Structural Constraints, Spandrels, and Exaptation 1187
adaptive utility in the Darwinian struggle for life ("final causes" in Aristotle's
terminology, with "final" referring to utilitarian, not temporal, ends). The Darwinian
functionalist, D'Arcy Thompson then claims, makes his key error in assuming that the
identification of utility (final cause) automatically specifies the process by which
such utility originates—a false inference from purpose to mechanism. But, as
Aristotle pointed out, a full explanation for natural objects and phenomena requires
the identification of several distinct kinds of causes. In particular, the final cause
(utility) of an object does not specify the efficient cause, or mechanism, that actually
(and actively) constructed the object ("efficient," that is, in the technical sense of
making or "effecting," rather than the more restricted vernacular sense of doing
something well).
When we identify "not sinking into the mud" as the adaptive value (final cause)
of webbing on the feet of shore birds, we have not proven thereby that the efficient
cause of webbing must be functionalist in nature, and explicitly tied to the purpose
(final cause) now served by this feature. After all, webbing might have arisen by any
one of numerous, and entirely plausible, non-functionalist mechanisms (or by
functionalist mechanisms unrelated to current utility for standing on mud)—and then
been happily and fortuitously available for cooptation to its current purpose. D'Arcy
Thompson preferred efficient causes of direct physical imposition (an improbable
alternative in this particular case), but his general point cannot be gainsaid. The
correct description of a final cause does not, by itself, identify the mechanism by
which this utility originated (p. 5):
The use of the ideological principle is but one way, not the whole or the only
way, by which we may seek to learn how things came to be, and to take their
places in the harmonious complexity of the world. To seek not for ends but for
"antecedents" is the way of the physicist, who finds "causes" in what he has
learned to recognize as fundamental properties, or inseparable concomitants,
or unchanging laws, of matter and of energy. In Aristotle's parable, the house
is there that men may live in it; but it is also there because the builders have
laid one stone upon another: and it is as a mechanism, or a mechanical
construction, that the physicist looks upon the world. Like warp and woof,
mechanism and teleology are interwoven together, and we must not cleave to
the one and despise the other.Moving from Aristotle to his own nation's greatest philosopher at the dawn of
modern science, D'Arcy Thompson then cites Bacon's famous disparagement of final
causes (as vestal virgins with empty downturned cups) falsely cited to explain
mechanisms of production (pp. 5-6)—and he blames a knee-jerk style of Darwinian
adaptationism for this common conflation in evolutionary science (that is, for
erroneous assumptions that functional utilities automatically identify structural or
mechanical origins by natural selection):