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
continued preservation of individuals with fuller and fuller
flank-membranes, each modification being useful, each being
propagated, until by the accumulated effects of this process of
natural selection, a perfect so-called flying squirrel was produced.
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