ember, when first collecting in the fresh waters of
Brazil, feeling much surprise at the similarity of the fresh-water insects,
shells, &c., and at the dissimilarity of the surrounding terrestrial
beings, compared with those of Britain.

But this power in fresh-water productions of ranging widely, though so
unexpected, can, I think, in most cases be explained by their having become
fitted, in a manner highly useful to them, for short and frequent
migrations from pond to pond, or from stream to stream; and liability to
wide dispersal would follow from this capacity as an almost necessary
consequence. We can here consider only a few cases. In regard to {384}
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 transport 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 continuous
mountain-ranges, which from an early period must have parted river-systems
and completely prevented 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 explained: 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 consequently 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