Structural Constraints, Spandrels, and Exaptation 1225
the actual pageant of life's history by allowing that no nomothetic theory (and ours is
'as good as it gets') can specify actual pathways without factoring in the historical
particulars that only your record preserves." (I have scarcely hid my conviction,
either in this book or elsewhere, that this truce always operated as a "lousy deal" for
the science of paleontology.)
In a minimal sense, Darwinian theory must grant this space to contingency—if
only because even the most basic and least sophisticated form of the theory holds that
organisms adapt to changing local environments (and do not follow preestablished
routes towards "progress" or any other goal). Since we all admit that local
environments change on an erratic and contingent vector through time, life's overall
pathway must be dominated by contingent factors, even if every immediate event of
natural selection could, in principle, achieve a deterministic explanation in local
environmental terms. (After all, this feature of Darwinism as emphasized in Chapter
2, and as long appreciated by intellectual historians, established the most radical
aspect of natural selection from the start—as contrary to all earlier evolutionary
speculations, with their assumptions about law like directionality, usually regulated
by divine intent.)
But if contingency resided only in this basic aspect of environmental scaling,
then the principle, though sound enough, would not run so deep in Darwinian
traditions. Rather, contingency gains its greatest force through the principle of quirky
functional shift: the discordance between historical origin and current utility, and the
consequent fallacy of direct inference from modern status to initial meaning.
Nietzsche emphasized the primary role of this discordance in the study of history by
writing (as quoted more fully on p. 1217) that "the development of a thing, a
tradition, an organ is therefore certainly not its progressus towards a goal," and that
the inevitability of functional shift makes any important historical sequence "instead
... a succession of more or less profound, more or less mutually independent
processes of subjugation exacted on the thing."
Even a unidirectional sequence of changing form with basically unaltered
function would require explicit knowledge of contingent environmental histories for
anything close to full or satisfactory explanation. The addition of quirky functional
shift, usually in several episodes for each organ in any complex phylogeny,
guarantees a cardinal role for historical explanation in any major lineage (again, as
Nietzsche recognized). In a personally favorite example, for combining the canonical
general case with a particular ending twist, the African black heron, Egretta
ardesiaca, uses its wings largely to shade the shallow water of its habitat, thus
providing a clear view of available prey (my thanks to E. Vrba for this example, as
discussed in Gould and Vrba, 1982).
Any intelligent person with a sense of history's length and meaning could
identify the author of the following statement as a modern Parsifal, or perfect fool:
"Aha, now I know why herons evolved wings—in order to eat, for they would starve
if they couldn't shade the water and see their food." Our savvy interlocutor would
offer the obvious refutation that most birds use their