No one questions that, overall, the record reflects a steady increase in the diversity and complexity of species, with the origin of new species and the extinction of established ones punctuating the passage of time. But the crucial issue is that, for the most part, the fossils do not document a smooth transition from old morphologies to new ones. "For millions of years species remain unchanged in the fossil record," said Stephen Jay Gould, of Harvard, "and they then abruptly disappear, to be replaced by something that is substantially different but clearly related."
The absence of transitional forms between established species has traditionally been explained as a fault of an imperfect record, an argument first advanced by Charles Darwin. The accumulation of sediments and the entrapment and fossilization of animal bones is, at best, a capricious process: as a result, geologists are familiar with the difficulties of reconstructing past events. According to the traditional position, therefore, if sedimentation and fossilization did indeed encapsulate a complete record of prehistory, then it would reveal the postulated transitional organisms. But it isn't and it doesn't.
This ancient lament was intoned by some at the Chicago meeting: "I take a dim view of the fossil record as a source of data," observed Everett Olson, the paleontologist from UCLA. But such views were challenged as being defeatest [sic]. "I'm tired of hearing about the imperfections of the fossil record," said John Sepkoski of the University of Chicago; "I'm more interested in hearing about the imperfections of our questions about the record." "The record is not so woefully incomplete," offered Steven Stanley of Johns Hopkins University; "you can reconstruct long sections by combining data from several areas." Olson confessed himself to be "cheered by such optimism about the fossil record," and he listened receptively to Gould's suggestion that the gaps in the record are more real than apparent. "Certainly the record is poor," admitted Gould, "but the jerkiness you see is not the result of gaps, it is the consequence of the jerky mode of evolutionary change."
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