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 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
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
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