1204 THE STRUCTURE OF EVOLUTIONARY THEORY
arguments that prevailed within evolutionary biology before the Modern Synthesis
reinstated natural selection at the center of the subject. In a brief commentary on the
extinction of dinosaurs, for example, D'Arcy Thompson reiterates the common claim
that natural selection, although a genuine force in evolution, can only play the minor
and negative role of eliminating the unfit, and not the central part of creating the fit.
"We begin to see," he states (p. 137), "that it is in order to account, not for the
appearance, but for the disappearance of such forms as these that natural selection
must be invoked." He then embellishes the argument with one of his characteristic
prose flourishes (pp. 137-138):
But there comes a time when "variation," in form, dimensions, or other
qualities of the organism, goes farther than is compatible with all the means at
hand of health and welfare for the individual and the stock; when, under the
active and creative stimulus of forces from within and from without, the active
and creative energies of growth pass the bounds of physical and physiological
equilibrium: Then, at last, we are entitled to use the customary metaphor, and
to see in natural selection an inexorable force, whose function is not to create
but to destroy,—to weed, to prune, to cut down, and to cast into the fire.But if this general and multifaceted critique of Darwinian functionalism—
arising from his contrary structuralist account of the direct production of adaptive
form by physical forces—harmonized well with major trends of thought in the
evolutionary biology of his generation, the second prominent implication that he
drew from his idiosyncratic theory of form could not have stood in more oppositional
relationship to an even deeper, and even more general, assumption of the
evolutionary sciences, both in D'Arcy Thompson's day and in our own. For D'Arcy
Thompson used his theory to disparage, indeed virtually to abolish, phylogenetic
reasoning and historical explanations in general. He did not, of course, deny that
phylogeny happened, and that a tree of life existed; and he did not challenge the fact
that each species stands atop a historical series of ancestral forms. But he did argue
that these common features of biological reality possessed virtually no explanatory
value for understanding either the morphology of individual species, or the
anatomical relationships among species, as depicted in his own diagrams of
transformed coordinates.
For if immediate physical forces shape the adaptive configurations of each
modern species directly, then what relevance can be found in extinct ancestral shapes
that responded to different physical forces of prior times, and that do not constrain the
current forms of their descendants? The sensible, rational, and constrained order of
related living forms in taxonomic groups represents a set of realizable positions
within the mathematical boundaries of laws that currently shape the relevant
organisms, not a set of phyletic constraints inherited from the past and now resident
within organisms in the form of inherited genetic and developmental patterns that
limit and channel the taxonomic structure of the living world.
For these reasons, I have labeled D'Arcy Thompson's "physicalist" and