Structural Constraints, Spandrels, and Exaptation 1207
the sequence of historic time. The forces that bring about the sphere, the
cylinder or the ellipsoid are the same yesterday and tomorrow. A snow-crystal
is the same to-day as when the first snows fell. The physical forces which
mould the forms of Orbulina, of Astrorhiza, of Lagena or of Nodosaria to-day
were still the same, and for aught we have reason to believe the physical
conditions under which they worked were not appreciably different, in that
yesterday which we call the Cretaceous epoch; or, for aught we know,
throughout all that duration of time which is marked, but not measured, by the
geological record.An epilog to an argument
We can, today, easily identify D'Arcy Thompson's primary error as an expression of
the venerable post hoc fallacy ("after this, therefore because of this"). He had
correctly noted a strong correlation, throughout organic nature, between the forms of
organisms and the shapes that inorganic objects assume under direct molding by
physical causes acting upon them. He therefore advocated the simplest hypothesis
that these physical causes had directly fashioned the organic forms (just as we would
unhesitatingly assert for inorganic objects). Here he made an empirical rather than a
logical error. That is, we cannot accuse D'Arcy Thompson of not recognizing the
potential fallacy of drawing a causal inference (direct physical production) from the
observation of a correlation (between realized organic forms and the idealized optima
constructed by these physical forces in the inorganic realm). He understood perfectly
well that biologists preferred a different and more complex explanation for the same
generality—that the inorganic objects may be directly crafted, but that organisms
generally achieve the same result by operation of a different kind of biological force,
natural selection working by differential reproductive success and survival of the
fittest. In the following passage, for example, D'Arcy Thompson separates the two
arguments: first, the false inference of direct organic production, followed by the
correct observation that organic forms obey physical laws (p. 10): "We want to see
how, in some cases at least, the forms of living things, and of the parts of living
things, can be explained by physical considerations, and to realise that, in general, no
organic forms exist save such as are in conformity with ordinary physical laws."
In other words, and using D'Arcy Thompson's favored Aristotelian terminology,
he tried to depict the physical laws to which organic shapes conform so well as the
actual efficient causes of these shapes. But, in general, Darwinians were right all
along. These physical laws are formal causes, or blueprints of optimal adaptive
designs for given circumstances of size, materials and ecology. The laws give us
insight into the adaptive values, or final causes, of organic designs. But the efficient
cause of good organic design is usually natural selection.
Ironically, this great student of Aristotle (D'Arcy Thompson wrote standard
translations, still in print, for two of Aristotle's biological treatises) guessed wrong
about the category of causes embodied in the correlation of