THE STRENUOUS MOLE 229 



by merely shaking or rocking his whole body from 

 side to side ; he does rock his body too, but at the 

 same time he gives the rapid vibratory motion to 

 the whole skin which discharges the wet. So it is 

 with the horse when he shakes off the wet or the 

 dust after rolling. 



But in the horse the twitching power does not 

 extend, or is not uniformly powerful, over the 

 whole surface ; it is feeble on the hind quarters, 

 and we can only suppose that in the horse, and 

 other large mammalians, the chief use of the 

 twitching act is to shake off dust, flies, and other 

 tormenting insects, and that the growth of the 

 hairy tail in the horse, used to switch insects off, 

 has made the twitching power less useful on this 

 portion of the body. In other words, when this 

 highly specialised tail had fully taken this office or 

 function on itself it caused the decay of the twitch- 

 ing muscle through disuse in those parts of his 

 body. 



We see, too, that the muscle has its greatest 

 power in that part of the body which is just out 

 of reach of the tail, and is also more difficult for 

 the animal to reach with his mouth — that is to 

 say, his back over the shoulders. A man riding 

 bare-back can feel it powerfully when the horse 

 shakes himself. "It is like riding on an earth- 

 quake," I heard a man say once ; to me, with no 

 experience of earthquakes, the sensation was like 

 that of an electric shock. 



In man we can imagine the loss of the twitching 



