1306 THE STRUCTURE OF EVOLUTIONARY THEORY
But, to show the frustration (and scientific nonoperationality) of such proposals,
Schindewolf actually stated—thus providing a favorite case that I have used for
decades to illustrate the difference between science and speculation—that he had
postulated cosmic radiation explicitly because such a cause would leave no empirical
sign (then known to geologists) in the record of strata and fossils. (For Schindewolf
had to admit that the empirical record revealed no direct evidence at all for a
catastrophic mechanism of mass extinction, and he therefore had to seek a potential
cause that would leave no testable sign of its operation! Can one possibly imagine an
unhappier situation for science? —to face the prospect of a plausible explanation that
does not, in principle, leave evidence for its validation.)
In a second example, well remembered by paleontologists of my generation,
Digby McLaren used his presidential address to the Paleontological Society in 1970
to hypothesize that a bolide impact had triggered the Devonian mass extinction. In
the light of Alvarez's later triumph with a similar explanation for the K-T event, one
might be tempted to view this address as prophetic. But, much as I admire both Digby
himself and iconoclasts in general, I'm sure McLaren would admit that he simply
"lucked out" in this case. For, like Schindewolf, McLaren could present no evidence
at all for his bolide, and simply slipped this proposal into the end of his talk in an
almost apologetic manner, after documenting the style and extent of the extinction
itself (the main focus of his paper). (Moreover, to this day and despite excellent
evidence for bolide triggering of the K-T event, we have no satisfactory explanation
of the Devonian extinction, and no credible data for its causation by impact.)
By contrast, the genesis of the Alvarez's hypothesis for the K-T mass extinction
could not have been more different, or more exemplary for science. For the K-T
bolide proposal began with an unanticipated empirical discovery— generated,
ironically, during a test for an opposite hypothesis, and therefore surely not gathered
under the aegis of any iconoclastic theoretical thoughts. Geologist Walter Alvarez,
trying to test an idea about latest Cretaceous sedimentation rates under traditional
gradualist views of the extinction, asked his father, the Nobel laureate in physics Luis
Alvarez, whether any isotopic signature might provide evidence for the following
conjecture: Walter wondered whether a false appearance of rapidity in extinction
might arise from an unusual slowdown in sedimentation rates, thereby compressing a
"standard" amount of extinction into an unusually short stratigraphic interval.
Luis proposed a measurement or iridium, an element virtually absent from the
earth's indigenous surficial rocks. (Presumably, the earth formed with iridium at
standard cosmic abundance, but this indigenous iridium, as such a heavy and
unreactive element, quickly sank well below the surface, especially since the earth's
crust was effectively molten early in the planet's history.) Luis therefore supposed
that throughout Phanerozoic time, iridium has entered the earth's surface only through
cosmic influx, and at the effectively constant rate of uniformitarian assumptions—the
"gentle cosmic rain from heaven" in radiation and tiny particles, as standard views
and terminology