Tiers of Time and Trials of Extrapolationism 1301
Thus, as it seems to me, the parallel, and, taken in a large sense, simultaneous,
succession of the same forms of life throughout the world, accords well with
the principle of new species having been formed by dominant species
spreading widely and varying; the new species thus produced being
themselves dominant owing to inheritance, and to having already had some
advantage over their parents or over other species; these again spreading,
varying, and producing new species. The forms which are beaten and which
yield their places to the new and victorious forms, will generally be allied in
groups, from inheriting some inferiority in common; and therefore as new and
improved groups spread throughout the world, old groups will disappear from
the world; and the succession of forms in both ways will everywhere tend to
correspond.But Darwin's success hinges upon the second and more important geological
argument—for his biological rationale only presents a theoretical defense, whereas he
must overturn a strong signal from a literal reading of the fossil record: the
appearance of true global simultaneity in mass extinction of entire groups and faunas,
at a rate far too fast for any biological mechanism based on ordinary competition. At
this crux, Darwin calls upon his standard argument from imperfection to "spread out"
this apparent moment into sufficient time for uniformitarian explanation.
Darwin admits the literal signal (p. 322): "Scarcely any paleontological
discovery is more striking than the fact, that the forms of life change almost
simultaneously throughout the world." But this impression must be an artifact
produced by the markedly incomplete preservation of more gradual and continuous
change in a woefully imperfect geological record (pp. 317-318): "The old notion of
all the inhabitants of the earth having been swept away at successive periods by
catastrophes is very generally given up, even by those geologists... whose general
views would naturally lead them to this conclusion. On the contrary, we have every
reason to believe, from the study of the tertiary formations, that species and groups of
species gradually disappear, one after the other, first from one spot, then from
another, and finally from the world."
Among the many relevant aspects of imperfection, Darwin stresses two
systematic factors that can compress a gradual transformation into a false appearance
of simultaneity. First, sediments do not accumulate continuously, even in
stratigraphic successions that look complete and uninterrupted. Strata pile up in
continuity only when their basin of deposition slowly subsides, and this geological
situation can occupy only a small percentage of total time. Thus, most intervals will
generate no sediments at all, and a group slowly petering out to extinction may seem
to disappear all at once because sedimentation ceased when the group still included
several declining species. If strata didn't begin to accumulate again until much later,
all these species may have slowly dribbled out of existence during the intervening
period of nondeposition: "We do not make due allowance for the enormous intervals
of time, which have probably elapsed between our consecutive formations—