In 1953, while I was a graduate student at Harvard University, I heard a lecture by Konrad Lorenz on ethology. The experience illustrates the principle that new fields are impelled by one to several great ideas expressible in a few words. The one offered by Lorenz that captured my imagination was the concept of the sign stimulus.

Animal behavior, Lorenz said, is organized into modules of fixed-action patterns, complex sequences of sensory and motor actions that accomplish something for the organism, such as capturing a prey or courting a mate.

A sign stimulus is a particular feature in another organism or in the inanimate environment that triggers a fixed-action pattern. Thus when a male stickleback fish on his territory sees a rival male nearby and rushes out to challenge the intruder, he is not responding to the whole image of a male stick-leback as human beings see it but to...

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