent period in 

 the level of the land, having caused rivers to flow into 

 each other. Instances, also, could be given of this 

 having occurred during floods, without any change of 

 level. We have evidence in the loess of the Rhine of 

 considerable changes of level in the land within a very 

 recent geological period, and when the surface was 

 peopled by existing land and fresh-water shells. The 

 wide difference of the fish on opposite sides of con- 

 tinuous mountain-ranges, which from an early period 

 must have parted river-systems and completely pre- 

 vented their inosculation, seems to lead to this same 

 conclusion. With respect to allied fresh-water fish 

 occurring at very distant points of the world, no doubt 

 there are many cases which cannot at present be ex- 

 plained: but some fresh- water fish belong to very 

 ancient forms, and in such cases there will have been 

 ample time for great geographical changes, and con- 

 sequently time and means for much migration. In 

 the second place, 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 fishes 

 confined exclusively to fresh water, so that we may 

 imagine that a marine member of a fresh- water group 

 might travel far along the shores of the sea, and subse- 



