THE LIVING BIRD 9 



point we shall have occasion to return in more detail 

 later. Obviously, this stability of body tempera- 

 ture leaves birds free to move whither they will, 

 other things being equal. It removes a specific 

 restriction and gives them that unlimited freedom 

 they could not otherwise enjoy. 



With temperature limitations virtually removed, 

 birds have been able to utilize to the full their chief 

 anatomical peculiarity, exceptionally perfect organs 

 of flight. Flight is not confined to birds and inci- 

 dentally not all birds can fly, but, apart from 

 insects, no other group of animals can boast mastery 

 of the air on a wholesale scale. Among other 

 vertebrates, bats only can truly fly, although certain 

 members of all other groups, fish, amphibia, reptiles 

 and mammals have learned to glide. Flying rep- 

 tiles, extinct long since have, however, existed in 

 the past. The earliest birds were little more than 

 this themselves, but they no doubt already had the 

 advantage, to some extent at least, of controlled 

 temperature. 



It is hardly necessary to go into details of wing 

 structure. Sufiice it merely to point out that while 

 in bats the fingers of the hand are enormously 

 elongaged with a patagium spread over them which 

 reaches back to the tail, in birds the fingers are 

 reduced in number as well as size, and numerous 



