d, 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 might travel far along the shores
of the sea, and subsequently become modified and adapted to the fresh
waters of a distant land.

Some species of fresh-water shells have a very wide range, and allied
species, which, on my theory, are descended from a common parent and
must have proceeded from a single source, prevail throughout the world.
Their distribution at first perplexed me much, as their ova are not
likely to be transported by birds, and they are immediately killed
by sea water, as are the adults. I could not even understand how some
naturalised species have rapidly spread throughout the same country. But
two facts, which I have observed--and no doubt many others remain to be
observed--throw some light on this subject. When a duck suddenly emerges
from a pond covered with duck-weed, I have twice seen these little
plants adhering to its back; and it has happened to me, in removing
a little duck-weed from one aquarium to another, that I have quite
unintentionally stocked the one with fresh-water shells from the other.
But another agency is perhaps more effectual: I suspended a duck's feet,
which might represent those of a bird sleeping in a natural pond, in
an aquarium, where many ova of fresh-water shells were hatching; and
I found that numbers