A HISTORY OF BIRDS 



CHAPTER I 



INTRODUCTORY 



General characters of structure — feathers, glands, the skeleton, the respira- 

 tory system, the lungs and air-sacs, the digestive system, the circulatory system, 

 the muscular system, the nervous system, the senses. 



Birds, in Broad Outline 



THOUGH the bird and the mammal are alike descend- 

 ants of the scaly reptile, they present no points in 

 common, save that both are " warm-blooded ". Birds 

 are essentially creatures of the air, mammals of the earth : yet, 

 be it noted, some birds have lost the power of flight, while 

 some mammals, in varying degrees, have acquired this ; albeit 

 only one group, the bats, have really mastered the art. 



Birds, in short, from the first were destined by Nature to 

 possess the air, while mammals were, on the other hand, de- 

 veloped along lines which made the earth their natural abiding 

 place. And this fundamental difference explains the fact that, 

 structurally, the birds present a remarkable degree of uniform- 

 ity, while the mammals, on the other hand, display a wonderful 

 diversity of form and structure. The mechanical requirements 

 of flight rendered this structural uniformity inevitable ; while, 

 on the other hand, the terrestrial life of the mammals left open 

 a wide range of variability in the matter of bodily shape, and 

 this in time manifested itself, as the need arose, as, in the 

 struggle for existence, some took to climbing trees, some to 

 burrowing in the earth, some to the rivers, and some to the open 

 sea, some to the burning deserts, and some to the wild fast- 

 nesses of the sombre mountains. 



