Fig. 22. — Feathers illustrating conditions where barbicels are unnecessary and 

 are hence reduced or entirely lost, causing downiness. 3/5 natural size, 

 (a) Primary of Pigeon — an important flight-feather; hence possessing a stiff 

 vane. (b) Under wing-covert of a Great Blue Heron; downy portion was over- 

 lapped by the adjoining feather, (c) Wing-covert of Owl; the downy edge makes 

 possible the all-important noiseless flight of this bird. (d) Feather of Ostrich; 

 the power of flight being lost, the feathers are downy throughout the entire vane. 



35 



