| 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,
to collect food more quickly, or, as there is reason to believe,
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
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,
by the accumulated effects of this process of natural selection, a perfect so-called flying squirrel was produced. |
| Now look at the Galeopithecus or
lemur, which formerly was
ranked amongst
→bats, but is now believed to belong to the Insectivora.
→An
extremely wide
from the corners of the jaw to the tail, and
the limbs
the elongated
This flank-membrane is furnished with an extensor muscle. Although no graduated links of
fitted for gliding through the air, now connect the Galeopithecus with the other
yet
→there is
no difficulty in supposing that such links formerly existed, and that each
→was developed in
the same
as
the
→OMIT
less perfectly gliding squirrels;
each grade of structure
useful to its possessor. Nor can I see any insuperable difficulty in further believing
that the
fingers and
of the Galeopithecus might
greatly lengthened by natural
and this, as far as the
|
