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. 
