290 THE WORLD OF LIFE CHAP. 



depends. Even in the smallest wing-feathers they are 

 probably a hundred thousand in number, since in the long 

 wing-feather of a crane the number is stated by Dr. Hans 

 Gadow to be more than a million. 



What are termed the " contour-feathers " are those that 

 clothe the whole body and limbs of a bird with a garment 

 of extreme lightness which is almost completely impervious 

 to either cold or heat. These feathers vary greatly in shape 

 on different parts of the body, sometimes forming a dense 

 velvety covering, as on the head and neck of many species, 

 or developed into endless variety of ornament. They fit 

 and overlap each other so perfectly, and entangle so much 

 air between them, that rarely do birds suffer from cold, 

 except when unable to obtain any shelter from violent 

 storms or blizzards. Yet, as every single feather is movable 

 and erectile, the whole body can be freely exposed to the 

 air in times of oppressive heat, or to dry the feathers rapidly 

 after bathing or after unusually heavy rain. 



A great deal has been written on the mechanics of a 

 bird's flight, as dependent on the form and curvature of the 

 feathers and of the entire wing, the powerful muscular 

 arrangements, and especially the perfection of the adjustment 

 by which during the rapid down-stroke the combined feathers 

 constitute a perfectly air-tight, exceedingly strong, yet highly 

 elastic instrument for flight ; while the moment the upward 

 motion begins the feathers all turn upon their axes so that 

 the air passes between them with hardly any resistance, and 

 when they again begin the down-stroke close up automatic- 

 ally as air-tight as before. Thus the effective down-strokes 

 follow each other so rapidly that, together with the support 

 given by the hinder portion of the wings and tail, the 

 onward motion is kept up, and the strongest flying birds 

 exhibit hardly any undulation in the course they are 

 pursuing. But very little is said about the minute structure 

 of the feathers themselves, which are what renders perfect 

 flight in almost every change of conditions a possibility and 

 an actually achieved result. 



But there is a further difference between this instrument 

 of flight and all others in nature. It is not, except during 

 actual growth, a part of the living organism, but a mechanical 



