Structural Constraints, Spandrels, and Exaptation 1189
life. By invoking an analogy to the limits of science in aesthetic and moral
arguments, D'Arcy Thompson allows that his favored theme of physical imposition
may not carry us as far as we (or at least he) would like to go. But he makes a strong
argument for this kind of minimalism (final causes generated by physical imposition,
thus obviating the need for special mechanisms to secure adaptation) as an
appropriate first approach (pp. 8-9):
How far, even then, mathematics will suffice to describe, and physics to
explain, the fabric of the body no man can foresee. It may be that all the laws
of energy, and all the properties of matter, and all the chemistry of all the
colloids are as powerless to explain the body, as they are impotent to
comprehend the soul. For my part, I think it is not so. Of how it is that the soul
informs the body, physical science teaches me nothing: consciousness is not
explained to my comprehension by all the nerve-paths and "neurons" of the
physiologist; nor do I ask of physics how goodness shines in one man's face,
and evil betrays itself in another. But of the construction and growth and
working of the body, as of all that is of the earth earthy, physical science is, in
my humble opinion, our only teacher and guide.The tactic and application of an argument
D'Arcy Thompson followed a definite strategy in attempting to carve out the largest
possible empirical role for his "minimalist" structural theory on the genesis of good
design and adaptive form in organisms. He would begin with his best "shot"—the
outward shapes of simple unicellular organisms—and then sally forth from this
plausible beginning. Again, he initiates the search in his overtly modest mode (p. 10):
"My sole purpose is to correlate with mathematical statement and physical law
certain of the simpler outward phenomena of organic growth and structure or form:
while all the while regarding, ex hypothesi, for the purposes of this correlation, the
fabric of the organism as a material and mechanical configuration."
The empirical chapters of Growth and Form embody this plan by first
elucidating a most promising principle (surface/volume ratios), applying it to a best
potential case (protistan form), and then moving from this position of initial strength
into ever less likely realms of application, always trying to capture the largest
possible domain for explaining final causes (adaptive forms) as automatic
consequences of the direct action of physical forces (efficient causes) upon yielding
organic material.
After a short introductory statement, presenting the basic argument as
summarized in my preceding pages, D'Arcy Thompson composes two lengthy
chapters to set a context for the empirical cases to follow. The first, entitled "on
magnitude" and devoted to an elegant explication, still read in many undergraduate
courses, of Galileo's principle of necessarily declining surface/ volume ratios as
geometrically similar objects increase in size, holds a central place in the logic of
D'Arcy Thompson's general theory. (Ironically, many of the most fervent admirers of
this chapter, especially those who encounter it