Structural Constraints, Spandrels, and Exaptation 1205
"externalist" form of structuralism as deeply out of harmony with the internalist and
historically based theories of structural constraint that are now enjoying such a
revival within evolutionary theory, and that constitute the subject of the rest of this
chapter. I have also not hidden my general approbation for these more popular
internalist and historicist versions. Paleontologists are, after all, historians at heart
and by profession, and I regard historical causation as the most powerful and
distinctive mode of reasoning—indeed the raison d'etre—of the evolutionary
sciences. But I must also confess my lifelong attraction to the prose, and the
chutzpah, behind D'Arcy Thompson's iconoclasm. His work has much to teach us,
although I do not think that his general theory of form can be validated as more than a
peripheral aid and secondary shaper within a primary nexus of historicism.
In any case, D'Arcy Thompson wrote many of his stylistically strongest and
philosophically most interesting disquisitions in a form that can only be labeled as
"riffs" for different taxonomic groups on the common subject of the irrelevance of
phylogeny in general, and the uselessness of Darwinian mechanisms in particular, for
explaining either their anatomical ground plan or the ordered array of forms defining
their taxonomic structure. He presents variants on the same riff for unduloid protists,
radiolarians, and the hard parts of foraminifers, sponge spicules, bird eggs, the
multifarious variations in shape among species of Mesozoic ammonoids, and the
spiral patterns of stems in climbing plants.
For example, in the final paragraph of his chapter on logarithmic spirals, D'Arcy
Thompson notes that the same set of varieties upon this universal curve populate the
seas throughout Phanerozoic times. Therefore, he can't help wondering whether
molluscan shells of this form can be meaningfully parsed into historical series of
descent, or even accorded particular adaptive values (p. 586):
Again, we find the same forms, or forms which (save for external ornament)
are mathematically identical, repeating themselves in all periods of the world's
geological history; and, irrespective of climate or local conditions, we see
them mixed up, one with another, in the depths and on the shores of every sea.
It is hard indeed (to my mind) to see where Natural Selection necessarily
enters in, or to admit that it has had any share whatsoever in the production of
these varied conformations. Unless indeed we use the term Natural Selection
in a sense so wide as to deprive it of any purely biological significance; and so
recognise as a sort of natural selection whatsoever nexus of causes suffices to
differentiate between the likely and the unlikely, the scarce and the frequent,
the easy and the hard: and leads accordingly, under the peculiar conditions,
limitations and restraints which we call "ordinary circumstances," one type of
crystal, one form of cloud, one chemical compound, to be of frequent
occurrence and another to be rare.In my opinion, D'Arcy Thompson's most powerful statement (also most likely to
include important elements of validity) occupies several pages at the end of his next
chapter 12 "on the spiral shells of the foraminifera." In this