Structural Constraints, Spandrels, and Exaptation 1185
The physicist proclaims aloud that the physical phenomena which meet us by
the way have their manifestations of form, not less beautiful and scarce less
varied than those which move us to admiration among living things. The
waves of the sea, the little ripples on the shore, the sweeping curve of the
sandy bay between its headlands, the outline of the hills, the shape of the
clouds, all these are so many riddles of form, so many problems of
morphology, and all of them the physicist can more or less easily read and
adequately solve: solving them by reference to their antecedent phenomena, in
the material system of mechanical forces to which they belong, and to which
we interpret them as being due...
Nor is it otherwise with the material forms of living things. Cell and tissues,
shell and bone, leaf and flower, are so many portions of matter, and it is in
obedience to the laws of physics that their particles have been moved, molded
and conformed... Their problems of form are in the first instance
mathematical problems, and their problems of growth are essentially physical
problems, and the morphologist is, ipso facto, a student of physical science.Our reluctance, D'Arcy Thompson claims, arises largely from conventional
beliefs about the "special" character of life, based on a traditional assumption that
organic shapes embody purposes and therefore demand teleological explanation,
whereas inorganic forms exert no action of their own, and can only be explained as
passive records of physical forces. We assert organic uniqueness by invoking both an
active and passive argument. The passive argument sets living things apart, without
specifying any uniquely biological causes or processes (p. 2):
The reasons for this difference lie deep, and in part are rooted in old traditions.
The zoologist has scarce begun to dream of defining, in mathematical
language, even the simpler organic forms. When he finds a simple geometrical
construction, for instance in the honey-comb, he would fain refer it to
psychical instinct or design rather than to the operation of physical forces;
when he sees in snail, or nautilus, or tiny foraminiferal or radiolarian shell, a
close approach to the perfect sphere or spiral, he is prone, of old habit, to
believe that it is after all something more than a spiral or a sphere, and that in
this "something more" there lies what neither physics nor mathematics can
explain. In short he is deeply reluctant to compare the living with the dead, or
to explain by geometry or by dynamics the things, which have their part in the
mystery of life.Biologists then advance the active argument to posit a set of distinctively
organic causes that, in their outcomes, mimic the same forms that physical forces, left
to their own devices, would impose upon any plastic material. At this point, D'Arcy
Thompson introduces his critique of Darwinism and of functionalist evolutionary
thought in general. In the paragraph following the last citation, D'Arcy Thompson
identifies the two main culprits in our erroneous convictions about special biological
forces behind good organic design: phyletic solutions (or any kind of historical
explanation), and adaptationist