288 THE WORLD OF LIFE CHAP. 



and difficult manner, yet so as to produce a marvellously 

 successful result The idea worked out was to reduce the 

 jointed bony framework of the wings to a compact minimum 

 of size and maximum of strength in proportion to the 

 muscular power employed ; to enlarge the breastbone so as 

 to give room for greatly increased power of pectoral muscles ; 

 and to construct that part of the wing used in flight in such 

 a manner as to combine great strength with extreme lightness 

 and the most perfect flexibility. In order to produce this 

 more perfect instrument for flight the plan of a continuous 

 membrane, as in the flying reptiles (whose origin was 

 probably contemporaneous with that of the earliest birds) 

 and flying mammals, to be developed at a much later period, 

 was rejected, and its place was taken by a series of broad 

 overlapping oars or vanes, formed by a central rib of 

 extreme strength, elasticity, and lightness, with a web on 

 each side made up of myriads of parts or outgrowths so 

 wonderfully attached and interlocked as to form a self- 

 supporting, highly elastic structure of almost inconceivable 

 delicacy, very easily pierced or ruptured by the impact of 

 solid substances, yet able to sustain almost any amount of 

 air-pressure without injury. And even when any part of 

 this delicate web is injured by separating the adjacent barbs 

 from each other, they are so wonderfully constructed that 

 the pressure and movement of other feathers over them 

 causes them to unite together as firmly as before ; and this 

 is done not by any process of growth, or by any adhesive 

 exudation, but by the mechanical structure of the delicate 

 hooked lamellae of which they are composed. 



The two illustrations here given (Figs. 108, 109) show 

 two of the adjacent fibre-like parts (barbs) of which the 

 web of a bird's feather is composed, and which are most 

 clearly shown in the wing-feathers. The slender barbs or 

 ribs of which the web of the feather is made up can be 

 best understood by stripping off a portion of the web and 

 separating two of the barbs from the rest. With a good 

 lens the structure of the barbs, with their delicate hooked 

 barbules interlocking with the bent-out upper margins of the 

 barbules beneath them, can be easily seen as in the view 

 and section here given. The barbs (B, B in the figures) are 



