CHEIROPTERA, OR BATS. 283 



be an air trespasser. In human beings the fore arm 

 is composed of two bones, called the radius and ulna, 

 and by their means the hand is able to rotate and 

 execute the different movements for which it was 

 designed. In the Bat, however, this rotating power 

 is not only needless, but would be absolutely injurious, 

 as it would make the limb incapable of acting as an 

 instrument of flight. The ulna, therefore, is prac- 

 tically absent, being represented only by a very tiny 

 piece of bone pressed against the radius, and, as a 

 consequence, the hand cannot rotate. 



The reader will now perceive that there is no 

 necessity for these bones to be hollow like those of 

 the birds, their great attenuation giving them the 

 requisite lightness. No new structure is therefore 

 required in order to convert a mammal into a winged 

 creature, a slight modification of existing structures 

 being amply sufficient for the purpose. A further 

 adaptation is to be seen in the thorax, which is very 

 large in proportion to the animal, and has the upper 

 portion of the breast bone greatly developed, so as to 

 afford an attachment to the powerful muscles which 

 are needed to move the wing. 



The next point is to furnish the developed limb 

 with a wide, flat, and light substance that will take 

 the place of the bird's feathers. This is managed in 

 an equally simple manner. In the first place, the toes 

 or fingers are webbed to the tips, like the feet of 

 frogs. This, however, would not afford a sufficient 

 surface, and so the webbing, if it may be so called, 

 extends from the thumb to the neck, and from the 

 fingers to the feet, so that when the limbs are ex- 



