inued
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 Lemuridae, 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 flappers, like the logger-headed duck
(Micropterus of Eyton); as fins in the water and front legs on the land,
like the penguin; as sails, like the ostrich; and functionally for no
purpose, like the Apteryx. Yet the structure of each of these birds is
good for it, under the conditions of life to which it is exposed, for
each has to live by a struggle; but it is not necessarily the best
possible under all possible conditions. It must not be inferred from
these remarks that any of the grades of wing-structure here alluded to,
which perhaps may all have resulted from disuse, indicate the natural
steps by which birds have acquired their perfect power of flight; but
they serve, at least, to show what diversified means of transition are
possible.

Seeing that a few members of such water-breathing classes as the
Crustacea and Mollusca are ad