Chap. XL] FOCAL ORGANS. 387 



the down-stroke than by the up-stroke ; for, in the 

 first place, the pressure of air on a concave surface is 

 always more effectual than that on a convex ; an 

 umbrella, for example, may be blown inside out, but 

 never outside in ; in the next place, the pressure of 

 the air against the separate feathers welds them into 

 a connected whole, while in the up-stroke the air not 

 only meets with a convex surface from which it may 

 flow away, but it is also able to escape between the 

 separate feathers. The influence, therefore, of gravity 

 is overcome by the greater value of the down-stroke, 

 and by the diminution of the pressure of air in the 

 less valuable up-stroke, which can be made more 

 rapidly than the down-stroke. A further inquiry 

 into the complicated question of the mechanism of 

 flight would lead us beyond the scope of this work. 

 In some cases the tail feathers, by being raised, de- 

 pressed, or turned a little to one side, are able to give 

 an upward, downward, or oblique course to the bird. 



CHAPTER XL 



VOCAL ORGANS. 



UNDER the head of vocal organs we may group all 

 those which produce distinct and definite sounds to 

 the human ear, or which may be supposed to similarly 

 affect the auditory nerves of other animals. 



These organs are never developed among the lower 

 Metazoa ; indeed, so far as we know at present, they 

 are confined to the Arthropoda and the Vertebrata, 

 Among the Crustacea they have been detected in 

 Palinurus. 



In several orders of Insects they are confined to 

 the male sex, and appear, therefore, to be means, as 



