680 THE STRUCTURE OF EVOLUTIONARY THEORY
- Each hierarchical level differs from all others in substantial and interesting
ways, both in the style and frequency of patterns in change and causal modes.
Nature's hierarchy, for all the commonality of its unifying principles (selection, for
example, acting at each level), does not display fractal structure with self-similarity
across levels.
As the theory of hierarchical selection develops, I predict that no subject
within its aegis will prove more fascinating than the varying strengths and
modalities among levels. Just as the study of allometry has recorded characteristic
and predictable scale-dependent differences in structure and function of organisms
at strongly contrasting sizes—a prominent subject in biology ever since Galileo
formulated the principle of surfaces and volumes in 1638, and so elegantly codified
in D'Arcy Thompson's masterpiece of both prose and concept, On Growth and
Form (1917, second edition, 1942)—so too does individuality as a tiny gene imply
substantially different properties for a unit of selection than "personhood" as a
large species or an even larger clade. Allometric effects across hierarchical levels
should greatly exceed the familiar (and extensive) differences between tiny and
gigantic organisms for two unsurprising reasons (see Gould and Lloyd, 1999, for a
detailed development of this argument). First, the size ranges among levels are far
greater still. Second, organisms share many common properties simply by
occupying a common level of evolutionary individuality despite an immense range
of size; but the levels themselves differ strongly in basic modes of individuality,
and therefore develop far greater disparity.
But this promise also implies a corresponding danger. In some famous lines
composed for a quite different, but interestingly related purpose, Alexander Pope
explored the paradox of man's intermediary status between two such disparate
extremes, both so desperately needed to know and to understand (the bestial and
the godly in Pope's concern)—but both so inscrutable as so far from our own
being:
Placed on this isthmus of a middle state,
A being darkly wise and rudely great...
He hangs between; in doubt to act or rest;
In doubt to deem himself a god, or beast...
Created half to rise, and half to fall;
Great lord of all things, yet a prey to all;
Sole judge of truth, in endless error hurl'd;The glory, jest, and riddle of the world!^
I appreciate this image of an "isthmus of a middle state"—a narrow standing place
linking two larger worlds of smaller and greater. Pope's dilemma may pack more
emotional punch in its moral meaning (since his greater and lesser worlds define
questions of value rather than geometry), but our problem features greater
intellectual depth—for, surely, a larger conceptual chasm separates the gene from
the clade in modes of evolutionary mechanics, than the bestial from the virtuous in
styles of human behavior.
The problem can be summarized with another, and much older, classical
quotation. "Man is," as Protagoras, wrote in his wonderfully ambiguous epigram,