ned all these things could it even
begin to raise the animal in the air. But observe how fallacious is this
argument. Although it is perfectly true that a wing could be of no use
_as a wing_ until sufficiently developed to serve the purpose of flight,
this is merely to say that until it has become a wing it is no use as a
wing. It does not, however, follow that on this account it was of no
prior use for any other purpose. The first modifications of the
fore-limb which ended in its becoming an organ of flight may very well
have been due to adapting it as an organ for increased rapidity of
locomotion of other kinds--whether on land as in the case of its now
degenerated form in the ostrich, or in water as in the case of the
expanded fins of fish. Indeed, we may see the actual process of
transition from the one function to the other in the case of
"flying-fish." Here the progressive expansion of the pectoral fins must
certainly have been always of use for continuously promoting rapidity of
locomotion through water; and thus natural selection may have
continuously increased their development until they now begin to serve
also as wings for carrying the animal a short distance through air.
Again, in the case of the so-called flying squirrels we find the limbs
united to the body by means of large extensions of the skin, so-that
when jumping from one tree to another the animal is able to sustain
itself through a long distance in the air by merely spreading out its
limbs, and thus allowing the skin-extensions to act after the manner of
a parachute. Here, of course, we have not yet got a wing, any more than
we have in the case of the flying-fish; but we have the foundations laid
for the possible development of a future wing, upon a somewhat similar
plan as that which has been so wonderfully perfected in the case of
bats. And through all the stages of progressive expansion which the skin
of the squirrel has undergone, the expansion has been of use, even
though it has not yet so much as begun to acquire the distinctive
functions of a wing. Here, then, there is obviously nothing "prophetic"
in the matter, any more than there was in the case of the swim-bladder
and the lung, or in that of the nerve-ending and the eye. In short, it
is the business of natural selection to secure the highest available
degree of adaptation for the time being; and, in doing this, it not
unfrequently happens that an extreme development of a structure in one