56 ANIMAL LIFE 



the arm to the toes. By its aid their leap off a branch 

 takes the form of a rise into the air, then a gentle 

 downward curve. Amongst the squirrels two forms 

 of such ' aeroplanes ' have been devised independently, 

 and by the aid of these, flying leaps can be made from 

 tree to tree without descending to the ground. But 

 it is only amongst the bats that true flight has been 

 attained. Like huge moths, these animals come out 

 every summer evening from the ivy, hollow tree, or 

 barns in which they lie hidden by day, hanging down- 

 wards from some projection, often in great clusters, 

 with their head enfolded by the wings. 



These wings are folds of skin stretched from the 

 altered hands and arms to the legs. To increase then- 

 extent the fingers are drawn out to an immense length 

 and the wing-membrane continued to the very tips, 

 leaving only the thumb free. The muscles for flight 

 are strongly developed, and, as in a bird, lead to the 

 formation of a keel-like breast-bone, from which they 

 arise (fig. 8, p. 41). 



The unerring certainty of a bat's flying and alighting 

 movements is a marvel of skill. In a room they 

 explore the furniture and recesses, sail through pas- 

 sages a few inches wide without touching anything, 

 dive under a sofa, and, turning a somersault, alight on 

 the webbing head downwards, ready for the next 

 flight. Then they spring clean into the air, even from 

 a flat surface, and are on the wing at once. 



The inclusion of the hinddimbs in the formation 

 of the wings has the effect of bending the knee out- 



