have ranged widely within the same country, and as the sea is
apparently a still more impassable barrier, that they never would have
extended to distant countries. But the case is exactly the reverse. Not
only have many fresh-water species, belonging to quite different
classes, an enormous range, but allied species prevail in a remarkable
manner throughout the world. I well remember, when first collecting in
the fresh waters of Brazil, feeling much surprise at the similarity of
the fresh-water insects, shells, etc., 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
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
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