The Structure of Evolutionary Theory

by Stephen Jay Gould

  Darwin used a brilliant argument to cut through this dilemma, thus mak­ing the study of evolution a practical science. He acknowledged Lamarck's implied claim that small scale adaptation to local environment defines the tractable subject matter of evolution. But he refuted the disabling contention that adaptation in this mode only diverted the “real” force of evolution into side channels and dead ends. And he revised previous evolutionary thinking in the most radical way — by denying that Lamarck's “real” force existed at all, and by encompassing its supposed results as consequences of the “subsid­iary” force accumulated to grandeur by the simple expedient of relentless action over sufficient time. Darwin established our profession not only by discovering a force — natural selection — that seems both powerful and true; he also, perhaps more importantly, made evolution accessible to science by granting to empiricists their most precious gifts of tractability and testability. The essence of Darwin's theory (specified in the next section) owes as much to his practical triumph at this immediate scale of daily work, as to his broadest perception that western views of nature had been seriously awry, and largely backwards.

  Darwin, as we all know, began the last chapter of the Origin with a claim that “this whole volume is one long argument” (1859, p. 459). Fine, but an argument for what? For evolution itself? In part, of course, but such a general theme cannot mark the full intent of Darwin's statement, for the bulk of the Origin moves well beyond the basic arguments for evolution's factuality, as Darwin proceeds to craft a defense for natural selection and for the philoso­phy of nature so entailed. “One long argument” for natural selection, then? Again, in part; but we now confront the obverse of my last statement: too much of the Origin details basic evidence for evolution, independent of any particular mechanism of change. Instead, we must ask what deeper subject underlies both the defense of evolution as a fact and the proposal of a mecha­nism to explain its operation? How should we characterize the “one long ar­gument” that pervades the entire book?

  Ghiselin (1969) correctly identified the underlying theme as the construc­tion, and defense by example, of a methodology — a mode of practice — for testing both the fact and mechanism of evolutionary change. But I cannot agree with Ghiselin that Darwin's consistent use of “hypothetico-deductive” reasoning constitutes his long argument (see Kitcher, 1985), for this style of scientific procedure, whatever its merits or problems, has been advocated as a general methodology for all scientific activity (see Hempel, 1965). Darwin, I [Page 99] believe, sought to construct and defend a working method for the special sub­ject matter of evolutionary inquiry — that is, for the data of history.

  Inferences about history, so crucial to any evolutionary work, had been plagued by problems of confidence that seemed to bar any truly scientific in­quiry into the past. Darwin knew that evolution would not win respect until methods of historical inference could be established and illustrated with all the confidence of Galileo viewing the moons of Jupiter. He therefore set out to formulate rules for inference in history. I view the Origin as one long illus­tration of these rules. Historical inference sets the more general theme under­lying both the establishment of evolution as a fact, and the defense of natural selection as its mechanism. The “one long argument” of the Origin presents a comprehensive strategy and compendium of modes for historical inference (see fuller exposition of this view in Gould, 1986). We must grasp Darwin's practical campaign on this battlefield in order to understand his radical phi­losophy, and to identify the features of his theory that count as essential to any definition of “Darwinism.”

  THE PROBLEM OF HISTORY

  Reading Darwin has been a persisting and central joy in my intellectual life. Lyell and Huxley may have been greater prose stylists, with more consistency in the ring and power of their words. Yet I give the nod to Darwin, and not only for the greater depth and power of his ideas. Darwin often wrote quite ordinary prose, page after page. But then, frequently enough to rivet the at­tention of any careful reader, his passion bursts through, and he makes a point with such insight and force (almost always by metaphor) that under­standing breaks like sunrise. Every evolutionist can cite a list of favorite Dar­winian passages, written on well-worn index cards for lectures (or, now, eter­nally embedded in PowerPoint files), posted on the office door or prominently displayed above the typewriter (now the computer terminal), or simply (and lovingly) committed to memory.

  Several of my favorite passages celebrate the broadened understanding of nature that derives from recognizing organisms as products of history, rather than objects created in their present state. Darwin writes (1859, 485–486):* [Page 100]

  When we no longer look at an organic being as a savage looks at a ship, as at something wholly beyond his comprehension; when we regard every production of nature as one which has had a history; when we contemplate every complex structure and instinct as the summing up of many contrivances, each useful to the possessor, nearly in the same way as when we look at any great mechanical invention as the summing of the labor, the experience, the reason, and even the blunders of numerous workmen; when we thus view each organic being, how far more interesting, I speak from experience, will the study of natural history become!

  By contrast, Darwin's chief quarrel with creationism resides not so much in its provable falseness, but in its bankrupt status as an intellectual argument — for a claim of creation teaches us nothing at all, but only states (in words that some people may consider exalted) that a particular creature or feature exists, a fact established well enough by a simple glance: “Nothing can be more hopeless than to attempt to explain the similarity of pattern in members of the same class, by utility or by the doctrine of final causes ... On the ordinary view of the independent creation of each being, we can only say that so it is; — that it has so pleased the Creator to construct each animal and plant” (p. 435).

  Moreover, and more negatively, creation marks the surrender of any at­tempt to understand connections and patterns. We express no causal insight whatever when we say that taxonomic order reflects the plan of a creator — for unless we can know the will of God, such a statement only stands as a re­dundant description of the order itself. (And God told us long ago, when he spoke to Job from the whirlwind, that we cannot know his will — “canst thou draw out leviathan with a hook?”) Darwin, an ever genial man in the face of endless assaults upon his patience, directed several of his rare caustic com­ments against the ultimate idea stopping claim that God so made it, praise his name. Darwin notes, for example, that horses are sometimes born with faint striping on their hides. A creationist can only assert that God made each equine species of zebras, horses, and asses alike, with such tendencies to vary and thereby to display, if only occasionally, the more comprehensive type. [Page 101] Evolution, on the other hand, supplies a true cause for an anomaly by posit­ing community of descent with retention of ancestral states by heredity — something that might be tested in many ways, once we understand the me­chanics of inheritance. (The following passage appears just before Darwin's summary to Chapter 5 on laws of variation.) Darwin lambastes the cre­ationist alternative as causally meaningless: “To admit this view is, as it seems to me, to reject a real for an unreal, or at least for an unknown, cause. It makes the works of God a mere mockery and deception; I would almost as soon believe with the old and ignorant cosmogonists, that fossil shells had never lived, but had been created in stone so as to mock the shells now living on the sea-shore” (p. 167).

  If we must locate our confidence about evolution in evidence for history — in part directly from the fossil record, but usually indirectly by inference from modern organisms — by what rules of reason, or canons of evidence, shall his­tory then be established? Darwin's “long argument,” in my view, can best be characterized as a complex solution to this question, illustrated with copious examples. We must first, however, specify the kinds of questions that can­not be answered. Many revealing statements in the Origin circumsc
ribe the proper realm of historical inference by abjuring what cannot be known, or usefully comprehended under current limits. Darwin, for example, and fol­lowing Hutton, Lyell and many other great thinkers, foreswore (as beyond the realm of science) all inquiry into the ultimate origins of things.* In the first paragraph of Chapter 7 on instincts, for example, Darwin writes (1859, p. 207): “I must premise, that I have nothing to do with the origin of the pri­mary mental powers, any more than I have with that of life itself.” Darwin in­voked the same comparison in discussing the evolution of eyes, one of his greatest challenges (and firmest successes). He states that he will confine his attention to transitions in a structural sequence from simple to complex, and not engage the prior issue — answerable in principle, but beyond the range of knowledge in his day — of how sensitivity to light could arise within nervous tissue in the first place (1859, p. 187): “How a nerve comes to be sensitive to light, hardly concerns us more than how life itself first originated.” Most crucially, and in a savvy argument that saved his entire system in the face of con­temporary ignorance on a central issue, Darwin argues over and over again that we may bypass the vital question of how heredity works, and how varia­tions arise — and only illustrate how evolution can occur, given the commonplace [Page 102] observation that sufficient variation does exist, and is inherited often enough:

  Whatever the cause may be of each slight difference in the offspring from their parents — and a cause for each must exist — it is the steady accumu­lation, through natural selection, of such differences, when beneficial to the individual, that gives rise to all the more important modifications of structure, by which the innumerable beings on the face of this earth are enabled to struggle with each other, and the best adapted to sur­vive (p. 170 — see also p. 131 for Darwin's argument that when we as­cribe variation to “chance,” we only mean to express our ignorance of causes).

  Having established a domain of testability by exclusion, Darwin laid out his methodology for history — never explicitly to be sure, but with such accu­mulating force by example that the entire book becomes “one long argu­ment” for the tractability of his new science. Those of us who practice the sci­ences of reconstructing specific events and unravelling temporal sequences have always fought a battle for appropriate status and respect, no less so to­day than in Darwin's time (see Gould, 1986), against those who would view such work as a “lesser” activity, or not part of science at all. History presents two special problems: (1) frequent absence of evidence, given imperfections of preservation; and (2) uniqueness of sequences, unrepeatable in their con­tingent complexity, and thereby distancing the data of history from such stan­dard concepts as prediction, and experimentation.

  We may epitomize the dilemma in the following way: many people define science as the study of causal processes. Past processes are, in principle, unobservable. We must therefore work by inference from results preserved in the historical record. We must study modern results produced by processes that can be directly observed and even manipulated by experiment — and we must then infer the causes of past results by their “sufficient similarity” (Steno's principle — see Gould, 1981c) with present results. This procedure re­quires, as Mill (1881) and other philosophers recognized long ago, a method­ological assumption of temporal invariance for laws of nature. Historical study manifests its special character by placing primary emphasis upon com­parison and degrees of similarity, rather than the canonical methods of sim­plification, manipulation, controlled experiment, and prediction.

  Darwin had done some paleontological work, particularly in his treatises on barnacles (1851-1858), and his important discoveries of South American fossil vertebrates (formally named and described by Owen, at Darwin's invi­tation). But Darwin was not primarily a paleontologist, and he did not intend to base his argument for evolution on the evidence of fossils — especially since he viewed the stratigraphic record, with its vast preponderance of gaps over evidence, as more a hindrance than an aid to his theory (see chapters 9 and 10 of the Origin). Thus, of the two major sources for historical reconstruction — direct but imperfect information from fossils, and indirect but copious data from modern organisms — Darwin preferred the second as his wellspring of [Page 103] documentation. The Origin therefore focuses upon the establishment of a methodology for making inferences about history from features of modern organisms — and then using these multifarious inferences to prove both the fact of evolution and the probability of natural selection as a primary mecha­nism of change.

  A FOURFOLD CONTINUUM OF METHODS FOR THE INFERENCE OF HISTORY

  Darwin, as a subtle and brilliant thinker, must be read on several levels. Con­sider just three, at decreasing domains of overt display, but increasing realms of generality: On the surface — a lovely, and not a pejorative, location for any student of nature — each book treats a particular puzzle: different forms of flowers on the same plant (1877), modes of formation for coral atolls (1842), formation of soil by worms (1881), styles of movement in climbing plants (1880a), the fertilization of orchids by insects (1862). At an intermediary level, as Ghiselin (1969) showed in his innovative study of the entire Darwin­ian corpus, each book forms part of a comprehensive argument for evolution itself. But I believe that we must also recognize a third, even deeper and more comprehensive layer of coordinating generality — Darwin's struggle to con­struct and apply a workable method for historical inference: a series of proce­dures offering sufficient confidence to place the sciences of history on a par with the finest experimental work in physics and chemistry. I have come to re­gard each of Darwin's books as, all at the same time, a treatment of a particu­lar puzzle (level one), an argument for an evolutionary worldview (level two), and a treatise on historical methodology (level three). But the methodological focus of level three has usually been overlooked because Darwin chose to work by practice rather than proclamation.

  Darwin recognized that several methods of historical inference must be developed, each tailored to the nature and quality of available evidence. We may order his procedures by decreasing density of available information. I recognize four waystations in the continuum and argue that each finds a pri­mary illustration in one of Darwin's books on a specific puzzle in natural his­tory. The Origin of Species, as his comprehensive view of nature, uses all four methods, and may therefore be read as a summation of his seminal contribu­tion to the methodology of historical science. I shall list, and then illustrate with examples from the Origin, these four principles ordered by decreasing density of information.

  Uniformity. Or working up by extrapolation from direct observations on rates and modes of change in modern organisms. Call this, if you will, the worm principle to honor Darwin's last book (1881), which explains the topsoil and topography of England by extrapolating the measured work of worms through all scales of time, from the weight of castings left daily on a patch of sod to the historical and geological realms of millennia to millions of years.

  Sequencing. Or the definition and ordering of various configurations, [Page 104] previously regarded as unrelated and independent, into stages of a single his­torical process. Here we cannot observe the changes between configurations directly and we must therefore work by recognizing them as temporally or­dered products of a single underlying process of change. Call this, if you will, the coral reef principle to honor Darwin's first book (1842) on a scientific subject. His successful theory proposes a single historical process for the for­mation of coral atolls by recognizing three configurations of reefs — fringing reefs, barrier reefs, and atolls — as sequential stages in the foundering of oce­anic islands.

  Consilience (concordance of several). We now reach a break in types of information. Methods 1 and 2 permit the reconstruction of histor­ical sequences, either by extrapolating up from the most palpable and testable of daily changes (method 1), or by ordering a series of configurations as tem­poral stages (method 2). In many cases, however, we cannot reconstruct se­quences, and m
ust infer history from the configuration of a single object or circumstance. Of the two major methods for inferring history from single configurations, consilience calls upon a greater range of evidence. This word, coined by William Whewell in 1840, means “jumping together.” By this term, Whewell referred to proof by coordination of so many otherwise unrelated consequences under a single causal explanation that no other organization of data seems conceivable. In a sense, consilience defines the larger method un­derlying all Darwin's inference from historical records. In a more specific con­text, I use consilience (see Gould, 1986) for Darwin's principal tactic of bring­ing so many different points of evidence to bear on a single subject, that history wins assent as an explanation by overwhelming confirmation and unique coordination. Call this, if you will, the different flowers principle to honor the extraordinary range of evidence that Darwin gathered (1877) to forge a historical explanation for why some taxa bear different forms of flowers on the same plant.

  Discordance (dissonance of one). Here we reach a rock bottom of minimalism — unfortunately all too common in a world of limited informa­tion. We observe a single object, but not enough relevant items to forge consilience about its status as the product of history. How can we work from unique objects? How shall we infer history from a giraffe? Darwin tells us to search for a particular form of discordance — some imperfection or fail­ure of coordination between an organism and its current circumstances. If such a quirk, oddity, or imperfection — making no sense as an optimal and immutable design in a current context — wins explanation as a holdover or vestige from a past state in different circumstances, then historical change may be inferred. Call this, if you will, the orchid principle (though I have also designated it as the panda principle for my own favorite example, perforce unknown to Darwin, of the panda's false thumb, Gould, 1980d), to honor Darwin's argument (1862) for orchids as products of history. Their intricate adaptations to attract insects for fertilization cannot be read as wonders of optimal design, specially created for current utilities, for they represent con­traptions, jury-rigged from the available parts of ordinary flowers. [Page 105]