1226 THE STRUCTURE OF EVOLUTIONARY THEORY
wings to fly (as does the egret, albeit as a "demoted" secondary utility in the species'
current habitat). Any knowledgeable biologist would then add— hence my fondness
for this example of double quirky functional shift—that feathers were initially
coopted for flight in a much older functional shift, a defining transition in avian
phylogeny from a different initial role, perhaps in thermoregulation. This old
hypothetical argument has now been fortified by two sources of evidence:
experimental data on the thermodynamic, with no accompanying aerodynamic,
advantages of tiny protowings (Kingsolver and Koehl, 1985); and historical data on
the probable origin of birds from the smallest-bodied lineage of running dinosaurs, a
group that might have experienced the greatest functional need for supplementary
thermoregulation, given their highest levels of activity combined with highest
surface/volume ratios (as a consequence of minimal body size), of any lineage within
the dinosaurian clade. Indeed, this example has served as the canonical illustration,
ever since Mivart's defining book of 1871, for "the problem of incipient stages of
useful structures," or quirky functional shift (and also as the eponym for my
designation, in nontechnical writing, as "the 5 percent of a wing problem").
Faced with this argument, our Parsifal might continue to reject contingency, and
embrace predictability, on the false assumption that natural knowledge, as
"scientific," must be so constituted. Even after we mock his previous conviction that
he knows why egrets developed wings when he understands their present use, we
discover that he has not generalized the message, for he now argues from the opposite
temporal end: "But if I, as a great scientist with full knowledge of evolutionary
theory, had visited the Earth in early Jurassic times and observed the avian ancestor
as a small running dinosaur using feathers on protowings for supplementary
thermoregulation, I surely could have concluded that this animal would evolve larger
wings and eventually enter a realm of cooptable utility for flight. I would also know
that, for 150 million years, the ancestors of African herons would use those wings for
flight, and then, on a continent to be called Africa [for our seer also knows the future
history of plate tectonics on Earth] this one little avian lineage would redeploy those
old thermoregulatory organs for yet another novel task of shading water." At this
point, and continuing the literary analogy, we could only hope that our Parsifal finds
the Holy Grail of quirky functional shift, and abandons his foolish ways for the path
of wisdom!
Needless to say, the actual history of any key organ in any major lineage far
surpasses this avian cartoon both in complexity and in number of episodes of
functional shifting. Just consider, for starters, the passage from a cartillaginous rod-
like element, functioning to support the agnathan gill, to the hyomandibula of
gnathostome fishes (used primarily to suspend the upper jaw from the cranium), to
the stapes of tetrapods (following fusion of the upper jaw to the skull, and responding
to a functional need for a different mode of sound perception in air vs. water).
This quirky historical character of major evolutionary change in particular
lineages—thoroughly explainable after the fact, however unpredictable in principle
beforehand—constitutes the greatest fascination of the subject for