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.
Now look at the Galeopithecus or flying lemur, which formerly was
falsely ranked amongst bats. It has an extremely wide flank-membrane,
stretching from the corners of the jaw to the tail, and including the
limbs and the elongated fingers: the flank membrane is, also, furnished
with an extensor muscle. Although no graduated links of structure,
fitted for gliding through the air, now connect the Galeopithecus with
the other Lemurid , yet I can see no difficulty in supposing that such
links formerly existed, and that each had been formed by the same steps
as in the case of the less perfectly gliding squirrels; and that each
grade of structure had been useful to its possessor. Nor can I see any
insuperable difficulty in further believing it possible that the
membrane-connected fingers and fore-arm of the Galeopithecus might be
greatly lengthened by natural selection; and this, as far as the organs
of flight are concerned, would convert it into a bat. In bats which
have the wing-membrane extended from the top of the shoulder to the
tail, including the hind-legs, we perhaps see traces of an apparatus
originally constructed for gliding through the air rather than for
flight.
If about a dozen genera of birds had become extinct or were unknown,
who would have ventured to have
surmised that birds might have existed which used their wings solely as
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