The Structure of Evolutionary Theory

by Stephen Jay Gould

  These issues and parsings have pervaded natural history since Plato and Aristotle argued about abstract form vs. teleology. Darwin made a seminal

  4-5. Because constraint and adaptation act either from the past or in the present, we may envisage three primary determinants of present form: present constraint, present adaptation, and inheritance due to past history of either constraint or adaptation.

  4-6. One of a group of “triangular” models constructed to express the major in­fluences upon the genesis of form. The three vertices of this triangle refer to the three influences depicted in Figure 4-5.

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  contribution by adding a dimension for history, and by formulating a theory that granted controlling relative frequency to adaptation. But he did not in­vent the issues, or the scheme of classification. The triumph of Darwinian functionalism did, however, erase much historical memory for the old alter­native of constraint. The next two chapters sink their rationale in a simple premise: Our current need to reinvigorate constraint as a vital topic in evolu­tionary explanation (see Stearns, 1986; Maynard Smith et al., 1985; Gould, 1989a, 1992b) — based upon advancing knowledge of genetic architecture, development and macroevolution — requires that we rediscover this legacy of structuralist thought,* and recognize that the entire history of evolutionary theory has been pervaded by an issue that simply would not disappear, if only because the dialectic of inside and outside, structure and function, design and adaptation, must be resolved at some fascinating interplay and synthesis, not as a victory for either pole in a debate without true sides.

  Two Ways to Glorify God in Nature

  We cannot comprehend the past from the vantage point of a newly constructed present reality. Once the 19th century had discovered evolution as the primary cause of relationships among organisms, the historical axis not only sprang into being as a pole of explanation, but quickly assumed a pri­mary status (Figs. 4-2 to 4-6). More than a century later, we can hardly imag­ine biology without this theme. What kind of questions could be posed be­fore history became an option for resolution? What kinds of explanations could be rendered when a biologist couldn't ask (or even conceptualize): “How has this feature changed from an ancestral state; what do its differing forms in various species tell us about phyletic relationships; what are the causal bases for both the origin and later alterations of this feature?”

  Immediate appearance in a fully formed state provides the only alternative to history — whether such “creation” is achieved by the direct hand of a di­vine agent, or by spontaneous organization from elements according to some unknown law or principle of nature. If basic taxa originated as we find them now, then the range of theoretical explanation remains wide. Species might be purposely ill designed to suit the black humor of a diabolical creator; or they might be cobbled together with no rhyme or reason by forces of universal randomness. The list of possibilities continues ad infinitum.

  But, in fact, Western cultural traditions greatly limited the range of acceptable [Page 261] alternatives. Very few creationists could imagine that species might be purposely ill formed, or constructed in a disorganized fashion. With these at­tributes — purpose and order — as part of a cultural heritage, the basic expla­nations for organic form could be reduced to two major alternatives, express­ing the primacy of one or the other overarching principle for a rational and benevolent world. These principles have been called structuralism and functionalism, order and teleology, laws of form and adaptation, Unity of Type and Conditions of Existence. These poles set the dichotomy that Darwin ex­panded by introducing history (see last section), but never really fractured be­cause the new axis of time could also be divided into structural vs. functional explanations for ancestral forms. This dichotomy continues to set an impor­tant agenda for evolutionary theory at the opening of a new millennium, especially since the overly adaptationist Modern Synthesis (representing a temporary triumph of the functionalist pole — see Chapter 7) has yielded to a pluralism of structuralist alternatives as partners rather than subsidiary forces (Chapters 10 and 11).

  In this light, I find it fascinating that the oldest tradition in modern natural history — the natural theology of so many pre-Darwinian biologists* — also existed in two primary versions, expressing the two poles of the same dichot­omy. Since Darwin built his evolutionary theory in continuity with the pole favored by a long English heritage — the adaptationism of William Paley — this subject cannot be dismissed as an arcane issue from a forgotten past, but re­mains a vital presence in our daily concerns (by our own fundamental evolu­tionary criterion of genealogy and phyletic heritage!). For we still struggle with adaptation and constraint just as Paley and Agassiz contrasted the com­parable positions in natural theology: “the creator foresaw the needs of each species and created just those organs that were necessary to carry them out” vs. “God had in the beginning established laws, and nature was left to unfold in accordance with them” (characterizations of Appel, 1987, p. 7). Do not Fisher vs. Wright, or Cain and Maynard Smith vs. Goodwin and Kauffman carry on the same debate, evolutionarily transmogrified of course?

  Natural theology held, as a central premise, that the works of nature not only demonstrated God's presence, but could also reveal his character as well. We could learn about him, not only persuade ourselves that he exists. Paley's full title (1802) reads: Natural Theology: or, Evidences of the Existence and Attributes of the Deity, Collected from the Appearances of Nature. From this shared premise, two traditions proceeded, both “preadapted” to a later evo­lutionary transformation. [Page 262]

  In this section I shall contrast the two great texts of these alternative traditions — Paley's Natural Theology (1802) with Agassiz's Essay on Classification (1857). The two works dovetail with remarkable symmetry in their op­position: Paley the British adaptationist vs. Agassiz the continental formalist. One might almost believe that the two works were explicitly written to flesh out (and fully clothe) the central dichotomy of form, with each awarded ex­actly half the totality. In a curious sense, this lack of contact almost allows the two texts to speak to each other — as if they formed a sand painting with one (Paley for temporal priority) filling in half the area up to an elaborate and jag­ged boundary, and the other then pouring sand of a different color right up to the previous boundary, leaving no space between at the contact. I am puzzled that these two texts have not been explicitly contrasted before.

  WILLIAM PALEY AND BRITISH FUNCTIONALISM: PRAISING GOD IN THE DETAILS OF DESIGN

  Just a few years before Paley wrote his Natural Theology in 1802, Coleridge's Ancient Mariner (1798) proclaimed his hard-won message to a wedding feast, and to the world:

  He prayeth best, who loveth best

  All things both great and small;

  For the dear God who loveth us,

  He made and loveth all.

  Paley probably appreciated the sentiments and surely longed to extend the argument. He entertained no doubt that all things proclaimed God's existence. But he believed that we must be able to learn more if we hope to use natural theology as a strategy of exegesis. That is, we must also be able to infer im­portant aspects of God's nature and character from the works of creation.

  The search to infer God's attributes from general features of natural objects led Paley to open his book with one of the most famous images in all English literature — a strong competitor with Adam Smith's “invisible hand” (a line also found in Paley, 1803, p. 344) and Darwin's tangled bank or tree of life. The good Reverend, crossing a heath on shank's mare, bumps his foot against a stone, feels the pain, but learns nothing about the origin of rocks because the object is too simple and too disordered to reveal a source of production. But if he should then kick a watch, he would surely know that the timepiece had been fashioned by a purposeful agent:

  When we come to inspect the watch, we perceive (what we could not discover in the stone) that its several parts are framed and put together for a purpose, e.g. that the
y are formed and adjusted as to produce motion, and that motion so regulated as to point out the hour of the day... The inference, we think, is inevitable; that the watch must have had a maker; that there must have existed, at some time and at some place or other, an artificer or artificers who formed it for the purpose which we find it actually [Page 263] to answer; who comprehended its construction, and designed its use (Paley, 1803, p. 203 — I am using my personal copy of the widely read 1803 edition for all quotes).

  Two features of the watch compel this conclusion. First, and less impor­tant, its complexity — for chance could not make anything so intricate: “What does chance ever do for us? In the human body, for instance, chance, i.e. the operation of causes without design, may produce a wen, or a wart, a mole, a pimple, but never an eye” (1803, pp. 67-68). Second, and far more impor­tant, the watch's design, its adaptation to a clearly perceived end.* A high de­gree of order might arise from laws of nature with no reference to final cause, but complexity for a clear purpose implies a designer. “There cannot be de­sign without a designer; contrivance without a contriver; arrangement, with­out anything capable of arranging” (p. 12). Thus does Paley attack his hypo­thetical opponent and partial straw man throughout his work? * “Nor would any man in his senses think the existence of the watch, with its various ma­chinery, accounted for, by being told that it was one out of several possible combinations of material forms; that whatever he had found in the place where he found the watch, must have contained some internal configuration or other; and that this configuration might be the structure now exhibited” (p. 6).

  The watch implies, by its utility, a mind capable of forethought, design and construction: “In the watch which we are examining, are seen contrivance, design; an end, a purpose; means for the end, adaptation to the purpose. And the question, which irresistibly presses upon our thoughts, is, whence this contrivance and design. The thing required is the intending mind, the adapt­ing hand, the intelligence by which that hand was directed” (p. 16).

  But organisms surely display more complexity and more purposeful design than any watch. Just as Darwin would exalt natural selection as vastly more powerful than artificial human selection in breeding or agriculture, so does [Page 264] Paley identifies God's work as incomparably superior to any human art. If the existence of the watch implies a skilled craftsman, how can we even conceive the more awesome skill of he who made all living things: “For every indica­tion of contrivance, every manifestation of design, which existed in the watch, exists in the work of nature; with the difference, on the side of nature, of being greater and more, and that in a degree which exceeds all computa­tion” (p. 19).

  In succinct epitome of the entire argument, Paley writes (p. 473): “The marks of design are too strong to be got over. Design must have a designer. That designer must have been a person. That person is God.”

  Since we often misuse the past for ridicule, Paley has emerged as every­body's favorite whipping boy from the bad old days of creationism. As a lively writer, he is, to be sure, eminently quotable. And he does sometimes stray into the kind of Panglossian perfectionism (or, rather, far-fetched ratio­nalization for beneficence within apparent evil) that Voltaire savaged with such glee in Candide.

  Paley, for example, does engage in “just-so” storytelling to support adaptationist explanation, though he presumably read this account of Babyrussa in a fallacious traveler’s report, and can only be charged with insufficient skep­ticism, not fabrication (Fig. 4-7):

  I shall add one more example for the sake of its novelty. It is always an agreeable discovery, when, having remarked in an animal an extraordi­nary structure, we come at length to find out an unexpected use for it. The following narrative furnishes an instance of this kind. The baby-rouessa, or Indian hog, a species of wild boar found in the East Indies, has two bent teeth, more than half a yard long, growing upwards, and (which is the singularity) from the upper jaw. These instruments are not wanted for defense, that service being provided for by two tusks issuing from the under jaw, and resembling those of the common boar. Nor does the animal use them for defense. They might seem therefore to be both

  4-7. Paley does not include a drawing of the skull of Babyrussa, but this figure comes from an equally interesting source — P. H. Gosse's Omphalos of 1857, his treatise arguing for the sudden and recent creation of the Earth, including all its fossils, which therefore only display an appearance of great age.

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  a superfluity and an incumbrance. But observe the event. The animal sleeps standing; and, in order to support its head, hooks its upper tusks on the branches of trees (pp. 270-271).

  More in the Panglossian mode, pain (an adaptation, Paley argues, for signaling distress to the mind so that we may care for our bodies) also shows God's benevolence on the theme of the old moron joke — we feel so good when the suffering stops! (On the subject of good in apparent noxiousness, compare John Ray (1735, p. 309) on why God made lice: “I cannot but look upon the strange instinct of this noisome and troublesome creature a louse, of searching out foul and nasty clothes to harbor and breed in, as an effect of divine providence, designed to deter men and women from sluttishness and sordidness, and to provoke them to cleanliness and neatness. God Himself hateth uncleanliness, and turns away from it.” Or, as Robert Burns would later generalize the lesson in “To a Louse”: “Oh wad some power the giftie gie us / To see oursels as ithers see us!”) “A man resting from a fit of the stone or gout, is, for the time, in possession of feelings which undisturbed health cannot impart. They may be dearly bought, but still they are to be set against the price. And, indeed, it depends upon the duration and urgency of the pain, whether they be dearly bought by suffering a moderate interruption of bodily ease for a couple of hours out of the four and twenty” (pp. 523-533).

  To complete the picture of joyous nature made by a loving God, signs of non-utility in sheer behavioral exuberance, particularly in the play of young creatures, testify to the sheer pleasure of being alive on such a wondrous planet:

  Swarms of newborn flies are trying their pinions in the air. Their sportive motions, their wanton mazes, their gratuitous activity, their continual change of place without use or purpose, testify their joy, and the exulta­tion which they feel in their lately discovered faculties . . . Other species are running about with an alacrity in their motions which carries with it every mark of pleasure. Large patches of ground are sometimes half cov­ered with these brisk and sprightly natures. If we look to what the waters produce, shoals of the fry of fish frequent the marshes of rivers, of lakes, and of the sea itself. These are so happy, that they know not what to do with themselves (pp. 490-491).

  (Paley's prose may be purple, but his purpose is sanguine. He argues, in stat­ing his primary case, that organic adaptation proves the personhood of God. But we want to know more. God could, after all, be a consummate crafts­man, but a crabby character. Paley's arguments on pain and natural happiness indicate that God is not only skillful, but also benevolent as well.)

  These statements, taken out of context (as usually done), promote an un­fair caricature of a subtle argument. Paley cannot be dismissed as an intellec­tual slouch. His Evidences of Christianity (1794) remained a required text for entrance to Cambridge University until the 20th century, and Darwin would never have chosen a cardboard dogmatist for a hero or, later, for an opponent [Page 266] worthy of overturning as the essential thrust of a revolutionary theory (see pp. 116–125). Paley's totality presents a subtle, coherently reasoned brief for an adaptationist natural theology.

  First of all, Paley cannot be caricatured as a Panglossian perfectionist. He states explicitly that we cannot use perfection as a criterion for identifying good design, or even as the necessary mark of divinity in craftsmanship: “It is not necessary that a machine be perfect, in order to show with what design it was made: still less necessary, where the only question is, whether it were made with any design at all” (p. 5).

  Paley
also provides, if only occasionally, positive arguments for imperfec­tion, as in feathers of an ostrich's wing. “The filaments hang loose and sepa­rate from one another, forming only a kind of down; which constitution of the feathers, however it may fit them for the flowing honors of a lady's head­dress, may be reckoned an imperfection in the bird, inasmuch as wings, com­posed of these feathers, although they may greatly assist in running, do not serve for flight” (p. 236). And he acknowledges that the creator's preference for utility lies revealed in the overwhelming relative frequency, not the ubiq­uity, of adaptation (but adding the conventional rider, still commonly ad­vanced today, that, if we look hard enough, we will discover uses for traits now judged “nonadaptive”). “Instances . . . where the part appears to be to­tally useless, I believe to be extremely rare: compared with the number of those, of which the use is evident, they are beneath any assignable propor­tions; and perhaps, have never been submitted to a trial and examination suf­ficiently accurate, long enough continued, or often enough repeated” (Paley, 1803, p. 64).

  In fact, Paley uses adaptationism primarily as a theoretical argument about depth of causality, not as an excuse to rhapsodize about happy nature. Oppo­nents who wish to see “physical law” as the source of form might cite sexual generation and embryology as leading examples. But these processes only provide the immediate physical continuity of efficient causation: “The truth is, generation is not a principle but a process” (p. 453). We need a deeper rea­son, a true principle, for the evident adaptation of form to function — in short, a final cause. Even if watches gave birth to new watches, Paley argues, we would not identify ontogeny as the ultimate source of timekeeping. Neither can embryology be the cause of optical excellence in the human eye, if only because “things generated possess a clear relation to things not generated” (p. 455) — the eye to external light and to the objects we need to see in this case. (We now recognize this otherwise persuasive argument as wrong only because life, unbeknownst to Paley, possesses history and mutability.)