the bird's
wing. But the bird's patagium is unimportant, and the bird's wing is on
an evolutionary tack of its own--a fore-limb transformed for bearing the
feathers of flight. Feathers are in a general way comparable to the
scales of reptiles, but only in a general way, and no transition stage
is known between the two. Birds evolved from a bipedal Dinosaur stock,
as has been noticed already, and it is highly probable that they began
their ascent by taking running leaps along the ground, flapping their
scaly fore-limbs, and balancing themselves in kangaroo-like fashion with
an extended tail. A second chapter was probably an arboreal
apprenticeship, during which they made a fine art of parachuting--a
persistence of which is to be seen in the pigeon "gliding" from the
dovecot to the ground. It is in birds that the mastery of the air
reaches its climax, and the mysterious "sailing" of the albatross and
the vulture is surely the most remarkable locomotor triumph that has
ever been achieved. Without any apparent stroke of the wings, the bird
sails for half an hour at a time with the wind and against the wind,
around the ship and in majestic spirals in the sky, probably taking
advantage of currents of air of different velocities, and continually
changing energy of position into energy of motion as it sinks, and
energy of motion into energy of position as it rises. It is interesting
to know that some dragon-flies are also able to "sail."

The web-wing of bats involves much more than the fore-arm. The double
fold of skin begins on the side of the neck, passes along the front of
the arm, skips the thumb, and is continued over the elongated palm-bones
and fingers to the sides of the body again, and to the hind-legs, and to
the tail if there is a tail. It is interesting to find that the bones of
the bat's skeleton tend to be lightly built as in birds, that the
breast-bone has likewise a keel for the better insertion of the pectoral
muscles, and that there is a solidifying of the vertebrae of the back,
affording as in birds a firm basis for the wing action. Such similar
adaptations to similar needs, occurring in animals not nearly related to
one another, are called "convergences," and form a very interesting
study. In addition to adaptations which the bat shares with the flying
bird, it has many of its own. There are so many nerve-endings on the
wing, and often also on special skin-leaves about the ears and nose,
that the bat flying