Some species of fresh-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.
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