The Structure of Evolutionary Theory

by Stephen Jay Gould

  14. 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, [Page 287] faced with observations of both directionality and repeatability up the stem, recognized the need for both poles of this dichotomy.

  Refinement of sap as a directional principle. Up and down, heaven and hell, brain and psyche vs. bowels and excrement, tuberculosis as a noble disease of airy lungs vs. cancer as the unspeakable malady of nether parts (see Susan Sonntag's Illness as Metaphor for a brilliant analysis of these conventional images). Almost irresistibly, we apply this major metaphorical apparatus of Western culture to plants as well — with gnarly roots and tubers as lowly objects of the ground, and fragrant, noble flowers as topmost parts, straining towards heaven. Goethe, by no means immune to such thinking in an age of Naturphilosophie, viewed the growth of a plant as progressing to­wards refinement from cotyledon to flower. He explained this directionality by postulating that, moving up the stem, each successive leaf modification progressively filters an initially crude sap. Inflorescence cannot occur until these impurities have been removed. The cotyledons begin both with minimal organization and refinement, and with maximal crudity of sap: “We have found that the cotyledons, which are produced in the enclosed seed coat and are filled to the brim, as it were, with a very crude sap, are scarcely organized and developed at all, or at best roughly so” (1790, No. 24).

  The plant then grows towards a floral apotheosis, but too much nutriment delays the process of filtering sap — as material rushes in and more stem leaves must be produced for drainage. A decline in nutriment finally allows filtering to attain the upper hand, and the sap becomes sufficiently pure for inflor­escence: “As long as cruder saps remain in the plant, all possible plant organs are compelled to become instruments for draining them off. If excessive nutri­ment forces its way in, the draining operation must be repeated again and again, rendering inflorescence almost impossible. If the plant is deprived of nourishment, this operation of nature is facilitated” (1790, No. 30). Finally, the plant achieves its topmost goal: “While the cruder fluids are in this man­ner continually drained off and replaced by pure ones, the plant, step by step, achieves the status prescribed by nature, We see the leaves finally reach their fullest expansion and elaboration, and soon thereafter we become aware of a new aspect, apprising us that the epoch we have been studying has drawn to a close and that a second is approaching — the epoch of the flower” (1790, No. 28).

  Cycles of expansion and contraction. If the directional force worked alone, then a plant's morphology would only express this smooth continuum of progressive refinement up the stem. Since, manifestly, plants do not display such a pattern, some other force must be operating.* Goethe describes [Page 288] this second force as cyclical, in opposition to the directional principle of refining sap. He envisages three full cycles of contraction and expansion during ontogeny. The interplay of these progressive and cyclical forces pro­duces the full pattern of a general refinement up the stem, but impacted by discontinuities and transitions that express no directional pattern (“contrac­tion” of stem-leaves to sepals by bunching together in a circlet, for example). The cotyledons begin in a retracted state. The main leaves, and their substan­tial spacing on the stem, represent the first expansion. The bunching of leaves to form the sepals at the base of the flower marks the second contraction, and the subsequent elaboration of petals the second expansion. The reduction of archetypal leaf size to form pistils and stamens marks the third contraction, and the formation of fruit the last and most exuberant expansion. The con­tracted seed within the fruit then starts the cycle again in the next generation. Put these three formative principles together — the archetypal leaf, progressive refinement of sap up the stem, and three expansion-contraction cycles of veg­etation, blossoming, and bearing fruit — and the vast botanical diversity of our planet falls under the chief vision of formalism: production of realized va­riety from interaction of a few abstract, general, and internally based (not ex­ternally imposed and adaptationally driven) morphological laws: “Whether the plant vegetates, blossoms, or bears fruit, it nevertheless is always the same organs with varying functions and with frequent changes in form, that fulfill the dictates of nature. The same organ which expanded on the stem as a leaf and assumed a highly diverse form, will contract in the calyx, expand again in the petal, contract in the reproductive organs, and expand for the last time as fruit” (1790, No. 115).

  This formalist commitment implies an aversion to primary explanation by adaptation, function or final cause. In accord with all the great formalists, Goethe often expressed his dislike of explanations based upon the externality of fit between form and function (though he delighted in the evident fact of such fit, as formalists also generally do, for such an admission poses no threat to the chief formalist argument for primacy of morphological order — see Chapter 11 on exaptation).

  Goethe's statements on final cause often attack the larger idea of manufac­ture for explicitly human ends — not the chief complaint of formalist mor­phology, but worth recording, if only for the power of Goethe's prose:

  For several centuries down to the present, we have been retarded in our philosophic views of natural phenomena by the idea that living organ­isms are created and shaped to certain ends by a teleological life force. ... Why should he not call a plant a weed, when from his point of view it really ought not to exist: He will much more readily attribute the exis­tence of thistles hampering his work in the field to the curse of an en­raged benevolent spirit, or to the malice of a sinister one, than simply re­gard them as children of the universal nature, cherished as much by her as the wheat he carefully cultivates and values so highly (from essay of 1790, in Mueller and Engard, 1952). [Page 289]

  But Goethe also attacked adaptationist primacy in the more focused realm of explaining morphology: “It is not a question of whether the concept of final causes is convenient, or even indispensable, to some people, or whether it may not have good and useful results when applied to the moral realm; rather, it is a question of whether it is an aid or a deterrent to physiologists in their study of organized bodies. I make bold to assert that it does deter them, therefore avoided it myself and considered it my duty to warn others against it” (2nd essay on plant metamorphosis, written in 1790, in Mueller and Engard, 1952, p. 80).

  Citing a perennial complaint, then and now, against adaptationist explanations — that such efforts tell good stories i
n the speculationist mode, but do not explain morphology — Goethe compares final causes with Linnaeus' fan­ciful descriptions of sexual anatomy in plants: “For example, Linne calls flower petals 'curtains of the nuptial bed,' a parable that would do honor to a poet. But after all! the discovery of the true physiological nature of such parts is completely blocked in this way, just as it is by the convenient and false es­pousal of the theory of final causes” (2nd essay on plant metamorphosis, written in 1790, in Mueller and Engard, 1952, pp. 79-80).

  Proper morphological explanations, Goethe asserts, must be sought on internal and formalist principles; external fit, though of great importance, can only be regarded as secondary: “In my opinion, the chief concept underlying all observation of life — one from which we must not deviate — is that a crea­ture is self-sufficient, that its parts are inevitably interrelated, and that noth­ing mechanical, as it were, is built up or produced from without, although it is true that the parts affect their environment and are in turn affected by it” (2nd essay on plant metamorphosis, written in 1790, in Mueller and Engard, 1952, p. 80).

  In a remarkable passage, that could serve as a credo for modern formalism as well, Goethe asserts his central claim for internalist primacy, while also specifying the vital, but secondary, role of adaptation. Internal formation acts as a primary source that “must find external conditions.” Adaptation may then shape a range of diversity from an underlying form, but the arche­typal pattern cannot be explained by these secondary modifications, and the adaptations themselves can only express a superficial restructuring of inher­ent order:

  Man, in considering all things with reference to himself, is obliged to assume that external forms are determined from within, and this assump­tion is all the easier for him in that no single living thing is conceivable without complete organization. Internally, this complete organization is clearly defined; thus it must find external conditions that are just as clear and definite, for its external existence is possible only under certain con­ditions and in certain situations.... An animal possesses external useful­ness precisely because it has been shaped from without as well as from within, and — more important and quite natural — because the external element can more readily adapt the external form to its own purposes [Page 290] than it can reshape the internal form. We can best see this in a species of seal whose exterior has taken on a great deal of the fish character while its skeleton still represents the perfect quadruped (2nd essay on plant metamorphosis, written 1790, in Mueller and Engard, 1952, p. 83).

  Goethe's views therefore provide a “test case” for a primary thesis of this book. We should, I believe, recognize the space of our intellectual world as inherently structured, by some combination of our evolved mental quirks and the dictates of logic, into a discontinuous array of possible, coherent posi­tions — hence the double entendre in the title of this book. These mental posi­tions express “morphologies,” just as organisms do. The chief components of these “morphologies” must reside together and interact to build the “es­sence” of any powerful intellectual system. The components of a theory's es­sence should be recognized as both deep and minimal; with other less impor­tant and potentially dispensable principles allied to them in secondary webs subject to “restructuring” by “adaptation.” (Thus I advocate a minimal set of three principles for defining the essence of Darwinism, while regarding other components of the usual Darwinian nexus as conjoined more loosely and less central intellectually.) These essential and minimal components remain corre­lated, although arising independently and in reiterated fashion, across lan­guages, centuries and cultural traditions. Such firm linkages define the struc­ture of these few nucleating positions in the intellectual landscape.

  In formalist or structuralist theories, the strongest correlation unites a commitment to generative laws of form with an aversion to adaptationist explanation as the primary goal of morphology. The two commitments need not conjoin in logic or empirical necessity; indeed, Darwin found a brilliant argu­ment to drive them asunder by identifying most (though not all) generating principles as past adaptations, and relegating remaining laws of form to a pe­ripheral or secondary status (see section 1 of this chapter). But almost every formalist theory of morphology also views adaptation as secondary tinkering rather than primary structuring.

  I regard Goethe as an exemplar of this approach to major scientific theories because he was, in an important sense, an outsider to the swirling debates of formalism vs. functionalism in his time. He understood, of course, his affinity with the formalism of German Naturphilosophie. But he did not attend the debates, publish in the journals, or use the lingo of developing scientific pro­fessionalism. He viewed himself as apart and neglected. In fact, he didn't even regard the debate between Cuvier and Geoffroy, which fascinated him so keenly at the end of his life (see pages 310–312), primarily as a struggle be­tween formalism and functionalism, but rather as a contest between the em­piricism of Cuvier and the intuitionism of transcendental morphology — and his explicit preference for Geoffroy invoked his poet's concern with the pri­macy of abstract ideas as much as his morphologist's attention to the primacy of form.

  In this context of Goethe's separation from the core of scientific contro­versy in 1830, we should not treat his own formalism as derivative, imitative, [Page 291] or simply imbibed from the stated mores of a recognized intellectual brother­hood. If his views also feature — as they do — a linkage of interest in laws of form with an antipathy to adaptationist explanation, then we may interpret the correlation as independently generated, at least in part, and therefore as good evidence for a link based upon intrinsic intellectual entailment in the “morphology” of formalism as a key “nucleating” idea in biology.

  Indeed, Goethe showed a strong appreciation for the morphology (and, in this case, the utility) of dichotomy in intellectual life. In discussing his under­standing of the division between Cuvier and Geoffroy, Goethe noted that each man defended not a single idea or a unitary position, but rather a nexus or complex of mutually entailed notions, causing a precipitation at one of two foci — with these two aggregations then opposing each other like the poles of a magnet. For Goethe, the systems of Cuvier and Geoffroy formed “two different doctrines, which are so ordinarily and so necessarily separated that little chance exists for finding them together in a single person. On the contrary, it is of their essence that they not be well allied” (Goethe, 1831, p. 181). For Darwin, discontinuity originates by historical contingency (fol­lowing extinction of intermediate forms) in a fully accessible and isotropic morphospace. Natura non facit saltum. But the universe of formalism — in ideas and in morphology — views discontinuity as inherent in the structure of inhabitable space.

  GEOFFROY AND CUVIER

  Cuvier and Conditions of Existence

  The struggle of Cuvier and Geoffroy continues to rivet our attention (from Russell, 1916, to the modern book of Appel, 1987) because this conflict fea­tures the two central elements of intellectual drama: a clash of two superior minds within the primal tale of professional ontogeny: two scholars begin as warm friends fired with the idealism of youth, and end as wily, cynical, politi­cally astute opponents. (The conventional view interprets Cuvier as a clear winner by virtue of such astuteness and Geoffroy as loser by naiveté and woolliness. I shall defend a different version of interesting, disparately styled, equality.)

  When the French revolutionary government established the Museum d'histoire naturelle as the world's finest in 1793, Etienne Geoffroy Saint-Hilaire, installed as the first curator for vertebrates, played a primary role in bringing Georges Cuvier to Paris, thus launching his scientific career. The two men enjoyed close friendship, sharing living quarters and making idealistic plans for the reform and flowering of natural history. In 1798, as Geoffroy embarked with Napoleon upon a long expedition to Egypt, he wrote to Cuvier: “Goodbye my friend, love me always. Do not cease to consider me as a brother” (in Appel, 1987, p. 73).

 
But their differences in temperament, intellect and style eventually and inevitably drove them apart. Cuvier became one of the most powerful, politi­cally conservative figures ever to operate in Western science. The oft quoted [Page 292] statement of the awestruck Charles Lyell, visiting Cuvier at the height of his influence, provides insight into the nature of his power:

  I got into Cuvier's sanctum sanctorum yesterday, and it is truly characteristic of the man. In every part it displays that extraordinary power of methodizing which is the grand secret of the prodigious feats which he performs annually without appearing to give himself the least trouble ... There is first the museum of natural history opposite his house, and admirably arranged by himself, then the anatomy museum connected with his dwelling. In the latter is a library disposed in a suite of rooms, each containing works on one subject. There is one where there are all the works on ornithology, in another room all on ichthyology, in another osteology, in another law books! etc.... The ordinary studio contains no bookshelves. It is a longish room comfortably furnished, lighted from above, with eleven desks to stand to, and two low tables, like a public of­fice for so many clerks. But all is for the one man, who multiplies himself as author, and admitting no one into this room, moves as he finds neces­sary, or as fancy inclines him, from one occupation to another. Each desk is furnished with a complete establishment of inkstand, pens, etc. . . . There is a separate bell to several desks. The low tables are to sit to when he is tired. The collaborators are not numerous, but always chosen well. They save him every mechanical labour, find references, etc., are rarely admitted to the study, receive orders and speak not (in Adams, 1938, p. 267).