384 
GEOGKAPHICAL DISTEIBUTION. 
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 Ehine 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- 
