SCOPE. Even the most favorably minded of contemporaries often admitted that
Darwin had developed a theory capable of building up small changes (of an
admittedly and locally "positive" nature as adaptations to changing environments)
within a "basic type"—the equivalent, for example, of making dogs from wolves or
developing edible corn from teosinte. But these critics could not grasp how such a
genuine microevolutionary process coukl be extended to produce the full panoply
of taxonomic diversity and apparent "progress" in complexification of morphology
through geological time. Darwin insisted on full sufficiency in extrapolation,
arguing that his micro-evolutionary mechanism, extended through the immensity
of geological time, would be fully capable of generating the entire pageant of life's
history, both in anatomical complexity and taxonomic diversity—and that no
further causal principles would be required.
Because primates are visual animals, complex arguments are best portrayed or
epitomized in pictorial form. The search for an optimal icon to play such a role is
therefore no trivial matter (although scholars rarely grant this issue the serious
attention so richly merited)—especially since the dangers of confusion, misplaced
metaphor, and replacement of rigor with misleading "intuition" stand so high. I
knew from the beginning of this work that I needed a suitable image for conveying
the central logic of Darwinian theory. As one of my humanistic conceits, I hoped
to find a historically important scientific image, drawn for a different reason, that
might fortuitously capture the argument in pictorial form. But I had no expectation
of success, and assumed that I would need to commission an expressly designed
figure drawn to a long list of specifications.
The specific form of the image—its central metaphorical content, if you
will—plays an important role in channeling or misdirecting our thoughts, and
therefore also requires careful consideration. In the text of this book, I speak most
often of a "tripod" since central Darwinian logic embodies three major propositions
that I have always visualized as supports—perhaps because I have never been
utterly confident about this entire project, and I needed some pictorial
encouragement to keep me going for twenty years. (And I much prefer tripods,
which can hold up elegant objects, to buttresses, which may fly as they preserve
great Gothic buildings, but which more often shore up crumbling edifices.
Moreover, the image of a tripod suits my major claim particularly well—for I have
argued, just above, that we should define the "essence" of a theory by an absolutely
minimal set of truly necessary propositions. No structure, either of human building
or of abstract form, captures this principle better than a tripod, based on its
absolute minimum of three points for fully stable support in the dimensional world
of our physical experience.)
But organic images have always appealed more strongly, and I preferred a
biological icon. If the minimal logic can be represented by a tripod pointing
downward, then the same topology can be inverted into a structure growing
upward. Darwin's own favorite image of the tree of life immediately suggested
itself, and I long assumed that I would eventually settle on a botanical
16 THE STRUCTURE OF EVOLUTIONARY THEORY