Tiers of Time and Trials of Extrapolationism 1297
Having disposed of this simple error in thought, Darwin must now face the real
dilemma that intermediates rarely occur in the fossil record either, where they should
exist in abundance: "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" (p. 280).
Darwin's general answer stands out in the title to his chapter, and he invokes the
crucial claim to solve each of the three issues that, in their increasing difficulty,
define the flow and logic of his treatment. The very next sentence, following the
quotation cited just above, states this comprehensive solution: "The explanation lies,
as I believe, in the extreme imperfection of the geological record" (p. 280). This
"argument from imperfection" became so indispensable to Darwin's dispersal of his
major challenge from the fossil record that, later in the chapter, he ventured one of
the most honest and revealing statements in all our scientific literature: a striking
admission that the needs of theory had provoked an understanding of paleontological
data that he otherwise might never even have considered. Such feedbacks always
occur in science, but the empirical ethos of our profession leads us to underplay, or
never to recognize in our own mental processing at all, this reverse flow from the
expectations of theory to the perception and interpretation of factuality. Darwin's
admission strikes me as wonderfully honest and self-scrutinizing, but also as
potentially triggering a trap of circular reasoning, as the dictate of theory, mandating
an expectation of imperfection, biases the reading of a fossil record that might
actually be displaying more genuine signal than the "noise" of absence: "But I do not
pretend that I should ever have suspected how poor a record of the mutations of life,
the best preserved geological section presented, had not the difficulty of our not
discovering innumerable transitional links between the species which appeared at the
commencement and close of each formation, pressed so hardly on my theory" (p.
302).
First, Darwin presents his easier defense of gross imperfection at global scales,
as he argues convincingly that we lack an adequate sample of life's full richness
because both natural limitations (non-deposition of strata during most intervals) and
insufficient human effort in a very young science (poverty of existing collections
relative to fossils that could be gathered, exploration of only a small and
geographically restricted percentage of the earth's fossil bearing strata) preclude any
adequate sampling of life's full richness.
But Darwin must then admit that such general reasons do not resolve the second,
and local, issue of why we do not find a "finely graduated organic chain" within
single formations that do seem to preserve a continuous record of strata: "From the
foregoing considerations it cannot be doubted that the geological record, viewed as a
whole, is extremely imperfect; but if we confine our attention to any one formation, it
becomes more difficult to understand, why we do not therein find closely graduated
varieties between the allied species which lived at its commencement and at its close"
(pp. 292-293).
Again, for this second issue, Darwin stresses the record's imperfection,