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 squirrels; each grade of
structure having 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 have
been greatly lengthened by natural selection; and this, as far as the
organs of flight are concerned, would have converted the animal into a bat.
In certain bats in which the wing-membrane extends from the top of the
shoulder to the tail and includes the hind-legs, we perhaps see traces of
an apparatus originally fitted for gliding through the air rather than for
flight.
If about a dozen genera of birds were to become extinct, who would have
ventured to surmise 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 as 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.
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