158 THE STRUCTURE OF EVOLUTIONARY THEORY
change in any case)? Or should they examine the large-scale phenomena of
taxonomic order or geographic distribution (issues of great import again, but lying
too far from immediate causation)? Instead, the best strategy, Darwin asserts, lies
in the study of adaptation, for adaptation is the direct and primary result of natural
selection; and the relative frequency of selection stands so high that almost any
adaptation will record its forming power.
Adaptation therefore becomes, for Darwin, the primary subject for practical
study of evolutionary mechanisms. Recall the basic methodological problem of a
science of history (see p. 102): science aims, above all, to understand causal
processes; past processes cannot be observed in principle; we must therefore learn
about past causes by making inferences from preserved results. Adaptation is the
common and coordinating result of nearly any episode of non-trivial evolutionary
change. Adaptation not only pervades nature with an overwhelming relative
frequency, but also embodies the immediate action of the primary cause of
change—natural selection. The adaptations of organisms therefore constitute the
bread and butter objects of study in evolutionary biology. Our first order approach
to change must pose the following question in any particular case: what adaptive
value can we assign; how did natural selection work in this instance? In a revealing
statement, Darwin rolls all exceptions, all ifs and buts, into a set of subsidiaries to
adaptation forged by natural selection—as either consequences of adaptation,
inherited marks of older adaptations, or rare products of other processes: "Hence
every detail of structure in every living creature (making some little allowance for
the direct action of physical conditions) may be viewed, either as having been of
special use to some ancestral form or as being now of special use to the
descendants of this form—either directly, or indirectly through the complex laws
of growth" (p. 200).
The primary anti-Darwinian argument of late 19th century biology proceeded
by denying a creative role to natural selection—but Darwin countered with a
strong riposte. If adaptation pervades nature and must be constructed by natural
selection, and if the steps of evolutionary sequences are generally so tiny that we
may seek their source in palpable events subject to our direct view and
manipulation, then we not only gain a theoretical explanation for evolutionary
change. We also obtain the practical gift of a workable research program rooted in
the observable and the resolvable.
But nothing so precious comes without a price, or without consequences.
Darwin's argument works; no logical holes remain. But the research program thus
entailed must embody attitudes and assumptions not necessarily true— or at least
not necessarily valid at sufficiently high relative frequency to make the world
exclusively, or even primarily, Darwinian. To accept Darwin's full argument about
the creativity of natural selection, one must buy into an entire conceptual world—a
world where externalities direct, and internalities supply raw material but impose
no serious constraint upon change; a world where the functional impetus for
change comes first and the structural alteration of form can only follow. The
creativity of natural selection makes adaptation central, isotropy of variation
necessary, and gradualism pervasive.