THE MACHINERY OF FLIGHT 87 



thus give the feather its elasticity, besides making it 

 impervious to air. Compare for a moment a great 

 flight-feather with one of those whose business is 

 merely to clothe the bird and retain its animal heat 

 — a poor, weak, fluffy thing. The barbicels have 

 another name, hamuli or hooklets, from their 

 shape. On the side of the barb which is nearer to 

 the base of the feather, the barbules have no bar- 

 bicels, but their endings resemble hairs, and these 

 hair-like endings are neatly folded together so as to 

 form a kind of hem. The opposite set of barbules 

 lie over these smooth-ending ones, and among them 

 they insert their hooklets. The hooklets slide along 

 the barbules at moments of strain and stress, and to 

 this, very largely, must be due the elasticity of the 

 feather. The number of barbules is enormous ; 

 according to Dr. Gadow, over a million in one large 

 flight-feather. 



Here is another very interesting point: on the 

 margin of the barbules opposite to that which is 

 armed with barbicels are rough knobs. These are, 

 I believe, the vestiges of barbicels which, being 

 useless, are in process of disappearing. The feather 

 is in fact thrice pinnate. The main shaft branches 

 into barbs, the barbs into barbules, the barbules 

 formerly branched, I believe, into barbicels on either 

 side. But the barbules on the side nearer to the base 

 of the feather have altogether lost their barbicels 

 and have now only hair-like endings, while the other 

 set have only one member of each pair properly 

 developed, the other member being represented only 

 by a mere vestige, so that there are only rough knobs 

 to match the barbicels. 



