108 THE STRUCTURE OF EVOLUTIONARY THEORY
correct and crucially important, for the logic works equally well in either direction.
Ancestral fishes maintained two systems for breathing—gills and lungs (as do
modern lungfish, taxonomically called Dipnoi, or "two breathing"). The original
lung probably played a subsidiary role in buoyancy; this function could be
enhanced, and the original use in breathing deleted, because gills could adopt the
entire respiratory burden. Darwin wrote (pp. 204-205): "For instance, 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."
As a second response, Darwin proceeded beyond conceivability and tried to
document actual sequences for supposedly impossible transitions—as in the
evolution of a light-sensitive spot into an "organ of extreme perfection" like the
vertebrate eye. These sequences cannot represent true phylogenies (since they
consist solely of living species), but they do constitute structural series illustrating
the conceivability of transitions. After admitting, for example, that the gradual
evolution of such a miracle of workmanship as the eye "seems, I freely confess,
absurd in the highest possible degree" (p. 186), Darwin presents a structural series
of disparate animals, including working configurations proclaimed impossible by
opponents: "Yet reason tells me, that if numerous gradations from a perfect and
complex eye to one very imperfect and simple, each grade being useful to its
possessor, can be shown to exist... then the difficulty of believing that a perfect and
complex eye could be formed by natural selection, though insuperable by our
imagination, can hardly be considered real" (p. 186).
Darwin applies this principle to behavior and its products, as well as to form.
For the exquisite mathematical regularity of the honeycomb, he writes (p. 225):
"Let us look to the great principle of gradation, and see whether Nature does not
reveal to us her method of work." (See also page 210 on complex instincts and
their explanation by the establishment of structural series.)
CONSILIENCE (CONCORDANCE OF SEVERAL). Darwin took great pride in
his formulation of natural selection as a theory for the mechanism of phyletic
change. But he granted even more importance to his relentless presentation of
dense documentation for the factuality of change—for only such a cascade of data
would force the scientific world to take evolution seriously. (The contrast between
the Origin as a compendium of facts, and Lamarck's Philosophie zoologique as a
purely theoretical treatise, strikes me as an even more distinguishing difference
than the disparate causal mechanisms proposed by the two authors.) Facts literally
pour from almost every page of the Origin, a feature that became even more
apparent following Darwin's forced change of plans, and his decision to compress
his projected longer work into the "abstract" that we call the Origin of Species—a
revised strategy that led him to omit almost every reference and footnote, and
almost all discursive discussion between bits of information. In some parts, the
Origin reaches an