1324 THE STRUCTURE OF EVOLUTIONARY THEORY
the smaller and much more common extinctions as equally random in their raison
d'etre (in opposition to the knee jerk view that local Darwinian determinism rules for
ecological moments in competition by wedging, whereas randomness can only enter
at higher levels, where the speed and intensity of an input can catch a Bauplane
unawares).
Such a model of fractal continuity in extinction, triggered by sudden impact at
all scales and levels, might be conceptualized as a "field of bullets" (Raup, 1991a)—
with agents of destruction raining from the sky and death as a random consequence of
residence in the wrong place at the wrong time (when each member of the population
expresses exactly the same properties as any other, and with each independent of all
others). One might conceptualize the agents of catastrophic destruction (the field of
bullets) in either of two ways:
First, the random shooter in the sky may, for each episode of the game, release a
varying number of simultaneous bullets of identical form, with continually decreasing
probability of a larger number (following the inverse curve of frequency and
magnitude). Thus, as a lazy or compassionate character, he hurls only one bullet most
often, but must occasionally release such a dense load that few inhabitants can escape
annihilation. Thus, the vast majority of moments feature none, one, or just a few
extinctions, easily equated with our usual idea of a "background," but with causes just
as random in their "selection" of targets, and just as sudden in their effects, as in the
largest event of mass extinction. Once in a great while, following the dictates of the
same distribution and its implied continuity in causality, bullets reach the extirpating
density of the nearly continuous sheet of arrows launched by the English
longbowmen at Agincourt in 1415, where the French suffered some 6000 deaths to an
English handful. Second, the random shooter might always release the same number
of projectiles, this time following the inverse curve by using smaller bullets (covering
a tiny percentage of territory) most of the time, and large bombs (flattening most of
life's field) only rarely, at the much lower frequency of mass extinctions.
In practice, Raup (1991b, 1992, 1996) derived a "kill curve" (his chosen term)
from the empirical compendium of generic level extinctions per geological stage
developed by J. J. Sepkoski, and widely used by the entire "taxon counting" school of
modern paleobiology (see Figure 12-1). The frequency distribution, based on
Sepkoski's data, assumes the expected inverse form, monotonic and strongly right
skewed, with about half the 106 geological units (with their average duration of 6
million years) plotting in the leftmost interval, and showing less than 10 percent
extinction of genera.
Raup's kill curve (Figure 12-2) follows the familiar form (the inverse
relationship of frequency and magnitude again) that generates such vernacular
concepts as the "100 year flood"—so often, and so tragically, misunderstood by so
many people who, for lack of education to undo one of the most stubborn of our
inherent mental foibles, do not grasp the basic meaning of probability and assume, for
example, that they may safely build their house on the floodplain because the 100
year deluge swept through the region five