From the first realization that organic life on this planet has evolved over geologic time, scientists have speculated about the time interval required for these changes to occur. For many years Darwin's idea of slow incremental changes, gradualism, was the accepted model. In the 1970s Stephen Gould (1941-2002) and Niles Eldridge (b. 1943) provided an alternative model of very rapid evolutionary change, followed by long periods of stability, which they called punctuated equilibria. It now appears that Henry Shaler Williams, a geology professor at Cornell University, arrived at the same interpretation almost one hundred years earlier, but only stated this belief in his class lecture notes. He, like Gould and Eldridge, noticed that the fossil record provides evidence that organisms evolved by very fast physical changes occurring in only a few generations and in a very short geological time interval. Then the organisms appear to undergo almost no change for long periods of time; i.e., long periods of stability through many generations. Exactly why Williams did not produce a formal publication of his concept remains a mystery, but his lecture notes from the early 1880s clearly demonstrate that he had developed the idea of punctuated equilibria.

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