Examples of biological rhythmicity mounted in the first half of the twentieth century, but the nature of biological rhythms–whether they were direct responses to rhythmic environmental stimuli or generated internally by biological clocks or oscillators–remained sharply disputed. When Frank A. Brown Jr. observed precise twenty-four-hour rhythms in the colour changes of fiddler crabs that were isolated from changes in environmental lighting that could synchronise them with the daily cycle, he rejected the internal clock as an explanation, owing to his belief that such a biological mechanism must operate chemically and consequently run faster at higher temperatures (Van’t Hoff’s rule), which was not borne out by his experiments. Colin Pittendrigh was committed to the idea that the rhythms were generated by unknown, temperature-compensated mechanisms, without offering any explanation for how this might be accomplished. At the core of their dispute, which was aired publicly from 1954 until Brown’s death in 1983, was a difference in belief: Brown’s fundamental reductionism, which required biological processes to conform to the laws of chemistry, vs. Pittendrigh’s trust in the power of evolution to select for such a temperature compensated biological clock, for which no plausible chemical model then existed.
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