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 



FIG. 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. 



