| Some species of fresh-water shells have
very wide
and allied
which, on
theory, are descended from a common
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
and
→are
immediately killed by
→sea water, as
I could not even understand how some naturalised species have
throughout the same country. But two facts, which I have observed— and
many others
→remain to
be
throw some light on this subject. When
suddenly
from a pond covered with duck-weed, I have twice seen these little plants adhering to
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
→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
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
molluscs, though aquatic in their nature, survived on the
feet, in damp air, from twelve to
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
to an oceanic
or to any other distant
→point. Sir Charles Lyell
|
