cessarily
correlated with such useful variations.

[Illustration: WINGBONES OF PTERODACTYLE, BAT, AND BIRD.
(_Copied, by permission, from Mr. Andrew Murray's "Geographical
Distribution of Mammals."_)]

On this theory the chances are almost infinitely great against the
independent, accidental occurrence and preservation of two similar series
of minute variations resulting in the independent development of two
closely similar forms. In all cases, no doubt (on this same theory), _some_
adaptation to habit or need would gradually be evolved, but that {64}
adaptation would surely be arrived at by different roads. The organic world
supplies us with multitudes of examples of similar functional results being
attained by the most diverse means. Thus the body is sustained in the air
by birds and by bats. In the first case it is so sustained by a limb in
which the bones of the hand are excessively reduced, but which is provided
with immense outgrowths from the skin--namely, the feathers of the wing. In
the second case, however, the body is sustained in the air by a limb in
which the bones of the hand are enormously increased in length, and so
sustain a great expanse of naked skin, which is the flying membrane of the
bat's wing. Certain fishes and certain reptiles can also flit and take very
prolonged jumps in the air. The flying-fish, however, takes these by means
of a great elongation of the rays of the pectoral fins--parts which cannot
be said to be of the same nature as the constituents of the wing of either
the bat or the bird. The little lizard, which enjoys the formidable name of
"flying-dragon," flits by means of a structure altogether peculiar--namely,
by the liberation and great elongation of some of the ribs which support a
fold of skin. In the extinct pterodactyles--which were _truly_ flying {65}
reptiles--we meet with an approximation to the structure of the bat, but in
the pterodactyle we have only one finger elongated in each hand: a striking
example of how the very same function may be provided for by a modification
similar in principle, yet surely manifesting the independence of its
origin. When we go to lower animals, we find flight produced by organs, as
the wings of insects, which are not even modified limbs at all; or we find
even the function sometimes subserved by quite artificial means, as in the
aerial spiders, which use their own threads to float with in the air. In
the vegetable kingdom the atmosph