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Teoria ta powstała w odpowiedzi na pytanie, jak to się dzieje, że istnieje naprawdę niewiele
zbiorników wodnych, w których nie występują wyższe formy życia (niż bakterie, grzyby
czy pierwotniaki), pomimo, że zbiorniki te nie mają stałego ani okresowego połączenia z
większym systemem wodnym, a czasem nawet są mocno izolowane (jeziorka wulkaniczne,
wysokogórskie itp). Teoria ta przemawia do mnie dużo bardziej, niż teoria tłumacząca ten
fakt udziałem trąb powietrznych, a jest i taka!
Myślę, że warto zacytować tu klasyka:
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.
Karol Darwin, O pochodzeniu gatunków (1859), Rozdział XII - Geographical Distribution.
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