136 THE REIGN OF LAW. 



animal heavier than the air to support itself against 

 the Force of Gravity, it must be enabled to strike 

 the air downwards with such force as to occasion a 

 rebound upwards of corresponding power. The 

 wing of a flying animal must therefore do some- 

 thing more than barely balance Gravity. It must 

 be able to strike the air with such violence as to 

 call forth a reaction equally violent, and in the 

 opposite direction. This is the function assigned 

 to the powerful muscles by which the wings of 

 birds are flapped with such velocity and strength. 

 We need not follow this part of the problem fur- 

 ther, because it does not differ in kind from the 

 muscular action of other animals. The connexion, 

 indeed, between the Wills of animals and the mech- 

 anism of their frame is the last and highest pro- 

 blem of all in the mechanics of Nature, but it is 

 merged and hid for ever in the one great mystery 

 of Life. But so far as this difficulty is concerned, 

 the action of an Eagle's wing is not more myste- 

 rious than the action of a Man's arm. There is a 

 greater concentration of muscular power in the 

 organism of birds than in most other animal 

 frames, because it is an essential part of the pro- 



