The Structure of Evolutionary Theory

by Stephen Jay Gould

  Appel notes an interesting source of Cuvier's accumulated influence: “Cuvier was able to remain on the Council [of State] through the Empire, three kings, and several ministries because he held no extreme opinions and was willing to support whatever regime was in power” (1987, p. 53). Yet, lest we view this chameleonic shifting merely as cynical and self-serving, much like the Vicar of Bray in the old song about maintaining office through all the vicissitudes of 17th century British politics, Appel points out the underlying consistency of a true political and biological conservative: after a bloody and traumatic revolution, any hierarchical order, proceeding from any source holding promise for stability, must be preferred over potential anarchy and populism.

  Appel designates three broad domains of difference between Cuvier and Geoffroy: Cuvier's conservative connection to substantial political power, his insistence (largely for rhetorical purposes, since science cannot operate in such a manner) that the profession restrict itself to reporting positive facts and shunning speculation, and his commitment to one of the purest forms of functionalism ever maintained in the study of morphology. Appel notes the evident connection between political elitism and the call for a descriptive, factually based science of experts:

  In a politically volatile country which had recently experienced trau­matic revolution, Cuvier justly feared that speculative theories, most of [Page 293] which had a materialist tinge, would be exploited in the name of science and undermine religion and promote social unrest. If science could be limited to experts and restricted to accumulating “positive facts” then it might achieve a measure of autonomy, while at the same time the questioning that might lead to heretical theories would be eliminated. As Cuvier became increasingly concerned about the danger posed by certain biological theories, he became increasingly insistent on the restraints imposed by proper scientific method (Appel, 1987, pp. 52-53).

  The third theme of morphological explanation, though supported by other roots, also melds into the Cuvierian totality of politics, method, and theory — for Cuvier's functionalism views organisms as discrete, untransformable en­tities, designed for specific conditions of life and no other. By contrast, Geoffroy held opposite attitudes on all three accounts — as an outsider in poli­tics, both academic and national; a dreamer and visionary in methodology, a man who explicitly exalted the power of ideas to guide and even to channel factual inquiry; and a resolute formalist in morphology, with a theory of ro­bust generation and transformation along lines set by overarching laws of structure and archetypal form.

  To grasp the purity of Cuvierian functionalism, we must break through a century's commitment to genealogical models of relationship. We are now so wedded (properly of course) to the homological basis of deep similarity by descent, that we can scarcely imagine any other theory of Bauplan. After all, what could the sequence of humerus, to radius and ulna, to carpals, meta-carpals and phalanges denote except inheritance by common descent when expressed over so broad a functional range as dolphin, dog and bat. Even the most rabid panselectionist would not identify phylum-level homologies (broad symplesiomorphies) as indications of current function. At most, fol­lowing Darwin (see pp. 253–260), they would view such features as originat­ing by adaptation in distant ancestors. Current function will then be ex­pressed in particular modifications of homologies within each line.

  Yet Cuvier actually believed that common features of current Bauplan recorded such immediate functional rules of correlation. Cuvier acknowledged that science does not yet understand organic physics well enough to know the logical basis of these rules, and must therefore work empirically from com­parative anatomy, but the regularities must be rooted in function and will, one day, be resolved analytically. Start with a carnivore's claw (or canine tooth, or any other tool of its trade), and all other items of anatomy follow by mechanical necessity. One part implies the next, and eventually the entire skeleton, according to correlations set by functional rules alone. Type records broad function; specific adaptation denotes local function. No part exists “in vain” or merely to indicate conformity to plan (vestigial organs, developmen­tal sequelae). Evolution becomes literally inconceivable because change in one part requires corresponding change in every other intimate detail — and no one can imagine a mechanism for such globally coordinated alteration. (Nor can one, even today, gainsay this excellent argument. If evolution were [Page 294] not mosaic, transmutation would be inconceivable, and would not occur pre­cisely for the reasons stated by Cuvier.)

  In Cuvier's remarkable output of publications, three works stand out as powerful, comprehensive documents that established professions and set a good part of the course of 19th century biology — the 5 volume Legons d'anatomie comparee of 1800-1805, the 4 volume Recherches sur les ossemens fossiles of 1812, and Le regne animal of 1817. The pivotal role of these three works has always been acknowledged, but their common philo­sophical grounding in Cuvier's overarching functionalism has not been ade­quately recorded.

  The Legons of 1800-1805 arranges natural history in functional terms by shunning the usual taxonomic order and proceeding instead by organ systems considered in operational rather than morphological terms. Volume one treats locomotion, functionally focused and defined (“les organes du mouvement”), while subsequent volumes proceed through sensation, diges­tion, circulation, respiration, voice, generation, and excretion.

  The very first lesson, functionally organized as “considerations sur Veconomie animale,” presents the heart of Cuvier's approach. His theory of function cannot be characterized as a crude, “democratic” adaptationism, part by part with each item separately optimized, but rather as a more subtle, hierarchical system that renders both structural regularities and correlations in functional terms. Primary functions, common to all organisms, lie at the base — origin by generation, growth by nutrition, and termination by death (see Russell, 1916, p. 31). Secondary functions — feeling and moving — build a layer above and set the morphology of organs for their manner of operation. These secondary, or “animal,” functions, with their neuromuscular expres­sion, determine a yet higher level of “vital functions” — digestive, circulatory, and respiratory, in that order. Feeling and movement require a set of organs to hold and process food; digestion then implies a system of distribution (cir­culation). Higher levels may then feed back “in a type of circle” (Cuvier, 1805, p. 47) to influence the logically prior foundation. Power of movement affects mode of generation and “fluide nerveux” of secondary status flows through channels of tertiary circulation. Above all, function holds pri­ority and determines structure; coordination and correlation among struc­tures records the hierarchical ordering of interrelated functions (see particu­larly Cuvier, 1805, pp. 45-60).

  Cuvier states the functional foundation of his morphology in bold terms (1805, p. 47): “The laws that determine the relationships of organs are founded upon this mutual dependence of functions, and upon the aid that they lend to each other. These laws have a necessity equal to laws of meta­physics and mathematics. For it is evident that a proper harmony among or­gans that act upon each other is a necessary condition of existence* for the [Page 295] creature to which they belong. If one of these functions were modified in a manner incompatible with modifications of other organs, this creature could not exist.”

  This statement of analytically necessary functional laws, and ineluctable correlation of parts, echoes the philosophy better known from the justly cele­brated Discours preliminaire of Cuvier's 1812 Recherches, the document that founded modern paleontology by establishing the fact of extinction and or­ganic succession through time. The laws of organic form have a purely func­tional basis. One anatomical part implies all others, for proper function (not abstract laws of structure) demands such interdependence.* Animals there­fore cannot undergo substantial change by evolution because such a complex and precisely coordinated transformation of all parts could not occur — espe­cially under functionalist theories of the independent and adaptati
onal ori­gin of each part (rather than the coordinated change of all parts of an arche­typal form along preestablished lines of possibility, thus making evolution far easier to conceive under the formalist philosophies that Cuvier rejected). Therefore, when geological conditions change drastically, many species die and can never reappear or continue in any way. The sequence of extinctions through time gives the earth a history by establishing a vector of directional change. Geology, now furnished with an alphabet, can finally become a sci­ence. Cuvier expresses the functional basis of correlation:

  Every organized individual forms an entire system of its own, all the parts of which mutually corresponds, and concurs to produce a certain definite purpose, by reciprocal reaction, or by combining towards the same end. Hence none of these separate parts can change their forms without a corresponding change in the other parts of the same animal, and consequently each of these parts, taken separately, indicates all the other parts to which it has belonged (from the standard Jameson transla­tion, 1818, p. 99).

  In short, the shape and structure of the teeth regulate the forms of the condyle, of the shoulder-blade, and of the claws, in the same manner as the equation of a curve regulates all its other properties (1818, p. 102) . . . Anyone who observes merely the print of a cloven hoof, may con­clude that it has been left by a ruminant animal, and regard the conclu­sion as equally certain with any other in physics or in morals (p. 105).

  The relationship of the third great work — Le regne animal of 1817 — to this functionalist nexus seems more obscure at first. Here Cuvier codifies the sys­tem of animal taxonomy that he first published in 1812 — the abandonment [Page 296] of the old bipartite division of vertebrate and invertebrate (and the equation of vertebrate classes with invertebrate phyla), for a system of four equal embranchements based on necessarily separate and untransformable anatom­ical plans: Radiata, Articulata, Mollusca, and Vertebrata. This appeal to lim­ited and untransformable anatomical designs as a basis for taxonomic order smacks of structuralism, but Cuvier, true to his guiding philosophy, presents a purely functional interpretation. Appel (1987, p. 45) explains: “The unity within an embranchement came not from a comprehensive unity of plan, but from a common arrangement of the nervous system, functionally the most important system of the animal. The forms of the other major systems remain constant throughout an embranchement because the other systems — respira­tion, circulation, etc. — were functionally subordinate to the nervous system and determined by the requirement of the nervous system. Animals within an embranchement could vary almost arbitrarily in their accessory parts, precisely because accessory parts were not necessitated by the choice of the nervous system.” Both unity and diversity therefore achieve a functional interpretation — unity by operational design, diversity by local adaptation. Conditions of existence set both major aspects of taxonomy.

  I emphasize a primary intellectual correlation throughout this chapter — formalism with commitment to internal constraint (in the positive sense of channeling change, not only the negative definition of restriction). To render this connection meaningful, the converse must also hold: functionalism must correlate with denial of constraint. Cuvier's arguments test and affirm this im­plication.

  In an overly broad (and therefore operationally meaningless) construction of constraint, all biologists acknowledge some restriction on organic form, if only because all conceivable shapes and sizes have not been realized. But we usually do not apply this term to nature's avoidance of obviously unworkable creatures (flying elephants or large dinosaurs with pencil-thin legs in Galileo's world of laws regulating the ratio of surface to volume), for no one disputes the underlying physical basis for their nonexistence. (For historically con­tingent reasons of modern professional life within a Darwinian functional­ist paradigm, we currently apply the term “constraint” primarily to inter­nal channels and limitations not set by adaptation — see my full argument for this usage in Chapter 10, pp. 1027–1037. That is, we apply the concept of “constraint” to sources of influence outside a favored explanation — see Gould, 1989a.)

  Thus, Cuvier cheerfully acknowledged limits set by function, but did not view such boundaries as constraining because aborted, unworkable creatures offend the very notion of a rational creating force. Instead, and thereby af­firming the link of functionalism to a denial of constraint, Cuvier clearly cher­ished his general theory as a principle for maximizing God's liberty to create (translated as “adaptation to alter” in the modern evolutionary version of functionalism). Cuvier wrote in an 1825 essay on “Nature”: “If we look back to the Author of all things, what other law could actuate Him but the neces­sity of providing to each being whose existence is to be continued the means [Page 297] of assuring that existence? And why could He not vary His materials and His instruments? Certain laws of coexistence of organs were therefore necessary, but that was all. For to establish others there must have been a want of free­dom in the action of the organizing principle, which we have shown to be only a chimera” (in Appel, 1987, p. 138).

  Cuvier, a severe rationalist (see Fig. 4-10 for an interesting and previously unpublished illustration of Cuvier's rationalism and hostility to florid metaphor),

  4-10. A remarkable note, written by Cuvier in his own hand, and indicating how much this rationalist thinker rejected and ridiculed silly metaphorical uses of poetic imagery as a substitute for rigor, or for saying anything of real sub­stance. Here Cuvier jotted down two such fatuous and metaphorical uses of “sphere” — obviously stored away for later use in satire or ridicule. (Author's collection.)

  Definition of life by M. Virey. Life is a circular movement, sustained and measured by time; time, that infinite sphere, of which God alone is the center, and where living beings are placed on the circumference, describing in their rapid or­bit, the circle of their destiny.

  Definition of poetry by Mme. de Stael. Poetry is the winged mediator, which moves distant nations and ancient times in a sublime sphere where admiration takes the place of sympathy.

  [Page 298]

  rarely waxed poetic about nature's abundances, but he surely rejoiced that organic form knew no limits beyond good design.

  While always remaining within the boundaries prescribed by necessary conditions of existence, nature abandons herself to all fecundity not limited by these conditions; and without ever departing from the small number of possible combinations for modification of important organs, she seems in all accessory parts, to be limitlessly endowed . . . Thus we find that as we move away from the principal organs, and approach those that are less important, varieties are multiplied; and when we arrive at the surface, where the nature of things ordains that the least important parts be placed, and where any damage is least dangerous, the number of varieties becomes so great that all the work of natural­ists has not succeeded in giving us any idea of its magnitude (1805, p. 58).

  Geoffroy's formalist vision

  Since the modes and practices of science inevitably reflect a surrounding so­cial environment, we should scarcely be surprised that the early to mid 19th century world of revolution in politics, and romanticism in art, literature, and music, also inspired a series of biological movements called Naturphilosophie in Germany and romantic, idealistic, transcendental, or philosophical anat­omy elsewhere. A scientific movement may begin under strong social influ­ence and little compulsion by data, but its empirical adequacy may ultimately rank high nonetheless. (Evolutionists, above all other professionals, should be optimally preprogrammed to appreciate the difference between reasons for origin, and assessment of eventual value — see pp. 1214–1218 particularly for Nietzsche's analysis of this vital issue in historiography.) Geoffroy, as the most important of the transcendental morphologists, heard the songs of his time, but he also composed a flawed symphony that plays better today than to the previous generation that built the Modern Synthetic theory of evolu­tion, and that improves even more when we recover and refurbish the origi­nal instruments of its initia
l performance.

  The story has been told many times and in many contexts (think of Don Quixote), but romantic dreamers often temporize and lose ground while practical schemers reap the benefits of accumulated diligence. Cuvier, three years younger than Geoffroy, began his Museum career in a clearly subor­dinate professional status. But while Geoffroy followed his bliss in Egypt, Cuvier built his career in Paris. Cuvier soon overtook his former protector, and Geoffroy brooded. (Cuvier, for example, entered the Academie des sci­ences, the forthcoming stage for the great 1830 debate, in 1795, while Geoffroy did not win membership until 1807.) By 1805, Cuvier had already published his Legons d'anatomie comparee in five volumes, while Geoffroy had produced no major counterweight. Geoffroy, strong in ambition what­ever his shortcomings in political acumen, knew that he needed a distinctive approach or discovery to secure his renown, and he found a guiding light in [Page 299] formalism, the “philosophic anatomique” of the book (1818) that would se­cure his reputation.

  Geoffroy began by applying the chief formalist notion of unity of type to the vertebrate skeleton. Reptiles, birds, and mammals presented minimal dif­ficulty, but fishes posed the key challenge to such a comprehensive view. Compared with terrestrial vertebrates, fishes seemed so different in their anat­omy of skull, fins, and shoulder girdle, and so disparate in mode of respi­ration, that any notion of a common plan must be deemed untenable if not fatuous prima facie. Cuvier had argued on functional grounds that the uniqueness of several skeletal elements in fishes testified to their fitness for swimming and breathing in water.

  Geoffroy published a group of memoirs on the anatomy of fishes in 1807, the first successes of his research program. Working primarily with bones of the shoulder girdle, he found a putative homologue of the furcula (wishbone) in birds. The functionalist credo that such a bone must exist “for” flight must therefore be false. Rather, the furcula in birds, and its homolog in fishes (op­erating as an additional rib in some species, and as an aid to opening the gills in others), must be specialized representatives of an abstract element in the ar­chetype of all vertebrates. The form of the archetype holds priority, whereas diversified functional utility only represents a set of secondary modifications, superimposed by conditions of existence upon the primacy of underlying form. Thus, in his first foray into formalism, Geoffroy codified the key idea of structural constraint: form exerts both logical and temporal priority upon function; good designs exist in abundance because the archetype includes this potential for secondary modification; function does not create form, rather form finds function: “Without a direct object in swimming animals, without a utility determined in advance, and thrown, so to say, by chance into the field of organization, the furcula enters into connection with the organs near it; and according to the manner in which this association is formed, it takes on uses which are in some sense prescribed by them” (Geoffroy, 1807, quoted in Appel, 1987, p. 87).