1196 THE STRUCTURE OF EVOLUTIONARY THEORY
In what may be his most famous example, D'Arcy Thompson proved a case of direct
construction in response to immediate gravitational forces by showing that internal
trabeculae in the head of the human femur strengthen the bone along the precise lines
of its greatest need for buttressing against compressive forces—for when bones break
and heal improperly, the trabeculae are absorbed and then reform along lines of stress
dictated by the limping walk of suboptimal reknitting. No one, in this case, could
make the usual claim for phyletic determination by natural selection (at least not for
these particular trabeculae in these unfavorable circumstances, although one might
identify selection as the basis for this underlying lability in trabecular formation).
D'Arcy Thompson writes (p. 687):
Our bone is not only a living, but also a highly plastic structure; the little
trabeculae are constantly being formed and deformed, demolished and formed
anew. Here, for once, it is safe to say that "heredity" need not and cannot be
invoked to account for the configuration and arrangement of the trabeculae:
for we can see them, at any time of life, in the making, under the direct action
and control of the forces to which the system is exposed... Herein then lies,
so far as we can discern it, a great part at least of the physical causation of
what at first sight strikes us as a purely functional adaptation.The admitted limitation and ultimate failure of an argument
As a common theme in the tragedies of human literature and history, entities of all
sorts (from bodies, to cities, to structures of ideas) often unravel at the height of their
apparent triumph, for the surface of success may fail to anchor any roots in the
general substrate below. Embodied within the very undeniability of D'Arcy
Thompson's explanation for the direct mechanical shaping of optimally positioned
trabeculae in the human femur, we can also locate the source of a strictly limited
applicability that D'Arcy Thompson himself eventually had to own.
After all, the trabeculae can be explained as direct consequences of immediate
mechanical forces because they cannot be construed as inherited aspects of a
phenotype that might be subject to natural selection, or to any process of truly
evolutionary modification for that matter. They represent labile responses of the
ontogenetic moment, and will therefore be subject to specification by immediate
forces (and, consequentially and crucially, not candidates for hereditary transmission
in our non-Lamarckian earthly biology). But when we study stable and inherited
features with equal claim to adaptive optimality—the main kinds of characters that
theories of adaptive evolution try to explain—how can we make an equally strong
case (or even a case of any plausibility at all) for their immediate construction by
physical forces acting upon the organism during growth? Immediate physical forces
may build my trabeculae, but how can they shape a set of stable and inherited traits
that made a first appearance when I was a tiny embryo in utero, long before the