154 THE STRUCTURE OF EVOLUTIONARY THEORY
times or more. (The deposition of the Chalk, an Upper Cretaceous formation,
persisted nearly to the period's end 65 million years ago.)
Moving to a biological example that underscores Darwin's hostility to
episodes of "explosive" evolutionary diversification (he used his usual argument
about the imperfection of the fossil record to deny their literal appearance and to
spread them out in time), Darwin predicted that the Cambrian explosion would be
exposed as an artifact, and that complex multicellular creatures must have thrived
for vast Precambrian durations, gradually reaching the complexity of basal
Cambrian forms. (When Darwin published in 1859, the Cambrian had not yet been
recognized, and his text therefore speaks of the base of the Silurian, meaning lower
Cambrian in modern terminology): "If my theory be true, it is indisputable that
before the lowest Silurian stratum was deposited, long periods elapsed, as long as,
or probably far longer than, the whole interval from the Silurian age to the present
day; and that during these vast, yet quite unknown periods of time, the world
swarmed with living creatures" (p. 307).
Paleontologists have now established a good record of Precambrian life. The
world did swarm indeed, but only with single-celled forms and multi-cellular
algae, until the latest Precambrian fauna of the Ediacara beds (beginning about 600
million years ago). The explosion of multicellular life now seems as abrupt as
ever—even more so since the argument now rests on copious documentation of
Precambrian life, rather than a paucity of evidence that could be attributed to
imperfections of the geological record (see Chapter 10, pp. 1155-1161). Darwin on
the other hand, predicted that complex, multicellular creatures must extend far into
the Precambrian. He wrote: "I cannot doubt that all the Silurian [= Cambrian]
trilobites have descended from some one crustacean, which must have lived long
before the Silurian [= Cambrian] age" (p. 306). Darwin also conjectured, again
incorrectly, that the ancestral verterbrate, an animal with an adult phenotype
resembling the common embryological Bauplan of all modern vertebrates, must
have lived long before the dawn of Cambrian times: "It would be vain to look for
[adult] animals having the common embryological character of the Vertebrata,
until beds far beneath the lowest Silurian strata are discovered" (p. 338).
Darwin struggled for clarity and consistency. He did not always succeed.
(How can an honest person so prevail in our complex and confusing world? I shall,
for example, examine Darwin's ambivalences on progress in Chapter 6.) Darwin
did not always keep the different senses of gradualism distinct. He frequently
conflated meanings, arguing (for example) that the validity of natural selection
(sense 2) required an acceptance of slow and continuous flux (sense 3). Consider
once again the following familiar passage: "It may be said that natural selection is
daily and hourly scrutinizing, throughout the world, every variation, even the
slightest... We see nothing of these slow changes in progress, until the hand of time
has marked the long lapse of ages" (p. 84).
This conflation came easily (and probably unconsciously) to Darwin, in large
part because gradualism stood prior to natural selection in the core of his beliefs
about the nature of things. Natural selection exemplified gradualism,