ADAPTATIONS OF BIRDS 57 



wards and backwards, instead of forwards as in 

 all other mammals, and this, together with the pre- 

 occupation of all the hand but the thumb, renders 

 walking difficult. On a flat surface some bats cannot 

 walk ; but others, such as the long-eared bat, progress 

 in the usual way — first a hind-foot is advanced, then the 

 fore-foot of the same side, followed by similar move- 

 ments of the opposite limbs. 



IV. The adaptations of birds for flight and 

 perching. — But it is amongst birds that we encounter 

 the greatest freedom and most sustained powers of 

 movement. To gain the dominion of the air by the 

 use of wings, whilst yet retaining the power of balancing 

 on the two legs, is a double problem of the utmost 

 difficulty, and in the solution every part of a bird's 

 body has undergone some adaptive change. What a 

 feat this is we may judge on considering the imperfect 

 attempts man has made towards constructing an 

 efficient flying-machine, though he has tried for a 

 century. But in a bird, so little cumbrous is the 

 mechanism that its sails can be folded evenly with the 

 contour of the body ; so efficient, that flight at express- 

 train speed and sailing in great winding circles with 

 outstretched, motionless wings are both performed 

 with the same easy mastery as perching or hopping ; 

 so well guided, that migratory birds sweep through 

 immense tracts of air and reach their destination with a 

 punctuality so sure that the Persian calendar is based 

 on their arrival.. 



The general shape of a bird fits it for rapid flight. 



