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 
