Structural Constraints, Spandrels, and Exaptation 1193
I choose this case (Figure 11-4) because D'Arcy Thompson invoked the
conformity of so many tiny organisms with the well-known and easily-produced
unduloid to draw an explicit contrast between his explanatory preferences and the
conventional Darwinian account of adaptive design (pp. 248-249) (note, especially,
how he highlighted these differences for resolving both the adaptive status of the
basic form itself, and the genesis of a rich set of taxonomically designated variants
upon the basic form):
Here we have an excellent illustration of the contrast between the different
ways in which such a structure may be regarded and interpreted. The
teleological explanation is that it is developed for the sake of protection, as a
domicile and shelter for the little organism within. The mechanical
explanation of the physicist (seeking only after the "efficient," and not the
"final" cause), is that it is present, and has its actual conformation, by reason
of certain chemicophysical conditions: that it was inevitable, under the given
conditions, that certain constituent substances actually present in the
protoplasm should be aggregated by molecular forces in its surface layer; that
under this adsorptive process, the conditions continuing favorable, the
particles should accumulate and concentrate till they formed a membrane,
thicker or thinner as the case might be; that this11 - 4. Single celled protists assuming the form of unduloids—and taken by D'Arcy Thompson
as proof of immediate physical construction rather than genetic encoding. See text for details.
From D'Arcy Thompson, 1917.