The Triumph Of Uniformitarianism
Even before Darwinian theory emerged, Catastrophism came under attack, principally from the Scottish geologist Charles Lyell who was following arguments made earlier by his fellow countryman James Hutton. In his Principles of Geology, published in three volumes in the 1830s, Lyell argued that the geological processes we observe today—such as erosion by wind and rain, earthquakes and volcanoes, and so on—are responsible for all geological changes that have occurred throughout Earth history. He also denied the existence of mass extinctions of species.
Lyell's scheme came to be known as Uniformitarianism. For a while, an intellectual battle pitted it against Catastroph-ism. Uniformitarianism won decisively, and Catastrophism was banished from the intellectual arena as a relic of earlier thinking. Catastrophism may have been defeated as an idea, but paleontologists continued to find evidence of mass dyings in the fossil record. Earth history evidently is not one of grad-ualistic progression, as Lyell and Darwin averred, but instead a litany of sporadic and spasmodic convulsions. Some of these events have moderate impact, with 15 to 40 percent of marine animal species disappearing, but a few others are of much larger extent, constituting the mass extinctions.
This last group —known as the Big Five—comprises biotic crises in which at least 75 percent of species became extinct in a brief geological instant. In one such event, which brought the Permian period and the Paleozoic era to a close, more than 95 percent of marine animal species are calculated to have vanished. This handful of major events, from oldest to most recent, include the following: the end-Ordovician (440 million years ago), the Late Devonian (365 million years ago), the end-Permian (250 million years ago), the end-Triassic (210 million years ago), and the end-Cretaceous (65 million years ago).
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