Twain remarked about a dachshund that it seemed to want another pair of
legs in the middle to prevent it sagging. Now, some lizards are so long
that they cannot keep from sagging, and their progress becomes a painful
wriggle. But if you must go by wriggling, then what is the use of legs
to knock against stems and stones? So some lizards have discarded two of
their legs and some all four. Zoologically they are not snakes, but
snakes are only a further advance in the same direction. That snakes did
not start fair without legs is clear, for the python has to this day two
tell-tale leg-bones buried in its flesh.

When we pass from reptiles to birds, lo! an astounding thing has
happened. That there were flying reptiles in the fossil ages we know,
and there are flying beasts in our own. But the wings of these are
simple mechanical alterations, which the imagination of a child, or a
savage, could explain.

The hands of a bat are hands still, and, though the fingers are hampered
by their awkward gloves, the thumbs are free. The giant fruit bats of
the tropics clamber about the trees quite acrobatically with their
thumbs and feet.

That Apollyonic monster of the prime, the pterodactyl, did even better.
Stretching on each little finger a lateen sail that would have served to
waft a skiff across the Thames, it kept the rest of its hands for other
uses. But what bearing has all this on the case of birds? Here is a
whole sub-kingdom, as they call it, of the animal world which has
unreservedly and irrevocably bartered one pair of its limbs for a
flying-machine. The apparatus is made of feathers--a new invention,
unknown to amphibian or saurian, whence obtained nobody can say--and
these are grafted into the transformed frame of the old limbs. The