1228 THE STRUCTURE OF EVOLUTIONARY THEORY
additional elaboration. Suffice to say that if a capacity for utilization in markedly
different ways did not lie within the inherent or formal structure of most primary
adaptations, then evolution would never be able to reach a novel "there" from its
present "here"—and life's history would stagnate in transient perfection (and then
expire when surrounding environments underwent their occasional substantial
alterations).
After all, natural selection cannot act as a magic wand for the immediate
construction of any urgent need. The adaptability—or, in the more general term now
finally receiving substantial and deserved attention from organismal biologists (see p.
1270), the "evolvability"—of any phenotype must depend, in large part, on a
flexibility for future change that simply cannot arise, if we understand the nature of
causality itself aright, by direct natural selection at the usual Darwinian level of
organismal phenotypes. Therefore, a large component of evolvability must be
attributed to inherent structural properties of features that originated by natural
selection for one reason, but also manifest a capacity for subsequent recruitment
(with minimal change) to substantially different and novel functions. The study and
systematization of these formal and structural reasons for evolvability sets an
important agenda, now largely unfulfilled but attracting considerable interest, for
evolutionary biology.
To return to my previous example, the agnathan ancestor that built a series of v-
shaped, backward-pointing gill arches, each made of several rod-like elements, for
pumping water to breathe and feed, evolved these features for its own immediate
needs, and not (obviously) with any forethought about modifiability into jaws that
might one day surround its unsupported mouth. But if the elements of the foremost
arch had not inherently possessed the form, the positioning, the coordination, and the
developmental potential to move to a more anterior position surrounding the mouth,
the gnathostome lineage would never have emerged, the agnathans might have
remained a relatively minor component of marine faunas (or become extinct entirely),
and terrestrial environments, to this day, might have remained the domain of plants
and insects—perfectly competent and "happy" ecosystems, building a lovely earth
teeming with life, but evolving nothing conscious to proclaim its aesthetic, extol its
virtues, or to record, perhaps even to seal, its doom. We must thank both this
contingent good fortune, and the latent structural possibilities of gill arches, for this
shot at our own particular brand of record keeping (even of "immortality" in some
operationally meaningful sense of the term).
The story, of course, continues from there (and for each lineage), with a constant
twinning of contingency and structural potentiality. If one marginal group of fishes
had not evolved a peculiar fin, with a branching central element orthogonal to the
body's antero-posterior axis (rather than parallel to the axis, as in most self-respecting
members of the clade and guild), no support firm enough to build the centerpiece of a
limb for terrestrial life might ever have emerged within the lineage of vertebrates.
And if these resulting tetrapods had never evolved their forelimb for terrestrial
locomotion, the celebrated convergence of aerodynamic form in the wings of bats,
birds and