286 THE STRUCTURE OF EVOLUTIONARY THEORY
no new organ but merely combines and modifies organs already known to us"
(1790, no. 38).
When parts become too distinct to show connection and reduction to the leaf
archetype in one species, Goethe uses the comparative approach to find transitional
forms in other taxa. The seedpod and sexual organs are manifestly unleaflike in
many plants, but Goethe establishes transitional series to species with, for example,
leaflike seedpods, or fertile stem leaves (as in ferns). Consider his exposition of the
comparative method for "difficult" seedpods: "Nature obscures the similarity to the
leaf most when she makes the seed containers soft and juicy or firm or woody;
however, the similarity will not escape our attention if we contrive to follow it in
all its transitional stages" (1790, No. 79). Or for the even more divergent
cotyledons that eventually grow into tolerably leaflike form:
- They are often misshapen, crammed, as it were, with crude matter, and
as much expanded in thickness as in breadth; their vessels are unrec-
ognizable and scarcely distinguishable from the mass as a whole. They bear
almost no resemblance to a leaf, and we might be misled into regarding
them as special organs. - Yet in many plants the cotyledons approach leaf form: they flatten out;
exposed to light and air, they assume a deeper shade of green; their vessels
become distinct and begin to resemble veins. - Finally they appear before us as true leaves: their vessels are capable of
the finest development; their similarity to the subsequent leaves will not
permit us to consider them separate organs; and we recognize them instead
as the first leaves of the stem (1790, Nos. 12-14).
If Goethe's system really advocated, as often misportrayed, a simple and
exclusive concept of the archetypal leaf, his theory could stake no claim for
interesting completeness—for this central principle cannot explain systematic
variation in form up the stem, and therefore could not operate as a full explanation
for both similarities and characteristic differences in the parts of plants. But, in his
most fascinating intellectual move, Goethe proposes a complete account by
grafting two additional principles onto the underlying notion of archetype: the
progressive refinement of sap, and cycles of expansion and contraction. We may
regard these principles as ad hoc or incorrect today, but the power of their
conjunction with the archetypal idea can still be appreciated with much profit.
These two additional principles embody both necessary sides of the primary
Western metaphor for intelligibility in any growing, or historically advancing,
system—arrows of direction and cycles of repeatability (I called these conjoined
principles "time's arrow" and "time's cycle" in my book on the discovery of
geological time—Gould, 1987b). We must, in any temporal process, be able to
identify both sources of story and order: vectors of change (lest time have no
history, defined as distinctness of moments), and underlying constant or cyclical
principles (lest the temporal sequence proceed only as one uniqueness after
another, leaving nothing general to identify at all). Goethe,