202 ANIMAL FORMS 



ancestors, and in becoming more perfectly adapted for an 

 aerial life have developed into our modern forms. 



In the modern birds the most important peculiarities, 

 those which separate them from all other animals, are 

 correlated with the power of flight. The body is spindle- 

 shaped, for readily cleaving the air. The fore limbs serve 

 as wings. The hind limbs, supporting the weight of the 

 body from the ground, are usually well developed. A series 

 of air-chambers usually exists in powerful fliers. This 

 serves a purpose analogous to that of the air-bladder of a 

 fish, giving buoyancy. But the most characteristic mark 

 of a bird, as above stated, is its feathers, universally present 

 and never found outside the class. Like the scales of 

 lizards, and probably derived from similar structures, thej 

 are of different forms, and serve a variety of purposes. 

 The larger ones, with powerful shafts, and forming the tail, 

 act as a rudder. Those of the wings give great expanse 

 with but little increase in weight, and are so constructed 

 that upon the down-stroke they offer great resistance to 

 the air, and push the bird forward, while in the reverse 

 direction the air slips through them readily. In flight 

 these movements of the wing may be too rapid for us to 

 follow, as in the humming-birds, though they are usually 

 much slower, two to five hundred a minute in many power- 

 ful fliers, such as the ducks, and frequently long-continued 

 enough to carry them many hundreds of miles at a single 

 flight. The remaining feathers are soft and downy, giving 

 roundness to the body and enabling it to cleave the air with 

 greater ease, and, being poor conductors of heat, they aid in 

 keeping the body at the high temperature characteristic of 

 birds. In most birds the body is not uniformly clothed in 

 feathers. Xaked spaces, usually hidden, intervene between 

 the feather tracts, and on the feet and toes scales exist. 



190. Molting. As we all know, the growth of feathers, 

 unlike that of hair and nails, is limited, and after they have 

 become faded and worn out they are shed, and new ones 



