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LIFE, IX 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 the 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 in which they are formed and 

 placed, is of equal service to birds when they must perch, 

 or otherwise remain at rest, so as to abide the blast, as 

 when they fly exposed to it. Perching or flying, when a 

 bird is in the wind it always faces the current; and thus 

 offers the least resistance both by its form and 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 



