Descriptions of East Asiatic Mammals 67 
quired unusual rigidity; others, like the finger joints, unusual 
elasticity. A Bat's wing has been produced by greatly lengthen- 
ing the bones of the hand and fingers to act as a framework on 
which is stretched the elastic skin of the hand. The skin of this 
hand-wing is continued along the sides of the body to the hind 
limbs, and usually to the tail. In this amazing transformation 
the thumb is the only digit to remain relatively unaltered; it is 
provided with a hook-like claw which enables the animal to 
cling to supports or drag itself along flat surfaces. 
Many changes in the body have accompanied this transforma- 
tion of hands into wings. Among them are great development 
of the powerful chest muscles that provide the motive power for 
the wings, the alteration of the hind legs and feet into mecha- 
nisms to hang by, the evolution of an assortment of special 
organs related to the senses of hearing, smell, and touch. 
The hind legs have rotated at the hips to such an extent that 
the knees bend backward. This helps to control the movement of 
those parts of the wing membranes which stretch from the 
"little fingers" to the ankles, and it also plays a part in the Bats' 
usual resting posture, hanging upside down by means of the five 
long hooked claws of the hind feet. 
The ears of many sorts of Bats are greatly lengthened or 
broadened, or both. In some they are joined together by a 
fleshy band above the crown of the head. Those of some of the 
Big-eared Bats are strengthened by rib-like folds and cartilages 
and can be partly folded. 
Bats have made all sorts of changes in their tails. Some have 
lost their tails ; others have them still, though greatly shortened; 
one family has the tail long, thin, and mouse-like. In most kinds 
it is combined in various ways with the flying membranes. 
The ability to fly carries with it certain penalties. The pro- 
found alteration of almost every part of the body needed to con- 
vert a running animal into a flying animal allows only awkward 
movements on the ground. The hind legs, as already stated, are 
