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
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 competing rodents or new beasts of prey immigrate, or old ones
become modified, and all analogy would lead us to believe that some at
least of the squirrels would decrease in numbers or become exterminated,
unless they also became modified and improved in structure in a
corresponding manner. Therefore, I can see no difficulty, more
especially under changing conditions of life, in the cont