152 THE STRUCTURE OF EVOLUTIONARY THEORY
Chapter 9 on geological evidence, where the uninitiated might expect to find a
strong defense for evolution from the most direct source of evidence in the fossil
record, reads instead as a long (and legitimate) apologia for a threatening
discordance between data and logical entailment—a fossil record dominated by
gaps and discontinuities when read literally vs. the insensible transitions required
by natural selection as a creative agent. Darwin, with his characteristic honesty,
states the dilemma baldly in succinct deference to his methodological need for
equating temporal steps of change with differences noted among varieties of
contemporary species: "By the theory of natural selection all living species have
been connected with the parent-species of each genus, by differences not greater
than we see between the varieties of the same species at the present day" (p. 281).
Darwin, as we all know, resolved this discordance by branding the fossil
record as so imperfect—like a book with few pages present and only a few letters
preserved on each page—that truly insensible continuity becomes degraded to a
series of abrupt leaps in surviving evidence:
Why then is not every geological formation and every stratum full of such
intermediate links? Geology assuredly does not reveal any such finely
graduated organic chain; and this, perhaps, is the most obvious and gravest
objection which can be urged against my theory. This explanation lies, as I
believe, in the extreme imperfection of the geological record (p. 280).
He, who rejects these views on the nature of the geological record,
will rightly reject my whole theory (p. 342).SLOWNESS AND SMOOTHNESS (BUT NOT CONSTANCY) OF RATE. Darwin also
championed the most stringent version of gradualism—not mere continuity of
information, and not just insensibility of innumerable transitional steps; but also
the additional claim that change must be insensibly gradual even at the broadest
temporal scale of geological durations, and that continuous flux (at variable rates to
be sure) represents the usual state of nature.
This broadest version of gradualism does not hold strong logical ties to
natural selection as an evolutionary mechanism. Change might be episodic and
abrupt in geological perspective, but still proceed by insensible intermediacy at a
generational perspective—given the crucial scaling principle that thousands of
generations make a geological moment. For this reason, Eldredge and I have never
viewed punctuated equilibrium, which does refute Darwinian gradualism in this
third sense, as an attack on the creativity of natural selection itself (Eldredge and
Gould, 1972; Gould and Eldredge, 1977, 1993). The challenge of punctuated
equilibrium to natural selection rests upon two entirely different issues of support
provided by punctuational geometry for the explanation of cladal trends by
differential species success and not by extrapolated anagenesis, and for the high
relative frequency of species selection, as opposed to the exclusivity of Darwinian
selection on organisms (see Chapters 8 and 9).