links. Consequently evidence of their former existence could be found
only amongst fossil remains, which are preserved, as we shall in a
future chapter attempt to show, in an extremely imperfect and
intermittent record.
_On the origin and transitions of organic beings with peculiar habits
and structure_. It has been asked by the opponents of such views as I
hold, how, for instance, a land carnivorous animal could have been
converted into one with aquatic habits; for how could the animal in its
transitional state have subsisted? It would be easy to show that within
the same group carnivorous animals exist having every intermediate
grade between truly aquatic and strictly terrestrial habits; and as
each exists by a struggle for life, it is clear that each is well
adapted in its habits to its place in nature. Look at the Mustela vison
of North America, which has webbed feet and which resembles an otter in
its fur, short legs, and form of tail; during summer this animal dives
for and preys on fish, but during the long winter
it leaves the frozen waters, and preys like other polecats on mice and
land animals. If a different case had been taken, and it had been asked
how an insectivorous quadruped could possibly have been converted into
a flying bat, the question would have been far more difficult, and I
could have given no answer. Yet I think such difficulties have very
little weight.
Here, as on other occasions, I lie under a heavy disadvantage, for out
of the many striking cases which I have collected, I can give only one
or two instances of transitional habits and structures in closely
allied species of the same genus; and of diversified habits, either
constant or occasional, in the same species. And it seems to me that
nothing less than a long list of such cases is sufficient to lessen the
difficulty in any particular case like that of the bat.
Look at the family of squirrels; here we have the finest gradation from
animals with their tails only slightly flattened, and from others, as
Sir J. Richardson has remarked, with the posterior part of their bodies
rather wide and with the skin on their flanks rather full, to the
so-called flying squirrels; and flying squirrels have their limbs and
even the base of the tail united by a broad expanse of skin, which
serves as a parachute and allows them to glide through the air to an
astonishing distance from tree to tree. We cannot doubt that each
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