Geoscience Reference
In-Depth Information
let the Hoffman paper speak for itself, he took the unusual step of
writing an editorial comment: "The analysis is certain to yield the
conclusion that, on the average, extinction peaks occur every four
stages. . . . Hoffman has undermined the assumption on which all
the excitement was based, the belief that there is a 26 million year
periodicity to be explained." Maddox continued, "Human nature be-
ing what it is, it seems unlikely that the enthusiasts for catastro-
phism will now abandon their quest."
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4
The trouble with Hoffman's third argument (and Maddox's
endorsement) is that they miss the point, as Stephen Jay Gould has
pointed out (and from whose article the rest of the discussion in this
section is drawn
15
). Hoffman wrote: "There is 0.25 probability that
any particular stage represents a peak of extinction. Peaks are, then,
expected to occur approximately every fourth stage."
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6
Read that
quotation carefully and think about what Raup and Sepkoski actu-
ally found. Hoffman is claiming that chance will produce peaks on
the average every 26 million years, approximately every fourth stage.
This is like saying that an honest coin, if tossed often enough, will
produce heads on the average 50 percent of the time. But Raup and
Sepkoski did not claim to have found a cycle with an
average
of
26 million years; they claimed to have found a peak
every
26 mil-
lion years, like a coin that, although it shows heads half the time,
gives this precise sequence: HTHTHTHTHTHTHTHTHTHT. . . .
Thus Hoffman's point is irrelevant to the arguments of Raup and
Sepkoski, who in any case had thoroughly tested the possibility
that their pattern was due to chance and rejected it at a very high
confidence level. Hoffman also manipulated Sepkoski's family data
using different time scales and extinction metrics, and came up with
20 different ways of gauging periodicity, on the basis of which he
claimed to have falsified Raup and Sepkoski's theory. When Sepkoski
subsequently combined all 20 of Hoffman's metrics, however, the
26-million-year periodicity reappeared, more robust than ever! Like
the newspaper account of the death of a very-much-alive Mark Twain,
the Hoffman-Maddox pronouncement of the demise of extinction
periodicity was an exaggeration.
Is C
RATERING
P
ERIODIC?
Though periodicity in the fossil record is still being criticized, Raup
and Sepkoski have responded well to their critics, and as Sepkoski
has added more data to his compendium, evidence for periodicity
has grown stronger.
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7
At the very least we can say that extinction
