The principle of use and disuse states that those parts of organisms’ bodies that are used grow larger. Those parts that are not tend to wither away. It is an observed fact that when you exercise particular muscles, they grow. Those that are never used diminish. 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 harder skin on your soles. It is easy to tell a farmer from a bank teller by looking at their hands alone. The farmer’s hands are horny, hardened by long exposure to rough work. The teller’s hands are relatively soft.

The principle of use and disuse enables animals to become better at the job of surviving in their world, progressively better during their lifetime as a result of living in that world. Humans, through direct exposure to sunlight, or lack of it, develop a skin color which equips them better to survive in the particular local conditions. Too much sunlight is dangerous. Enthusiastic sunbathers with very fair skins are susceptible to skin cancer. Too little sunlight, on the other hand, leads to vitamin-D deficiency. The brown pigment melanin which is synthesized under the influence of sunlight, makes a screen to protect the underlying tissues from the harmful effects of further sunlight. If a suntanned person moves to a less sunny climate, the melanin disappears, and the body is able to benefit from what little sun there is. This can be represented as an instance of the principle of use and disuse: skin goes brown when it is “used’, and fades to white when it is not.

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