bject with its wings, while a man's hand is well
adapted for the performance of such a function. Nature's motto in her
whole realm seems to be: "Every creature after its kind."

When we look at the structure of the flying birds, we see at once that
they were formed for swift locomotion through the air, just as plainly
as the lithe skiff was made to glide over the water or the carriage to
spin over the land. In the first place, the body of the bird is
comparatively light--that is, in proportion to the width, strength, and
extent of its wings. By its thick, light, airy covering of feathers
its body is made still more buoyant, besides presenting a larger
surface to the supporting air with very little additional weight. The
tail, too, with its long, closely woven quills spread out like a fan,
not only serves the purpose of a rudder for guiding the aerial craft,
but is still more useful in helping to sustain the bird's weight in the
up-buoying element.

It is interesting to note that the feathers on the bodies of the flying
birds are arranged in tracts, with intervals here and there of quite,
or almost, bare skin, called "apteria." Now, when a bird is carefully
skinned, it will be seen that the feathered spaces have their own
special slips of muscles inserted into the roots of the feathers, and
when these muscles are contracted, they serve to raise the feathers,
and must, therefore, be of some subsidiary value in flying, by making
the bird's body more buoyant. Suggestive, indeed, is the fact that the
plumes of the non-flyers are not arranged in tracts, but are evenly
distributed over the body.

Nor is that all that Nature has done to carry out her evident purpose
of making the bird a natural "flying machine." The body of the bird
contains numerous air sacs, all connected with the lungs, and these,
when inflated, are a great help in flying by making the bird light.
More than that, many of the bones, though strong, have thin walls and
are hollow, the cavities being connected with the lungs and air sacs,
from which they are also filled with air, contributing another element
of lightness to the aerial navigator. That the bird's bones are
capable of being permeated with air can be demonstrated by actual
experiment, and is, therefore, a scientifically established fact. It
is easy enough to prove it in this way: Take a dead bird that has been
beheaded, pass a syringe into its windpipe, tie it carefully so that
the air c