The Structure of Evolutionary Theory

by Stephen Jay Gould

  Evolution does not establish an ultimate divide for all transitions in the his­tory of biology. Several themes pass right through this great revision, only al­tering their terms and explanations. Formalism vs. functionalism may be the most prominent and persistent of issues too grand even for evolution to undo (or fully resolve). Paley and Agassiz once fought this battle in grand style; Dawkins and Goodwin cannot cast so broad a conceptual net, or muster the same stylistic panache today, but they pursue the same conflict. Paley vs. Agassiz remains relevant to modern evolutionists by the primary criterion of genealogical continuity.

  If Paley and Agassiz represent the yin and yang of totality for the analysis of form, then Darwin, though a pluralist who understood both poles, did ulti­mately cast his lot with the Paleyan yin, in filial piety with a British tradition that has spanned centuries, and still continues today. This imbalance, and the struggle for redress that now commands so much discussion in contemporary evolutionary biology, defines one of the three major issues that led me to write this book. The formalist alternative, as embodied in the subject now generally called “constraint,” provides a counterweight to stabilize the sec­ond leg in Darwin's essential tripod of support — the primacy of adaptation in asserting the creativity of natural selection at overwhelming relative fre­quency among the causes of evolutionary change.

  The past holds sufficient interest and capacity for illumination all by itself, and no justification in terms of present enlightenment need ever be given. Still, as a practicing scientist, I do favor the use of history as a current guide — while I struggle not to wrench the meaning and motivation of arguments from the primary matrix of their own time. I don't know how else to proceed when tides of history overwhelm a worthy subject for little reason beyond the vagaries of fashion and contingency. Scientists too often become convinced that inexorable logic or irrefutable data have closed a subject forever. Even worse, given our propensity for historical ignorance, we often collectively forget that an alternative ever existed at all. In such cases, I know no better tactic for reopening an important subject than the record of history — the proof that brilliant scientists (so worthy of our admiration that we cannot be­little their concerns) devoted their concentrated attention to an issue that never achieved true settlement, but only veered towards transient “resolu­tion” by sociological complexities of shifting preferences, rather than logic of proof or exigencies of data. I believe that structuralist and formalist ap­proaches to anatomy fell out of favor for such invalid reasons of fashion, and that the full range of this primary dichotomy must now be reestablished. And I unabashedly call upon the great formalists of history to state their case; while I ask modern evolutionists to make the proper translation to mod­ern terms. [Page 280]

  To reassert the importance of both poles in this dichotomy, I again cite my primary candidate for the unenviable title of most worthy “invisible man” — an important and influential thinker and educator in his day, but now entirely forgotten. I tried to resurrect the Reverend James McCosh, president of Princeton University, in Chapter 2 (pp. 116-118), and I now again want to call upon his fine book, published in 1869 in collaboration with George Dickie: Typical Forms and Special Ends in Creation. The Greek inscription on the title page — typos kai telos (type and purpose) — epitomizes the argu­ment. The two poles of the dichotomy inhere in all natural objects, and full explanation demands attention to both:

  In taking an enlarged view of the constitution of the material universe, so far as it falls under our notice, it may be discovered that attention, at once extensive and minute, is paid to two great principles or methods of procedure. The one is the Principle of Order, or a general plan, pattern, or type, to which every given object is made to conform with more or less precision. The other is the Principle of Special Adaptation, or partic­ular end, by which each object, while constructed after a general model, is, at the same time, accommodated to the situation which it has to oc­cupy, and a purpose which it is intended to serve. These two principles . . . meet in the structure of every plant and every animal (McCosh and Dickie, 1869, p. 1).

  McCosh also recognized the contingent and socially embedded nature of national preferences. He notes that English tradition — from Robert Boyle and John Ray through Paley to the Bridgewater Treatises — has favored the adaptationist theme. Thus, he argues, recent discoveries in formalist mor­phology have been viewed as threatening by some biologists (McCosh cites the French and German schools of ideal or transcendental morphology, espe­cially in their English translation through the work of Richard Owen, whom I treat later in this chapter): “The arguments and illustrations adduced by Brit­ish writers for the last age or two in behalf of divine existence, have been taken almost exclusively from the indications in nature of special adaptation of parts. Hence, when traces were discovered in the last age of a general pat­tern, which had no reference to the comfort of the animal or the functions of the particular plant, the discovery was represented by some as overturning the whole doctrine of final cause; not a few viewed the new doctrine with sus­picion or alarm” (McCosh and Dickie, 1869, pp. 6-7).

  But McCosh regards this perceived threat as false, and urges that formalist insights be welcomed — for full explanation demands attention to both poles. McCosh expresses the two key ideas in religious terms as natural illustrations of “lofty wisdom” (formalism) and “providential care” (functionalism). We call the same themes constraint and adaptation, but the image of exquisite balance remains every bit as valid today:

  We do not know whether to admire most the all-pervading order which runs through the whole of nature, through all the parts of the plant and [Page 281] animal, and through the hundreds of thousands of different species of plants and animals, or the skillful accommodation of every part, and of every organ, in every species, to the purpose which it is meant to serve. The one leads us to discover the lofty wisdom which planned all things from the beginning, and the enlarged beneficence reaching over all with­out respect of persons; whereas the other impresses us more with the providential care and special beneficence which, in attending to the whole, has not overlooked any part, but has made provision for every in­dividual member of the myriads of animated beings (p. 439).

  Though mercilessly savaged for intellectual mediocrity by W. S. Gilbert and other satirists and activists, the British peerage did turn out an occasional scholar or two. The Duke of Argyll might have won his title fair and square if Gilbert's ultimate recommendation had ever been instituted. (The Fairy Queen in Iolanthe, royally pissed off at a group of nobles, threatens: “peers shall teem in Christendom, and a Duke's exalted station be attainable by competitive examination!”) In a presidential address to the British Associa­tion, the good Duke, as a prominent critic of evolution and author of several books still worth reading today, argued that relations between both necessary poles of the dichotomy still persisted as a key issue in Darwin's new biology: “What is the meaning of that great law of adherence to type and pattern, standing behind, as it were, and in reserve, of that other law by which organic structures are specially adapted to special modes of life? What is the relation between these two laws; and can any light be cast upon it derived from the history of extinct forms; or from the conditions to which we find that existing forms are subjected?” (quoted in McCosh and Dickie, 1869, p. 68).

  Since then, countless events, from meanderings of history to permanencies of empirical discovery, have rocked this subject back and forth. But equi­librium at a center of dynamic tension, not of complacent rest, may foster our best biological understanding, and the Duke's question could not be more current, more a propos.

  Unity of Plan as the Strongest Version of Formalism:

  The Pre-Darwinian Debate

  MEHR LICHT ON GOETHE'S LEAF

  A prevalent myth of our time proclaims that broad and interdisciplinary vi­sions, though held in disrepute today, were once valued in a more ecumenical age that celebrated the “Renaissance man.” But the m
otto that “a cobbler should stick to his last”* dates from the 4th century bc, and people who wander [Page 282] outside their primary field have always attracted suspicion or ridicule. In 1831, near the end of a long life, a poet who had ventured into science de­plored his failure to obtain a fair hearing, but defended his forays as inter­nally necessary for a broad and searching intellect:

  The public was taken aback, for inasmuch as it wishes to be served well and uniformly, it demands that every man remain in his own field. This demand is well grounded, for a man who wishes to achieve excellence, which is infinite in its scope, ought not to venture on the very paths that God and nature do. For this reason it is expected that a person who has distinguished himself in one field, whose manner and style are generally recognized and esteemed, will not leave his field, much less venture into one entirely unrelated. Should an individual attempt this, no gratitude is shown him; indeed, even when he does his task well, he is given no spe­cial praise. But a man of lively intellect feels that he exists not for the public's sake, but for his own. He does not care to tire himself out and wear himself down by doing the same thing over and over again. More­over, every energetic man of talent has something universal in him, caus­ing him to cast about here and there and to select his field of activity ac­cording to his own desire (1831 essay, in Mueller and Engard, 1952, p. 16.9).

  We might ignore this statement, if its author stood among the many hope­fuls whom history fails to memorialize in either their chosen or their adopted professions. But the writer cited above, J. W. von Goethe, wrote a thing or two of enduring merit! Moreover, and in retrospect, his ventures into science far transcended the brief forays of an amateur dabbler.

  In any case, Goethe did not suffer complete neglect from scientists during his lifetime. In 1831, the great anatomist Etienne Geoffroy Saint-Hilaire praised Goethe's science as the work of “a poet trying to sing the grandeur of the universe in another form” (“un poete s'essayant de chanter sous une autre forme les grandeurs de l'univers” — 1831, p. 189). Geoffroy continued (1831, p. 193): “If Goethe had not already amassed enough titles to be proclaimed the greatest genius of his century, he would have added, to his crown of great poet and profound moralist, the fame of a wise naturalist — due to him for the profundity of his views, and for the philosophical force of his opinions on the subject of botanical analogies.”*

  But Geoffroy's praise (see Fig. 4-8) cannot be reckoned as entirely disinterested, for Goethe had just favored his side in the greatest brouhaha of early 19th century zoology — the celebrated 1830 debate with Cuvier before the [Page 283] Academie des sciences. Geoffroy needed all the help he could get (and he would later recruit other literary figures, including the novelists Balzac and George Sand, to his cause as well). Cuvier, after all, was no ordinary oppo­nent, and the subject of their argument — the age-old dispute of formalism and functionalism — could not have been more central to natural history.

  Geoffroy, with good cause, viewed Goethe as the doyen and spiritual leader of formalist morphology. Not only had Goethe coined the word “morphol­ogy,” but he had, long before, defended for plants the central proposition that Geoffroy championed for animals as the starting point for his anatomical views — the reduction of form to a single generating archetype (the leaf for Goethe, the vertebra for Geoffroy). While the young Geoffroy worried about establishing a career and surviving a revolution, Goethe was traveling in It­aly and developing the theory of his 1790 work, Versuch die Metamorphose der Pflanzen zu erkldren (Fig. 4-9). (This work, little more than a pamphlet, consists of 123 numbered and almost aphoristic paragraphs. I shall quote by number from the standard translation of Mueller and Engard (1952). But I have read and own a copy of the original, which I highly recommend to any­one who appreciates the fusion of great writing and fascinating science.)

  Goethe had been strongly interested in morphology throughout his life, and his preferences had always tended towards formalism, particularly to­wards the strongest version of the argument (and subject of this section) — the vision of a single, generating archetypal form, setting both the bounds and the possibilities of realized morphology. His two most famous forays into ani­mal anatomy both rested upon a formalist foundation: (1) his early support for the vertebral theory of the skull, a conviction that he traced to 1791 when he examined “a battered sheep skull from the sand of the dune like Jewish

  4-8. A letter from Etienne Geoffroy Saint-Hilaire, written in January 1831 to a Mr. Payaud, with Goethe featured in the penultimate paragraph. (Geoffroy pre­sumably enclosed some writings of Goethe along with this letter to Payaud.) The text of the paragraph reads as follows: “Goethe est ci-joint. Sa bonhomie qui n'exclue pas la force et la justesse d'esprit, vous frappera.” (Goethe is enclosed. His good nature, which does not preclude strength and fairness of spirit, will strike you.) (Author's collection.)

  [Page 284]

  4-9. Title page of Goethe's 1790 pamphlet on the growth and archetypal expressions of plants. (Author's collection.)

  cemetery in Venice” (1823 essay, in Mueller and Engard, 1952, p. 237); and (2) his discovery of the human premaxillary bone in 1784, based on its pres­ence in other mammals and his convictions about unity of type. (Goethe called this bone the intermaxilare; others referred to it as “Goethe's bone.” In an essay written in 1832, the year of his death, Goethe recalled this discov­ery as “the first battle and the first triumph of my youth” (Goethe, 1832, p. 573).)

  But Goethe chose botany for his most extensive study in formalism, and probably his finest contribution to science. In this important work, Goethe applied to plants the same vision that Geoffroy and Owen would later ad­vance in trying to reduce the great complexity and diversity of animal (or at least vertebrate) form to the single generating pattern of an archetypal verte­bra (see Geoffroy, 1831, for a homage to Goethe). For Goethe, the leaf rep­resented an archetypal form for all plant parts growing from the central [Page 285] stem* — from cotyledons, to stem leaves, to sepals, petals, pistils and stamens, and fruit.

  The common epitome of Goethe's system — all is leaf — should not be taken literally as the actual reduction of all serial diversity to the actual form of a stem leaf. Such a reading would contravene the Platonic character of arche­types in formalist theory. The “leaf” represents an abstract generating princi­ple, from which stem leaves depart least in actual expression. Goethe writes: “We ought to have a general term with which to designate this diversely metamorphosed organ and with which to compare all manifestations of its form . ... We might equally well say that a stamen is a contracted petal, as that a petal is a stamen in a state of expansion; or that a sepal is a contracted stem leaf approaching a certain stage of refinement, as that a stem leaf is a se­pal expanded by the influx of cruder saps” (1790, No. 120).

  Goethe expressed the epitome of his system in measured tone within his es­say (1790, No. 119): “The organs of the vegetating and flowering plant, though seemingly dissimilar, all originate from a single organ, namely, the leaf.” In private, he became more effusive: “[I have traced] the manifold spe­cific phenomena in the magnificent garden of the universe back to one simple general principle” (1831 essay, in Mueller and Engard, 1952, p. 168). To friends, as to the philosopher J. G. Herder, he could become positively effu­sive (dare I say florid): “The archetypal plant as I see it will be the most won­derful creation in the whole world, and nature herself will envy me for it. With this model and the key to it, one will be able to invent plants..., which, even if they do not actually exist, nevertheless might exist, and which are not merely picturesque or poetic visions and illusions, but have inner truth and logic. The same law will permit itself to be applied to everything that is liv­ing” (letter of 1787, quoted in Mueller and Engard, 1952, p. 14).

  Goethe dissects and compares, trying to find the leaf-like basis of appar­ently diversified and disparate structures. The anastomosed sepals, forming the calyx at the base of a flower, must be leaves that fail to separat
e when a cutoff of nutriment stops expansion of the stem: “If the flowering were re­tarded by the infiltration of superfluous nutriment, the leaves would be sepa­rated and would assume their original shape. Thus, in the calyx, nature forms [Page 286] 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:

  12. They are often misshapen, crammed, as it were, with crude matter, and as much expanded in thickness as in breadth; their vessels are unrec­ognizable 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.

  13. 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.