1308 THE STRUCTURE OF EVOLUTIONARY THEORY
where no more need be said: Raup: "This time it's different you know." Me, in reply:
"Yes, of course, the iridium."
In the retrospect of a mere twenty years between initial proposal and such
substantial success (although by no means total, in either its own hopes or terms), we
may identify several reasons to honor the conventional criteria used by scientists to
judge the strength and importance of hypotheses—criteria based on empirical
affirmation, fruitful extension, and widening intellectual scope, rather than on such
nonoperational notions as progress towards absolute truth. Science is, as P. B.
Medawar stated in the title to his finest book, the Art of the Soluble:
- At their initial decision to publish, the Alvarezes had detected an iridium
spike only at two nearby localities in Denmark and Italy, and couldn't even be
confident in their theory's crucial prediction of a worldwide enhancement.
(Fortunately for them, evidence for a third and virtually antipodal spike from New
Zealand arrived in time for inclusion in the original publication.) But, within a
decade, affirmation had accumulated from so many collateral sources—all
independent of, and unpredictable from, the iridium spike itself—that the case for
impact had effectively been sealed. These additions ranged from shocked quartz in
the K-T boundary layer throughout the world (with silica tetrahedra arranged in an
unusual manner only associated, so far as we know, with high pressures of impact,
including the initial discovery of such forms in nuclear bomb craters), to the
"smoking gun" of a gigantic crater of exactly appropriate age—the Chicxulub
structure off the Yucatan peninsula in Mexico. As this evidence accumulated, the
alternative volcanist scenario (with iridium recruited from the earth's interior in
extrusive events of a magnitude never witnessed in historical times), which had
provided such good material for fruitful debate, yielded the floor (although volcanic
action initiated by impact may have played an important role in the full scenario of
explanation for mass extinctions). Needless to say, other exciting hypotheses
generated by the impact debate—particularly Raup and Sepkoski's claim for a 26
million year periodicity in extinction, with a subsequent set of wondrous
astronomical hypotheses as potential explanations, including the actions of a
previously unknown dwarf companion star to the sun—have not fared so well
(although a few jurors, with a few good arguments, are still holding out). - Enhanced and surprising interdisciplinary communication offers no guarantee
of scientific rectitude or success, but we can only celebrate the veritable orgy of
exciting, and at least intellectually fruitful, discussion and collaboration inspired by
the impact hypothesis among scientists in subdisciplines that had never read each
other's work, hardly even knew the names of the most reputable leaders in the
disparate domains, and could barely speak
(me for the punctuational themes that Glen notes above, Raup for his work on random
processes, vertebrate paleontologist Dale Russell for his prior, if speculative, writing on a
potential role for impacts). Always look to reasons of personal interest, rather than general
wisdom, in such cases—and remember the wise words of W. S. Gilbert's most honorable, if
unlikable, man: King Gama of Princess Ida: "A charitable action I can skillfully dissect;
And interested motives I'm delighted to detect."