1224 THE STRUCTURE OF EVOLUTIONARY THEORY
malleus and incus of the ear because the new dentarysquamosal joint was already "up
and running," thus avoiding the specter of an inconceivable intermediate with an
unhinged jaw.
Interestingly, Darwin's own favorite example for coupling these two principles
of redundancy, repeated many times in the Origin of Species and still resident in most
biology textbooks today, is not only wrong, but backwards. He invoked this coupling
to explain the supposed conversion of piscine air bladders into lungs. But the same
argument works just as well, in reverse order, for the actual transformation of lungs
in plesiomorphic fishes to swim bladders in highly derived teleosts that did not
originate until the Triassic (Liem, 1988). Darwin wrote (1859, pp. 204-205): "A
swim-bladder has apparently been converted into an air-breathing lung. The same
organ having performed simultaneously very different functions, and then having
been specialized for one function; and two very distinct organs having performed at
the same time the same function, the one having been perfected whilst aided by the
other, must often have largely facilitated transitions." (Ancestral fish lungs can in-
deed also function for buoyancy, whereas, and more obviously, gills work as well as
lungs for breathing. Modern lungfishes retain both systems, as their formal name,
Dipnoi (or "two breathing") testifies.)
The two great historical and structural implications of quirky
functional shift
This principle of functional shift deserves far more prominence, and explicit
recognition, than it has ever received among evolutionary theorists. I have tried to
emphasize its vital role in establishing the contingency and unpredictability of
evolutionary change by an adjectival strategy of designation as "quirky functional
shift." In operational terms, we should acknowledge, most of all, the property of
major functional alteration based upon far more limited (in extreme cases, virtually
absent) structural change—another way of expressing the structuralist concept of
inherent flexibility in natural forms and designs (to different degrees that should be
subject to specification on a case by case basis). In any event, however textually
underemphasized, this principle has always played two important roles in standard
Darwinian theory:
AS THE GROUND OF CONTINGENCY FOR LIFE'S HISTORY. Thoughtful
Darwinians, no matter how confidently they have identified natural selection as the
exclusive cause of evolutionary change, have always recognized that their theory
necessarily underpredicts the actual pathways of life's history—and that explanations
for the byways of individual lineages (and major aspects of the highways as well) can
only be located in the factual record of particulars. This concept potentiated the tacit
truce that, until recent years, held between paleontologists and Darwinian theorists
under the Modern Synthesis. Under accepted terms, the theorists said to the
paleontologists: "give up your old claims about special macroevolutionary
mechanisms, and admit our contention that microevolutionary population genetics
and natural selection hold full theoretical sufficiency. We will then grant you control
over