Hutton: "The present is the key to the past." Catastrophism was rejected; Lyell and subsequent geologists needed no "help from a comet."
Of course, the slow processes that we see today at the earth's surface—wind- and water-driven erosion and deposition, the advance and retreat of glaciers and the sea—can be projected back into the past, and, in that sense, the present is at least a part of the key to the past. Thus a case could be made for retaining the uniformitarianism of process. But the whole edifice has caused such damage, and is today so misleading, that the case for abandoning uniformitarianism is much the stronger. Strict adherence to uni-formitarianism clearly played a role in delaying for half a century the recognition of continental drift and its modern version, plate tectonics. As for meteorite impact, Marvin writes, that "Uniformitarian-ism . . . probably has been the single most effective factor in preventing geologists from accepting the idea ... as a process of any importance in the evolution of the earth."2 If we measure from the date of Gilbert's erroneous conclusion about the origin of Meteor Crater in 1891, to 1980, the year of the first Alvarez paper, unifor-mitarianism and anticatastrophism cost geology nearly 90 years. But continental drift and meteorite impact are arguably the two most important processes that have affected the history of the earth. How much value remains in a paradigm that helped to retard the recognition of both for generations?
All scientists, geologists included, study cause and effect and then project to cases where only effect can be seen. But that is all that the uniformitarianism of process amounts to, and it is drastically incomplete. Is there any longer a reason for geologists, alone among all scientists, to give an exalted name to the standard modus operan-di of science? Doing so is more apt to misdirect geologists of the future, and to load them with the baggage of the past, than to assist them in understanding the history of our planet.
Where do the traditional uniformitarian explanations of mass extinction—changes in climate and sea level—stand today? Has additional evidence been uncovered since the Alvarez theory appeared in 1980 that lends them greater credence? No, just the opposite. While the Alvarez theory has grown stronger, they have grown weaker. I noted earlier that David Jablonski found that major mass extinctions failed to correlate in any way with known changes in sea level, global climate, and mountain building.3 Recently, John Alroy of the Smithsonian compared the appearance and extinction of mammals with the ups and downs of global climate over the past
80 million years.4 He found that until 65 million years ago, mammal diversity was low, and that in the aftermath of the Chicxulub impact it fell even lower. The number of species then rose sharply to reach a plateau about 50 million years ago, where it has remained since. Almost no correlation exists, however, between climate and the appearance and extinction of mammals. Instead, the appearance of new species was largely controlled by the number already present: When mammals were few to start with, more new species appeared. The driver of mammalian diversity thus seems to be not climate but the number of vacant ecological niches. Alroy's study does not rule out the possibility that some mass extinctions have been caused by rare extremes of climate. But taking the view that it is what a species does know that cannot hurt it, Alroy noted that cyclical changes in the position and shape of the earth's orbit relative to the sun produce changes in climate every 20,000, 40,000, and 100,000 years. (Most geologists believe these are the causes of the repeating ice ages.) Species, which live on the average for a few million years, have of necessity survived scores of changes in climate and sea level.
In summary, it seems fair to say that, nearly two centuries after Hutton, there is precious little positive evidence that changes in climate and sea level cause mass extinctions. It is up to the proponents of the claim that they produce such evidence.
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