The Structure of Evolutionary Theory

by Stephen Jay Gould

  In most general terms, and in order to form a more perfect union among evolution's hierarchy of structural levels and tiers of time, this revised theory rests upon an expansion and substantial reformation of all three central prin­ciples that build the tripod of support for Darwinian logic: (1) the expansion of Darwin's reliance upon organismal selection into a hierarchical model of simultaneous selection at several levels of Darwinian individuality (gene, cell lineage, organism, deme, species and clade); (2) the construction of an inter­active model to explain the sources of creative evolutionary change by fusing the positive constraints of structural and historical pathways internal to the anatomy and development of organisms (the formalist approach) with the external guidance of natural selection (the functionalist approach); and (3) the generation of theories appropriate to the characteristic rates and modali­ties of time's higher tiers to explain the extensive range of macroevolution­ary phenomena (particularly the restructuring of global biotas in episodes of mass extinction) that cannot be rendered as simple extrapolated consequences of microevolutionary principles.

  And yet, as an epilog to this epilog and, honest to God, a true end to this interminable book, I risk a final statement about contingency, both to expli­cate the appeal of this subject, and to permit a recursion to my starting point in the most remarkable person and career of Charles Robert Darwin. Al­though contingency has been consistently underrated (or even unacknowl­edged) in stereotypical descriptions of scientific practice, the same subject re­mains a perennial favorite among literary folk, from the most snootily arcane to the most vigorously vernacular — and it behooves us to ask why.

  Our greatest novelists have reveled in this theme, as Tolstoy devoted both prefaces of War and Peace to explaining why Napoleon's defeat in Moscow in 1812 rested upon a thicket of apparently inconsequential and independent details, and not upon any broad and abstract claim about the souls of nations or the predictable efficacy of Russia's two greatest generals, November and December. And Wuthering Heights would have lost both its story line and ex­istence if poor Heathcliffe had not overheard, and utterly misunderstood, a conversation not intended for his ears in any case. And where would our oc­casionally philosophical movies find a subject if they couldn't mine the con­tingent fascinations of alternative and unrealized histories, either of little towns (It's a Wonderful Life) or of otherwise inconsequential people (the Back to the Future trilogy). And how could satire flourish if contingency movies couldn't generate an opposing parody (Groundhog Day), based upon a day that, in its repetition, cannot be changed at all, even by the most portentous [Page 1341] act of murder or suicide that its utterly frustrated protagonist can devise to extract himself from this nightmare of no novelty — until, of course, he finally understands the wisdom behind the only consistent definition that a philosophical determinist can possibly devise for liberty: Spinoza's concep­tion of freedom as “the recognition of necessity.”

  If we then ask why literary, but not scientific, people have taken such a shine to contingency, I doubt that we need probe much beyond the most obvi­ous of all reasons, the framework for the conventional stereotype of each dis­cipline, and the putative difference between them as well. Science supposedly rests upon the objective generality of nature's laws and the utter insignificance of a practitioner's personality, or even his identity (beyond our vulgar and personal need to count coup, and also to count the prospects of future fund­ing, prizes, privileges and parking places). Why else have we been trained to write our professional papers in the unstylish passive voice, as if “I” didn't exist at all, and every datum “was discovered” in some disembodied manner? After all, although some particular somebody has to do it, the “it” is out there, and objectively knowable. Thus, it will be found, and within a narrow range of predictable time, largely dependent upon the development of tech­nologies that initially make the discovery possible.

  The equally silly and simplistic stereotype of the “other” side holds that lit­erary people view the world as completely inchoate and unstructured (be­yond the ideologically uninteresting, if practically portentous, compendium of observed regularities, suggesting, for example, that we will splatter if we fall off the roof of a 20-story building, or crunch if we happen to insert our­selves between a speeding vehicle and a concrete wall). Therefore, the argu­ment continues, we make our own way in a subjective and unconstraining world. We alone are the architects and responsible agents of both our per­sonal and our collective destinies.

  As exaggerated as these characterizations may be, they do reflect some gen­uine cultural, and even partly justifiable, differences between two important, even noble, enterprises in their uncaricatured state. And, in this case, science could learn an important lesson from the literati — who love contingency for the same basic reason that scientists tend to regard the theme with suspicion. Because, in contingency lies the power of each person, no matter how appar­ently insignificant he may seem, to make a difference in an unconstrained world bristling with possibilities, and nudgeable by the smallest of unpredict­able inputs into markedly different channels spelling either vast improvement or potential disaster.

  And so, if Joshua Lawrence Chamberlain, former professor at Bowdoin College, and now commander of the gallant 20th Regiment of the State of Maine had not led one of the last successful bayonet charges in the history of warfare (because he had run out of ammunition and could only hope to pre­vail by a bluff of this sort), thus preventing the outflanking of the Union line (which could easily have been outflanked and overtaken, if the Confederates had grasped the desperate military situation of their adversaries), the South would probably have won at Gettysburg, leading to potential victory in the [Page 1342] war, a sundering of the United States, the balkanization of our continent, and the end (with markedly negative consequences for human history) of the world's most promising experiment in democracy. And if George Bailey had never been born (an alternative scenario that his guardian angel constructed for his consideration), the history of his town would have been equally sensi­ble but altogether less pleasant for everyone actually loved by this apparently insignificant man. And so both the historical Mr. Chamberlain and the fic­tional Mr. Bailey (of America's most beloved movie) learned that one ostensi­bly small and meaningless life can make all the difference, sometimes for an entire world at a tipping point (in the admittedly grandiose and a bit extreme, but still not so utterly implausible, fable at the beginning of this paragraph), and more often for the few people whom we love and whom we yearn to serve as a source of comfort. The literati embrace contingency because no other theme so affirms the moral weight, and the practical importance, of each human life.

  Thus, to end where this book began with Charles Darwin and his personal importance to our understanding of this grandest earthly enterprise, the tree of life, I must side with the literati and insist that my decision to focus this book on Darwin and the logic of his explanatory system for life's history and evolution's mechanism does not merely record an idiosyncratic or antiquar­ian indulgence. I will grant one point to my scientific colleagues and freely al­low that if Charles Darwin had never been born, a well-prepared and waiting scientific world, abetted by a cultural context more than ready for such a re­construction of nature, would still have promulgated and won general accep­tance for evolution in the mid 19th century. At some point, the mechanism of natural selection would also have been formulated and eventually validated, perhaps by Wallace himself who might then have expanded his few pages of speculation, written during a malarial fit on Ternate, into the same kind of factual compendium that Darwin composed, and that guaranteed the tri­umph of this view of life.

  So why fret and care that the actual version of the destined deed was done by an upper class English gentleman who had circumnavigated the globe as a vigorous youth, lost his dearest daughter and his waning faith at the same time, wrote the greatest treatise ever composed on the taxonomy of barna­cles, and eventually grew a white beard, lived a
s a country squire just south of London, and never again traveled far enough even to cross the English Channel? We care for the same reason that we love okapis, delight in the fos­sil evidence of trilobites, and mourn the passage of the dodo. We care because the broad events that had to happen happened to happen in a certain partic­ular way. And something almost unspeakably holy — I don't know how else to say this — underlies our discovery and confirmation of the actual details that made our world and also, in realms of contingency, assured the minutiae of its construction in the manner we know, and not in any one of a trillion other ways, nearly all of which would not have included the evolution of a scribe to record the beauty, the cruelty, the fascination, and the mystery.

  Yes, the Renaissance would have unfolded — indeed, Europe already bathed [Page 1343] in its midst — if Michelangelo had never been born. But how much poorer would our world have been without the magnificent statue of Moses, furious and disconsolate as he holds the tablets of the law while his people dance about the golden calf, still presiding in the Church of San Pietro in Vincoli; and without the gigantic fresco of the Last Judgment, revealing all our blessed humanity in all our earthly sins, and still covering, in brilliant restoration, a full wall of the Sistine Chapel?

  No difference truly separates science and art in this crucial respect. We only perceive a division because our disparate traditions lead us to focus upon dif­ferent scales of the identity. The art historian looks right at Moses and knows the importance of its individuality. The scientist tends to gaze upon a world ready for evolution, and then discounts the centrality of a single, admittedly fascinating, individual named Charles Darwin. But if Darwin had never been born, we would have suffered the equivalent of a Renaissance without Moses or the Last Judgment — a biological revolution without the Origin of Species; without the invocation of Julia Pastrana, the bearded circus lady with two sets of teeth, to illustrate correlation of growth; without the Galapagos fauna to embody the principle of imperfection to prove the pathways of history; without pigeons to illustrate artificial selection; without barnacles to punc­ture half our pride with their dwarfed males upon the hermaphrodites.

  Most of all, we would have experienced the same biological revolution without the stunning clarity, illustrated by wonderfully apposite metaphors, of a complex central logic so brilliantly formulated, and so bristling with im­plications extending nearly forever outward, at least well past our current reckoning. In this alternate world, we would probably be honoring a differ­ent and far less compelling founder by occasional visits to a statue in a musty pantheon, and not by constant dialogue with a man whose ideas live, breathe, challenge, taunt, and inspire us every day of our lives, more than a century af­ter his bones came to rest on a cathedral floor at the foot of whatever persists in the material being of Isaac Newton.

  We would be enjoying an evolutionary view of life, but not the specific grandeur of “this view of life.” What can be more ennobling than a factual reality — the uniquely actualized result among innumerable potentials that did not obtain the most precious privilege of emergence into concrete existence? And what a stunning piece of good fortune, that this actuality came to us with all the grace, the moral weight, and the intellectual power of Darwin's particular struggles and insights, clothing the structure of his thought in that apotheosis of human achievement — wisdom, which the Book of Proverbs, citing the same icon that Darwin would borrow more than two millennia later, called Etz Chayim, the tree of life. “Length of days is in her right hand,” for “she is a tree of life to them that lay hold upon her; and happy is every one that retaineth her.”

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