might travel far along
the shores of the sea, and {385} 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 duckweed 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 of the extremely minute and just-hatched
shells crawled on the feet, and clung to them so firmly that when taken out
of the water they could not be jarred off, though at a somewhat more
advanced age they would voluntarily drop off. These just hatched molluscs,
though aquatic in their nature, survived on the duck's feet, in damp air,
from twelve to twenty hours; and in this length of time a duck or heron
might fly at least six or seven hundred miles, and would be sure to alight
on a pool or rivulet, if blown across sea to an oceanic island or to any
other distant point. Sir Charles Lyell also {386} informs me that a Dyticus
has been caught with an Ancylus (a fresh-water shell like a limpet) firmly
adhering to it; and a water-beetle of the same family, a Colymbetes, once
flew on board the 'Beagle,' when forty-five miles distant from the nearest
land: how much farther it might have flown with a favouring gale no one can
tell.

With respect to plants, it has long been known what enormous ranges many
fresh-water and even marsh-species have, both over continents and to the
most remote oceanic islands. This is strikingly shown, as remarked by Alph.
de Candolle, in large group