Field and Laboratory

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As a farmer boy, I often observed that when a litter of pigs was born, some of them were stronger than others, and these stronger pigs, because of their superior power, were able to get more milk from their mother than the weaker pigs. Of course this meant that the stronger pigs grew faster than the weaker; and the faster they grew, the more milk they were able to get, and the more milk they got, the faster they grew. It might seem that this cycle would be endless. It had, however, its limitation; for there is a limit to the amount of milk that even a strong pig can digest, and for various reasons, pigs never grow to be as large as elephants. This homely example illustrates the principle of reciprocal reinforcement and its limitations.

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