sh-water shells have very wide ranges, and allied
species which, on our 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 the ova, as well as the adults,
are immediately killed by sea-water. I could not even understand
how some naturalised species have spread rapidly throughout the same
country. But two facts, which I have observed--and many others no
doubt will be discovered--throw some light on this subject. When ducks
suddenly emerge from a pond covered with duck-weed, I have twice seen
these little plants adhering to their backs; and it has happened to me,
in removing a little duck-weed from one aquarium to another, that I have
unintentionally stocked the one with fresh-water shells from the other.
But another agency is perhaps more effectual: I suspended the feet of a
duck 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
if blown across the sea to an oceanic island, or to any other distant
point, would be sure to alight on a pool or rivulet. Sir Charles Lyell
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 been blown by 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 illustrated,
according to Alph. de Candolle, in those large groups of terrestrial
plants, which have very few aquatic members; for the latter seem
immediately to acquire, as if in consequence, a wide range. I think
favourable means of dispersal explain this fact. I have bef