The principle of use and disuse states that those parts of an organism's body that are used grow larger. Those parts that are not used tend to wither away. It is an observed fact that when you exercise particular muscles they grow; muscles that are never used shrink. By examining a man's body we can tell which muscles he uses and which he does not. We may even be able to guess his profession or his recreation. Enthusiasts of the 'body-building' cult make use of the principle of use and disuse to 'build' their bodies, almost like a piece of sculpture, into whatever unnatural shape is demanded by fashion in this peculiar minority culture. Muscles are not the only parts of the body that respond to use in this kind of way. Walk barefoot, and you acquire tougher skin on your soles. It is easy to tell a farmer from a bank clerk by looking at their hands alone. The farmer's hands are horny, toughened by long exposure to rough work. If the clerk's hands are horny at all, it amounts only to a little callus on the writing finger.
