, 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, to lessen 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 possible 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 become 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.

Now look at the Galeopithecus or so-called flying lemur, which was
formerly ranked among bats, but is now believed to belong to the
Insectivora. An extremely wide flank-membrane stretches from the corners
of the jaw to the tail, and includes the limbs with the elongated
fingers. This flank-membrane is furnished with an extensor muscle.
Although no graduated links of structure, fitted for gliding through the
air, now connect the Galeopithecus with the other Insectivora, yet there
is no difficulty in supposing that such links formerly existed, and that
each was developed in the same manner as with the less perfectly gliding
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