Even more obvious is the change of structure in the case of masts of
vessels, which originally bore the sails for propelling the ship. When
steam engines were employed to give motive power, masts did not disappear.
They now provide the derrick supports of trading steamers; in battleships
their function is changed to that of fighting tops and signal yards. Even
the poles carried by canal boats to bear windmills must be regarded as the
reduced vestiges of masts originally constructed to carry sails; and their
adaptive evolution, like that of countless structures in animals, has been
accomplished by degeneration.

* * * * *

The birds are another class of backboned animals which exhibit identical
principles of relationship. A heron has long legs and wide-spreading toes,
which keep its body out of the water as it stalks about the marshes where
it seeks its food; its bill is a long slender pincers. Compare it with an
eagle; the latter has a short and heavily hooked beak to tear flesh, while
its stout legs bear strongly curved talons to hold its struggling prey.
Swimming birds like the swan and duck and loon possess feet which are
constructed in general like those of the former examples, but they are
webbed and shortened to serve as paddles. In the penguin we find a
counterpart of the seal among mammals; its feathers are much reduced and
its fore limbs are no longer wings enabling the animal to fly, but they
are paddles which it uses when it swims in pursuit of fish. Finally the
ostrich and wingless bird of New Zealand--the _Apteryx_--have wings that
are useless vestiges, which, in the latter case, are hidden under the
brushlike feathers covering the body. It is unnecessary to add more
examples, for even these few illustrations establish exactly the same
principles of relationship and evidences of evolution that are to be found
in the series of mammalia.