al 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 {180} 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 structure is of use to each kind of
squirrel in its own country, by enabling it to escape birds or beasts of
prey, or to collect food more quickly, or, as there is reason to believe,
by lessening the danger from occasional falls. But it does not follow from
this fact that the structure of each squirrel is the best that it is
possible to conceive under all natural conditions. Let the climate and
vegetation change, let other competin