522 
MAMMALIA. 
femoral membrane ; but it never reaches the toes of tbe feet, 
which are short, and have nails. 
It is owing to this membranous sail that Bats direct their 
course through the air in the same manner as Birds. When they 
are at rest, they fold their wings around them, enveloping their 
bodies as if in a mantle, similar to our closing an umbrella to 
diminish its volume when it is no longer required. This com- 
parison is still more exact when we observe that the long filiform 
digits of the animal perfectly correspond to the ribs or rods of the 
umbrella. 
After what has been said, it will be understood that Bats are 
not adapted for terrestrial locomotion. Their movements have 
been observed, and the following has been remarked. When 
they wish to move on the ground, they project as far as 
possible the hooked nail that terminates one of their thumbs, 
and implant it in the ground; then, exercising a degree of 
traction of this point, it draws the body forward in the same 
direction by the play of the arm muscles, at the same time the 
posterior limbs act from behind, to aid this movement. The 
other thumb then executes the same manoeuvre, and the body 
advances at the same rate, but no longer in the same direction. 
It is easy to see that the creature is carried now to the right, then 
to the left, according as it fixes itself on one or other of its thumbs, 
and in this way the Bat walks, not in a straight line, but in a 
series of zigzags, in which the axis of the animal represents the 
real direction. 
An English naturalist, White, who had studied Bats in cap- 
tivity, wrote in error against the common opinion that they only 
moved on the ground with much difficulty. He stated that, on 
the contrary, they could run with considerable rapidity. Despite 
this affirmation, we refuse to believe in the agility of the Cheirop- 
tera, and to look upon them as walking Mammals ; therefore we 
think that there is exaggeration in the statement of the English 
naturalist. 
It is certain that Bats do not descend to the ground under 
ordinary circumstances. Besides the reason we have already 
given for this, there is another, which is that when placed on 
the ground they find themselves in a very inconvenient position 
to resume their flight. Their case is then almost the same as 
