54 MAMMALS

CHIROPTERA.—In the zoological series the
next order is that of the bats, or Chzroptera, the
hand-winged mammals, the only mammals that have
the power of true flight. The hands and arms are
large in proportion to the rest of the frame, and very
light in structure, the bones being remarkably hollow
and long and slender. Were those of a man to be in
the same proportion, his fathom—that is, the distance
he can stretch from finger-tip to finger-tip—would
measure thirty feet instead of six. The wing is an
expansion of the skin, stretching generally from the
shoulder along the arms and fingers, and so down to
the hind legs or the tail. The thumb is always free,
and carries the claw by which the bat climbs ; the
toes are always free, and are all clawed. The most
peculiar thing in the skeleton is the knee-joint, which
is turned backwards like the. elbow, so that a bat
cannot walk readily, and rarely settles on the ground.
Another point worth notice is the weakness of the
hip-girdle compared with that of the shoulders, whose
strength bears eloquent witness to the immense power
necessary for sustained flight. No mammal has so
highly developed a skin as a bat ; it is so richly pro-
vided with nerve filaments that the sense of touch
must be many times greater than that of any other
vertebrate. As a matter of fact, a bat deprived
of sight and smell and hearing has been found
capable of flying easily about a room without knock-
ing against a maze of silken threads, which had been
stretched across it so as to leave spaces only just
large enough for his open wings to pass. This power
of discovering the proximity of objects without sce-