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subsequently become modified and adapted 1861 1866 1869
subse- quently become modified and adapted 1859 1860
could, it is probable, become adapted without much difficulty 1872

are 1859 1860 1861 1866 1869
ova, as well as the adults, are 1872

sea-water, 1860 1861 1866 1869
sea water, as 1859
sea-water. 1872

remain to 1859 1860 1861 1866 1869
no doubt will 1872

a duck's feet 1861 1866
a duck's feet, which might represent those of a bird sleeping in a natural pond, 1859
a ducks feet, which might represent those of a bird sleeping in a natural pond, 1860
a ducks feet 1869
the feet of a duck 1872

would be sure to alight on a pool or rivulet, 1859 1860 1861 1866
OMIT 1869 1872

point. 1859 1860 1861 1866
point would be sure to alight on a pool or rivulet. 1869
point, would be sure to alight on a pool or rivulet. 1872

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
a
very wide
ranges,
range,
and allied
species,
species
which, on
our
my
theory, are descended from a common
parent
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;
birds,
and
the
they
they
are immediately killed by sea-water,
are
as are
as are
the
the
adults.
adults.
I could not even understand how some naturalised species have
spread rapidly
rapidly spread
throughout the same country. But two facts, which I have observed— and
no doubt
no doubt
many others remain to be
discovered—
observed—
throw some light on this subject. When
a
a
ducks
duck
suddenly
emerge
emerges
from a pond covered with duck-weed, I have twice seen these little plants adhering to
their
its
backs;
back;
and it has happened to me, in removing a little duck-weed from one aquarium to another, that I have
quite
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 in an aquarium, where many ova of fresh-water shells were hatching; and I found that numbers of the extremely minute and
just hatched
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
just-hatched
molluscs, though aquatic in their nature, survived on the
ducks
duck's
feet, in damp air, from twelve to
twenty-hours;
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
the sea
sea
to an oceanic
island,
island
or to any other distant point. Sir Charles Lyell
also
also
informs me that a
Dyticus
Dytiscus