_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.--p. 181.
For instance, a swim-bladder has _apparently_ been converted into an
air-breathing lung.--p. 181.