ing out others; with the jar and
shock often transmitted very far to other wedges in many lines of direction.
I have focused upon Darwin's commitment to the metaphor of the wedge
because this unappreciated core belief, more than the notion of natural
selection itself, shaped Darwin's conventional view about progress and
predictability in the long-term history of life. Darwin, like any honest
man in a world of such inordinate complexity, struggled hard but failed
to resolve several crucial issues in the interpretation of nature. No
question troubled him more than the common assumption, so crucial to Victorian
Britain at the height of industrial and imperial success, that progress
must mark the pathways of evolutionary change.
Darwin clearly understood that the basic mechanics of natural selection
offered no statement about progress, for the theory only speaks of local
adaptation to changing environments. (No theme is more distinctive and
radical in Darwin's thought, and no subject has circulated more frequently
through these columns during the past fifteen years.) But Darwin, as an
eminent, if critical, Victorian himself, could not let go of progress
so cleanly. He wished to validate predictable advance as a major theme
of the fossil record, but knew that the bare bones of natural selection
could not rationalize this belief. How then, could progress be affirmed
as a fact of life's history if the fundamental theorem of organic change--natural
selection--did not imply progress by itself?
To resolve this troubling discordance between the mechanics of his basic
theory and his fundamental impression of pattern in life's history, Darwin
called upon a second, basically ecological principle encompassed by the
metaphor of the wedge. Nature, Darwin believed, is full of species("a
surface covered with ten thousand sharp wedges ... all packed closely
together"). All potential addresses are occupied, but new challengers
continually arrive to compete for space. They can succeed in a full world
only by driving other species out in overt competition for limited resources
("the one driven deeply in forcing out others"). It's a tough,
crowded world out there, and successful creatures claw their way to the
top and remain there by constant vigilance and conquest. The Origin
of Species contains several passages about progress in the history
of life, and all are validated, not by the bare bones mechanism of natural
selection, but by the second principle of the wedge, the vision of a full
world ruled by overt competition among organisms: The more recent forms
must, on my theory, be higher than the more ancient; for each new species
is formed by having had some advantage in the struggle for life over other
and preceding forms.... I do not doubt that this process of improvement
had affected in a marked and sensible manner the organization or the more
recent and victorious forms of life, in comparison with the ancient and
beaten forms.
But what actual evidence do we have that long-term trends in the history
of life arise by continuous and intense biological competition in a perennially
crowded world? (Other models of the operation of natural selection are
surely plausible. Perhaps most species do not fall victim to overt wedging
by superior competitors, but to changes in the physical environment that
are too rapid or extensive to elicit an adaptive response.) I am persuaded
that some cases in Darwin's preferred mode of organic competition have
been documented. Biologist Geerat Vermeij (who lays out the brief for
Darwin's view most elegantly in his recent book Evolution and Escalation,
Princeton University Press, 1987), for example, has demonstrated a geological
trend for thicker and stronger crab claws matched by ever more efficient
defenses (spines, knobs, and thick shells) in the snails that crabs love
to eat. I accept the interpretation of this lock-step escalation as an
"arms race."
But
just as we needed so great a scholar as Aristotle, in so weighty a place as the
Nicomachean Ethics, to teach us that one swallow does not make a spring
(yes, he said spring, not summer), a case or two in the fossil record does not
establish a pattern. Directional trends produced by wedging do occur, but they
scarcely cry for recognition from every quarry and hillslope. The overwhelming
majority of palcontological trends tell no obvious story of conquest in competition.
Why did the large and gorgeously complex ammonites crash to oblivion some 65 million
years ago, while their rarer and apparently simpler closest cousins, the nautiloids,
survived to our own day as the "ship of pearl" immortalized by Oliver
Wendell Holmes(and Eugene O'Neill) as the builder of "more stately mansions."
Why did all members of three great groups of crinoids die 225 million years ago,
leaving all subsequent time to a fourth group that survives today but sports no
feature ever identified as an improvement over the three losers? Why did a similar
replacement of one group by another occur in reef-building corals at the same
time, and why can no theme of improvement in competition be discerned here either?
Why, for that matter, did dinosaurs die and mammals prevail after 100 million
years of reptilian success and mammalian marginality? (If mammals were competitively
superior, they certainly displayed no hurry in wedging out their "predecessors
in the race for life," for 100 million years is almost two-thirds of mammalian
history. Moreover, recent views on the sleekness and anatomical sufficiency of
dinosaurs speak strongly against any notion of wedging by rat-sized mammals.)
I chose these four cases for two reasons. First, I think that they are
far more characteristic of the fossil record--more numerous and more
significant in import--than putative examples of progress by wedging.
Second, they represent the kind of event that most directly challenges
the metaphor of the wedge and therefore the favored rationale for progress
by rigorous competition among organisms--"changing of the guard"
across episodes of mass extinction. In fact, these four cases provide
a small sample of events, a pair for each, in the two most celebrated
mass extinctions: the Permian debacle that may have wiped out more than
95 percent of marine invertebrate species some 225 million years ago,
and the Cretaceous event that removed remaining dinosaurs and gave mammals
a chance some 65 million years ago.
Any reader with a good feel for the history of paleontology must now
be intensely puzzled. How can I be suggesting that a study of mass extinction
will alter our view about directionality in the history of life when widespread
and coincident death of species is, perhaps, the oldest recorded fact
of the stratigraphic record? The boundaries of the geological time scale,
the alphabet of my profession, are defined by events of mass extinction.
The two episodes cited in my cases separate the three great eras of life's
multicellular history: the Permian extinction between the Paleozoic and
the Mesozoic, and the Cretaceous extinction between the Mesozoic and the
Cenozoic. How can something so canonical, and so long appreciated, now
threaten to disrupt another cherished view about biologically based competition
leading to progress in a crowded world?
A resolution to this historical puzzle rests upon a conventional interpretation
of mass extinction that makes peace with, or even supports, the notion
of biotic struggle as the driving vector of life. Mass extinction is a
specter haunting the metaphor of the wedge, but the ghostbusters of denial
and accommodation have held the fort--until recently. Darwin himself
chose the more vigorous response of denial, trying his darndest to dissolve
mass extinction
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