THE VERTEBRATES 429 



BIRDS 



354. Adaptations of Birds. No group of animals is so 

 easily defined as that of the birds, for even a small child 

 knows that an animal with feathers is a bird. 



Their most important adaptations are those connected 

 with fitting the wings, legs, and bills for locomotion and for 

 obtaining food. The whole structure of birds' bodies is 

 arranged in adaptation to flying. The general outline of 

 the body, the peculiar structure of wings, the great develop- 

 ment of internal air-sacs connected with the respiratory 

 organs all are specialized with reference to aerial loco- 

 motion. 



However, adaptation of anterior limbs for flying is not 

 limited among vertebrates to birds, for there were ancient 

 flying reptiles (Pterodactyls), and bats are flying mammals. 

 The so-called " flying fishes " do not really fly, but simply 

 use their large fins for gliding through the air for relatively 

 short distances when they leap from the water. 



Bats and birds, then, are the only living vertebrates 

 able to fly, and the great differences in their structures 

 make it certain that they have developed independently. 

 Only the birds among vertebrates have proved perfectly 

 adapted to life in the air. That they are well fitted ' is 

 shown by their success in developing more species than any 

 other group of vertebrates, and also countless numbers of 

 individuals. 



Biologists who have studied the flight of birds cannot help 

 marveling at their locomotor mechanism. It is simply 

 astounding that an animal as large as a homing pigeon can 

 fly faster than a limited express train, and average such speed 

 from daylight till dark. The recent attempts at perfecting 

 flying machines have made us wonder more than ever at the 

 flying power of the birds. 



It is interesting to note that the largest and heaviest birds 



