Structural Constraints, Spandrels, and Exaptation 1203
"types" to have arisen, now and then. Our argument indicates, if it does not
prove, that such mutations [occur] on comparatively few definite lines, or
plain alternatives, of physico-mathematical possibility.If D'Arcy Thompson ended the final chapter on transformed coordinates with his
most trenchant critique of Darwinian functionalism, he also lets us know, within the
body of the chapter, that he built his entire theory as an alternative to the major
implication of Darwinism for the daily practice of evolutionary biology and our
conventional manner of conceptualizing organisms—for Darwinism implies
separability of traits, and the subsequent potential for their independent optimization
by natural selection, whereas the very possibility of relating creatures by such simple
transformation grids points to structural channeling by overarching laws of growth (p.
727):
This independent variability of parts and organs... would appear to be
implicit in our ordinary accepted notions regarding variation; and, unless I am
greatly mistaken, it is precisely on such a conception of the easy, frequent, and
normal independent variability of parts that our conception of the process of
natural selection is fundamentally based ... But if, on the other hand, diverse
and dissimilar fishes can be referred as a whole to identical functions of very
different coordinate systems, this fact will of itself constitute a proof that
variation has proceeded on definite and orderly lines, that a comprehensive
"law of growth" has pervaded the whole structure in its integrity, and that
some more or less simple and recognisable system of forces has been at work.In discussing morphological variety among Radiolaria, where he suspects that
realized taxonomic diversity most closely approaches the filling of all slots permitted
by generating laws of form, D'Arcy Thompson extends his critique to the ultimate
step of even doubting whether many configurations, as occupants of geometrically
attainable positions in a predictable series, even need to be interpreted as adaptive at
all (p. 607):
In few other groups do we seem to possess so nearly complete a picture of all
possible transitions between form and form, and of the whole branching
system of the evolutionary tree: as though little or nothing of it had ever
perished, and the whole web of life, past and present, were as complete as
ever. It leads one to imagine that these shells have grown according to laws so
simple, so much in harmony with their material, with their environment, and
with all the forces internal and external to which they are exposed, that none is
better than another and none fitter or less fit to survive. It invites one also to
contemplate the possibility of the lines of possible variation being here so
narrow and determinate that identical forms may have come independently
into being again and again.D'Arcy Thompson bases these critiques of natural selection directly upon his
own idiosyncratic theory of form. But, in his more conventional participation in the
general debate of his time, he also presents the standard anti-Darwinian