Structural Constraints, Spandrels, and Exaptation 1197
gravitational forces to which these features are adapted could play any role in
building my body by direct imposition?
D'Arcy Thompson had to face this critical problem head on—and he did so in a
forthright manner by putting the best possible face upon adversity via two arguments
that fatally compromised his dream of winning generality for his idiosyncratic theory
of structuralist evolution from the outside. First and foremost, he simply admitted that
his principle of direct imposition couldn't explain the complex forms of the
multicellular phyla that, for however parochial a reason, have always defined the
central subject and puzzlement of morphology. D'Arcy Thompson continued to
maintain—and he may well have been right in some cases—that good matches
between simple organic conformations (primarily the outward forms of unicellular
creatures) and geometric shapes of well known mathematical definition and easily
accomplished mechanical construction probably illustrate his favored principle of
direct imposition by physical forces. But he had to admit that he could not apply this
line of reasoning to the basic form of a horse or a tuna.
Interestingly, he "came clean" on this point right after his strong argument about
the production of unicellular unduloids by forces of surface tension (cited on p.
1193). He begins by quoting a conventional defense of phylogenetic reasoning by E.
Ray Lankester, then refutes the argument for his unduloids, but cannot deny its
application to large and complex multicellular forms (pp. 251-252):
"The fact that we are able to classify organisms at all in accordance with the
structural characteristics which they present is due to the fact of their being
related by descent." But this great generalisation is apt in my opinion, to carry
us too far. It may be safe and sure and helpful and illuminating when we apply
it to such complex entities, —such thousandfold resultants of the combination
and permutation of many variable characters, —as a horse, a lion or an eagle;
but (to my mind) it has a very different look, and a far less firm foundation,
when we attempt to extend it to minute organisms whose specific characters
are few and simple, whose simplicity becomes much more manifest when we
regard it from the point of view of physical and mathematical description and
analysis, and whose form is referable, or (to say the least of it) is very largely
referable, to the direct and immediate action of a particular physical force.But D'Arcy Thompson truly throws in the towel during the most poignant and
appropriate round—right at the end of his last empirical chapter, as he reaches the
apex of complexity in his analysis of vertebrate skeletons (chapter 16 "on form and
mechanical efficiency"), and just before he recoups relevance in his brilliant final
chapter on the theory of transformed coordinates. He begins by admitting that he
cannot describe the skeleton as "a resultant of immediate and direct physical or
mechanical conditions" precisely because the very biological principle that he has
tried to deny (or at least to underplay) throughout the book—the phyletic inheritance,
rather than immediate