g {181} 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 Lemuridae, yet
I 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 was useful to its
possessor. Nor can I see any insuperable difficulty in further believing it
possible that the membrane-connected fingers and forearm 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.
{182}

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 r