32 The Bird 
rows of barbules, and these give rise to a series of curved 
hooks, known as barbicels, which work into opposite 
series of grooves, so tightly that air cannot force its way 
through the feather. When the wings are pressed down- 
ward, the phenomenon flight is made possible by the 
accumulated resistance which the flight-feathers offer to 
the air. At the lower end of our pigeon’s feather, bar- 
bicels are present only near the quill. Therefore the 
Fic. 20.—Two interlocked barbs from the vane of a Condor’s wing-feather, show- 
ing barbules and barbicels. Magnified 25 diameters. 
tips of the barbs are loose and fluffy, unconnected and 
useless for flight. This is the condition in all down and 
in the feathers of the ostrich and cassowary. We might 
naturally think that feathers stiffened by so many close 
rows of interlocking barbicels would be useful in many 
ways beside flight. But fluffy feathers are evidently just 
as efficient in keeping warmth in and rain out as the 
other kind; so Nature, economical to the most micro- 
scopic degree, has lessened the number of, or has never 
provided, barbules and barbicels wherever a feather is 
not needed for flight or steering. 
