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, they 
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. Naked 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 
