298 INTRODUCTION TO ZOOLOGY. 



have grown from infancy to manhood, without ever having 

 heard of a bird. He sees that the light snow-flake is unable 

 to remain suspended in the air; that the still lighter thistle- 

 down, when no longer supported by the breeze, has a tendency 

 to fall to the ground; and yet he is told that there are tenants 

 of the air, countless as those of earth and water; that some, 

 of considerable size and weight, can journey on their way 

 above the clouds, and with a facility and speed far exceeding 

 that of the swiftest-footed animal. He may, indeed, from 

 observing that cork and light bodies, when plunged in water, 

 rise to the surface, conceive the possible existence of a lighter 

 substance than air, capable, by the same laws of nature, of 

 rising above the earth; if a philosopher, he may even discover 

 the inflammable and lighter gas by which a balloon ascends, 

 with the weight of a man attached; but how shall he lift a 

 substance heavier than the air and how guide its proi 

 through the air ? Show him the weighty body of an Eagle 

 or a Swan;* tell him their living history, and he may rea- 

 sonably doubt your fact, and deny that these things could be." 



To understand the nature of the mechanism by which 

 flight is effected, let us attend, in the first instance, to the 

 structure of the skeleton of birds; and next, to the peculia- 

 rities connected with their respiration. 



Skeleton. The neck of birds is, in general, longer and 

 more moveable than that of quadrupeds. As it is by means 

 of the beak that their food is picked up from the earth, the 

 neck, or cervical part of the vertebral column, is longer in pro- 

 portion as the bird is more elevated by the length of its legs. 

 In swimming birds, which, like the Swan, plunge their head 

 into the water to take their prey, the length of the neck sur- 

 passes that of the trunk. The number of vertebra? differs 

 much, according to the different species of bird. It is com- 

 monly twelve or fifteen; but in the Sparrow it is only nine, 

 while in the Swan it reaches the extraordinary number of 

 twenty-three. It is to this bountiful provision that this bird 

 owes much of its grace and elegance; and this characteristic 

 feature is therefore justly noticed by the poet: 



' The Swan, with arched neck 



Between her white wings, mantling proudly, rows 

 Her state with oary feet." PARADISE LOST, Book vii, 



* The Wild Swan weighs 2" 



