ance, to confine our
attention to the arm, not only is the limb modified in the whale for
swimming, but in another mammal--the bat--it is modified for flying, by
having the fingers enormously elongated and overspread with a membranous
web.

In birds, again, the arm is modified for flight in a wholly different
way--the fingers here being very short and all run together, while the
chief expanse of the wing is composed of the shoulder and fore-arm. In
frogs and lizards, again, we find hands more like our own; but in an
extinct species of flying reptile the modification was extreme, the wing
having been formed by a prodigious elongation of the fifth finger, and a
membrane spread over it and the rest of the hand. (Fig. 5.) Lastly, in
serpents the hand and arm have disappeared altogether.

[Illustration: FIG. 5.--Wing of Reptile, Mammal, and Bird. Drawn
from nature (_Brit. Mus._).]

Thus, even if we confine our attention to a single organ, how wonderful
are the modifications which it is seen to undergo, although never losing
its typical character. Everywhere we find the distinction between
homology and analogy which was explained in the last chapter--the
distinction, that is, between correspondence of structure and
correspondence of function. On the one hand, we meet with structures
which are perfectly homologous and yet in no way analogous: the
structural elements remain, but are profoundly modified so as to perform
wholly different functions. On the other hand, we meet with structures
which are perfectly analogous, and yet in no way homologous: totally
different structures are modified to perform the same functions. How,
then, are we to explain these things? By design manifested in special
creation, or by descent with adaptive modification? If it is said by
design manifested in special creation, we must suppose that the Deity
formed an archetypal plan of certain structures, and that he determined
to adhere to this plan through all the modifications which those
structures exhibit. But, if so, why is it that some structures are
selected as typical and not others? Why should the vertebral skeleton,
for instance, be tortured into every conceivable variety of modification
in order to subserve as great a variety of functions; while another
structure, such as the eye, is made in different sub-kingdoms on
fundamentally different plans, notwithstanding that it has throughout to
perform the same function? Will any one have