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Signs of Life
Response to Stimuli

Response to stimuli

Don't try pushing these fellows around!

Living things are capable to rapid changes in the environment. These rapid changes, called stimuli, provoke reactions in living organisms that are often very different from those expected from nonliving things.

For example, if a 10-pound rock (a nonliving object) is pushed across a smooth surface (the stimulus), it will respond in a predictable manner by moving a predictable distance in a straight line (the response).

The force of the push and the result it produces can be reduced to a mathematical formula which can then be used to make accurate predictions about the behavior of this rock, and then other rocks, under similar sets of circumstances.

If, however, the same force or stimulus is applied to a large growling dog, the dog may move away or, equally well, turn and attack the person applying the force!

The response obtained is certainly not predictable from simple mechanical laws. Similarly, if a bird is pushed it will probably fly. Push a frog and it will jump. The range of responses to exactly the same stimulus varies widely for each organism and among different organisms.

This makes it almost impossible to generate universal laws to predict the behavior of living organisms in any given situation. The variety of unpredictable responses is a sign of life.

© 2003, Professor John Blamire