308 
LIFE, IN ITS HIGHER FORMS. 
pithy substance, hollowed at the lower end into a horny 
tube, containing the blood-vessels by which it is sustained ; 
and the vane, a double series of parallel thin plates, one on 
each side the shaft, set at an angle to it, which are them- 
selves furnished at their edges with a similar though 
smaller series. In all feathers which are destined to strike 
the air, these branchlets are hooked into one another, so 
as to present a continuous surface of astonishing firmness. 
The relation which the general clothing-plumage of the 
body bears to flight — though less direct and obvious than 
that of tho quills — is by no means small. “ From the 
mode in which the feathers, and all their parts, are laid 
upon the bird, it presents a smooth surface upwards and 
forwards, so that the animal can move in either of these 
directions, with very little resistance from the friction of 
the air. When it moves in either of them, the resistance 
of friction does not increase so rapidly as the rate of motion ; 
because the pressure smooths the feathers, and causes the 
air to take less hold on them. This property, which arises 
in part from the texture of the upper surface of the feathers, 
but chiefly from the way iu which they are formed and 
placed, is of equal service to birds when they must perch, 
or otherwise remain at rest, so as to abido the blast, as 
when they fly exposed to it. Perching or flying, when a 
bird is in the wind it always faces the current ; aud thus 
offers the least resistance both by its form aud its feathers. 
“ When, however, the feathers are taken in the opposite 
directions, they offer as much increase of resistance as they 
offer diminution when they are taken above or in front. 
The wings are always more or less hollow on the under 
sides, and they take hold of the air by millions of fibres; 
