Punctuated Equilibrium and the Validation of Macroevolutionary Theory 757
clock: "The amount of organic change in the fossils of consecutive formations
probably serves as a fair measure of the lapse of actual time" (1859, p. 488). I also
show in Chapter 2 that Darwin's conviction about extreme slowness and steadiness
of change can be grasped, perhaps best of all, as the common source of his major
errors—particularly his fivefold overestimate for the denudation of the Weald, and
his conjecture that complex metazoan life of modern form must have undergone an
unrecorded Precambrian history as long, or longer, than its known Phanerozoic
duration.
Despite this strong belief in geological gradualism, Darwin knew perfectly
well—as all paleontologists always have—that stasis and abrupt appearance
represent a norm for the observed history of most species. I needn't rehearse
Darwin's solution to this dilemma, for his familiar argument represents more than a
twice-told tale. Following the lead of his mentor, Charles Lyell, Darwin attributed
this striking discordance between theoretical expectation and actual observation to
the extreme imperfection of the fossil record.
(As discussed more fully on pages 479-484, this argument served as the
centerpiece for LyelPs system, and for the entire uniformitarian school. But then,
what alternative could they embrace? The literal appearance of the geological
record so often suggested catastrophe, or at least "moments" of substantial change,
especially in faunal turnover. To assert a gradualism of geological rate against this
sensory evidence, one had to declare the evidence illusory by advancing the
general claim—quite legitimate as a philosophical proposition—that science must
often work by probing "behind appearance" to impose the expectations of a valid
theory upon an empirical record that, for one reason or another, cannot directly
express the actual mechanisms of nature. Moreover, the "argument from
imperfection" holds substantial merit and cannot be dismissed as "special
pleading." Like most chronicles of history, and far more so than many others, the
geological record is extremely spotty. To cite Lyell's famous metaphor once again,
if Vesuvius erupted again and buried a modern Italian city atop Pompeii, later
stratigraphers might find a sequence of Roman ruins capped by layers of volcanic
ash and followed by the debris of modern Italy. Taken literally, this sequence
would suggest a catastrophic end to Rome followed by a saltation, linguistically
and technologically, to the industrial age—an artifact of nearly 2000 years of
missing data that would have recorded the evolution of Italian from Latin and a
gradual passage from walled cities to traffic jams.)
To quote the two most famous statements on this subject from the Origin of
Species, Darwin summarizes his entire argument by closing Chapter 9 with Lyell's
metaphor of the book (1859, pp. 310-311):
For my part, following out Lyell's metaphor, I look at the natural geological
record, as a history of the world imperfectly kept, and written in a changing
dialect; of this history we possess the last volume alone, relating only to
two or three countries. Of this volume, only here and there a short chapter
has been preserved; and of each page, only here and there a few lines. Each
word of the slowly-changing language, in which the history