I came to evolution in a roundabout way. Sure, as a kid I had seen the dinosaurs at the American Museum of Natural History—and had heard a bit about evolution in high school. But I was intent on studying Latin and maybe going to law school.
But evolution got in the way. I was dating my now wife, and through her getting to know members of the Columbia anthropology faculty. At the time (early 1960s), anthropology to me meant Louis Leakey and his adventures collecting human fossils at Olduvai Gorge—rather than, say, Margaret Mead and her adventures studying cultures of the South Pacific. A summer spent asking embarrassing personal questions in my halting Portuguese in a small village in northeastern Brazil ended my quest to study evolution through anthropology. I was far more taken with the Pleistocene fossils embedded in the sandstone that formed the protective cove for the fishing boats. By summer’s end I was determined to become a paleontologist.
Little did I know that paleontologists (with a few exceptions) had had virtually nothing to do with the development of evolutionary biology since Darwin’s day. Vertebrate paleontologists, to be sure, tended to be trained in zoology departments and to have at least a passing interest in evolution. But the undergraduate courses in paleontology at Columbia were in the Geology Department. I took my undergraduate degree in geology at Columbia, staying on for a PhD and writing my dissertation on the evolutionary career of the Devonian trilobite Phacops rana.
Evolution in those days was firmly in the hands of geneticists—who were at that very moment collectively like deer caught in the headlights of the onrushing revolution in molecular biology. DNA was threatening the comfortable world of population genetics—and there simply was little intellectual time or psychic energy for genetics-minded biologists to pay any attention to the results of a study on the evolution of a small cadre of long-dead and all-but-forgotten trilobites.
Indeed, had I read the introduction to my distinguished predecessor George Gaylord Simpson’s famous 1944 book Tempo and Mode in Evolution, I might have seen that paleontology was a decidedly rocky road for walking the evolutionary walk. Simpson had wryly encapsulated the tension between geneticists and paleontologists when he wrote:
Not long ago paleontologists felt that a geneticist was a person who shut himself in a room, pulled down the shades, watched small flies disporting themselves in bottles, and thought that he was studying nature. A pursuit so removed from the realities of life, they said, had no signficance for the true biologist. On the other hand, the geneticists said that paleontology had no further contributions to make to biology, that its only point had been the completed demonstration of the truth of evolution, and that it was a subject too purely descriptive to merit the name “science.” The paleontologist, they believed, is like a man who undertakes to study the principles of the internal combustion engine by standing on a street corner and watching the motor cars whiz by.
And yet it was the evolutionary process—and not just the simple facts of evolutionary history—that I longed to study. It seemed very much that paleontology, like anthropology, was the wrong choice if evolution were to be the focus of all my work.
I had been shocked to find very little change in the 5 million years or so of history recorded by the main lineage of my Devonian trilobite. I had been led to believe—along with the rest of the evolutionary-minded world—that such periods of time would engender almost as a matter of course some degree of palpable and lasting evolutionary change. I had pulled the fat out of the fire (one needs results, after all, to claim a PhD from a dissertation study) by saying that not only my little group of trilobites but most species in the history of life showed great stability for most of their histories; I then said that the idea of geographic speciation—as championed by the geneticist Theodosius Dobzhansky and the systematist Ernst Mayr (writing after Dobzhansky but somehow receiving most of the credit)—could account for the fact that evolution seems to occur relatively rapidly as new species split off from their long-stable ancestors.
Published in 1971 with a turgid title in the main journal devoted to evolutionary biology, my results sank pretty much without a trace. Repackaged a year later, with additions, in a jointly authored paper with a former fellow student with a knack for catchy phrases, the theory of “punctuated equilibria” was born. That colleague, of course, was Stephen Jay Gould—who would never tolerate the subversion of paleontology to the interests of any other field. At last, a good choice!
Our paper seemed to annoy virtually everyone—starting with Tom Schopf, the editor of the collection of essays in which it appeared. It just seemed too anti-Darwinian: the denial of natural selection’s inexorably changing entire species through time was too much for Darwinians—geneticists and even paleontologists—to take. To be sure, some paleontologists came up to us and, looking furtively around, admitted sotto voce that their own data showed similar patterns of stability “punctuated” by sudden events of evolutionary change. But for the most part we were held to be wrong—and, unkindest cut of all, traitors to the Darwinian tradition.
The creationists of the day got into the act as well. In a clear demonstration of how thoroughly political the creationist movement has always been in the United States, Ronald Reagan told reporters, after addressing a throng of Christian ministers during the 1980 presidential campaign, that evolution “is a theory, a scientific theory only, and it has in recent years been challenged in the world of science and is not yet believed in the scientific community to be as infallible as it once was believed.” The creationist who managed to get to Reagan’s handlers later bragged to me that those scientists in question were none other than Gould and me. The syllogism ran something like this: (1) Darwin said that evolution is slow, steady, and gradual; (2) some scientists say that evolution consists of rapid bursts of change interrupting vastly longer periods of evolutionary stagnation; ergo, (3) some scientists don’t follow Darwin, meaning (4) some scientists oppose evolution. Then, as now, at least in the public domain, “Darwin” is code for “evolution.” The two are virtual synonyms.
I take being called anti-Darwinian very personally. It has always hurt, for I have always thought of myself as more or less a knee-jerk neo-Darwinian, someone who thinks the basic mechanism underlying evolutionary change, including the origin, modification, and maintenance of adaptations, resides squarely in the domain of natural selection. And I have always felt that, with one or two major exceptions, my version of how the evolutionary process works lines up very well with Darwin’s. Take natural selection, for example: I see natural selection just as Darwin originally did—as the statistical effect that relative success in the economic sphere (obtaining energy resources, warding off predators and disease, etc.) has on an organism’s success in reproducing. This conservative view contrasts strongly with the modern tendency to see natural selection as a matter of competition among genes to leave copies of themselves to the next generation—a position I take to be hopelessly teleological, obfuscating the real interactive dynamics of economic and reproductive organismic behavior driving the evolutionary process.
But, of course, there are those sticking points: Darwin (or so the cartoon version of him goes) enjoined us to expect evolution for the most part to be slow, steady, and gradual—whereas to me the fossil record screams loudly that such, for the most part, is not the case. And as I’ll discuss a bit later in this essay, there is an even more dramatic parting of the ways between us which I discovered only last year in the course of preparing the exhibition “Darwin” (which opened at the American Museum of Natural History in New York on November 19, 2005) and its companion volume, Darwin: Discovering the Tree of Life (Norton, 2005).
But I never thought the fact that Darwin—from where I stand as a paleontologist—got some of his story wrong somehow made me an anti-Darwinian. For I have admired the man ever since I took my paperback copy of the sixth edition of On the Origin of Species to read while waiting for Louis Leakey to show up and give a lecture on human evolution on the Columbia campus. I had arrived early to get a good seat, and Louis was late—so I got my first real chance to sample Darwin’s prose. I was fearful of the complexity of the great man’s mind, and of the alien nature of his Victorian prose. But I needn’t have worried, for Darwin proved accessible to the readers of his day—even lay readers—and he remains so today.
But I am no historian—rather just a simple soldier on the fields of evolution. Lots of my friends have spent more time reading Darwin and collecting his books than (until recently!) I have—and of course, as closely associated with Steve Gould as I have been since the 1960s, I basically just left the historical part of our discipline up to him.
So I was scared all over again when the offer came along to develop a major exhibition on the life and works of Charles Robert Darwin. I jumped at the chance but was quaking in my boots as I embarked on that particular part of the evolutionary road. There was nothing for it but to plunge in and become immersed in Darwiniana; getting on with it is so often the key to conquering one’s fears.
The exhibition, only recently opened as I write, is by all measures and accounts a huge success. The great thing about museum exhibitions is that they are three-dimensional; you can walk into them, be surrounded by them, and feel a part of them. And perhaps above all, exhibitions are about stuff. We are awash in genuine Darwiniana (the historian David Kohn, a key figure in the exhibition’s development, calls them the “crown jewels”). We have Darwin’s Bible and geological hammer, his specimens and his notebooks, his walking stick and Down House table, his letters and his manuscripts. And so much more.
The exhibition tells Darwin’s story well. Yet it is notorious how little actual verbiage makes it to the walls of an exhibition—or for that matter into the typical script of an hour-long sitcom. I decided to write a companion book primarily because the greater space would force me to understand Darwin’s story in richer detail than could ever be presented in an exhibition—the better to tell the bare bones of the story on the exhibition’s walls. But the book itself raised the additional angst of trying to find a fresh take on Darwin’s story—a story that the so-called “Darwin Industry” had analyzed so assiduously over the past thirty or forty years.
Fortunately—for me, but I think also for anyone looking to probe the inner workings of a proven creative mind—Darwin left an incredibly rich (if at times terribly cryptic) paper trail. We can see Darwin go from a callow, albeit enthusiastic, youth to a hard-working young man at times almost delirious with the sights, sounds, and smells of the natural world (“The mind is a chaos of delight,” Darwin wrote of his first encounter with the tropics in Bahia, Brazil, in his February 28, 1832, diary entry); to an open-minded young naturalist “letting nature come to him” (in the words of my colleague Joel Cracraft), noting repeated regularities in the natural phenomena around him; to an analytic thinker as the patterns built up and began to seep into his conscious mind as a suspicion (life has evolved!); to a hypothetico-deductive sharp analyst who saw further patterns as natural—predicted—outcomes if life had indeed evolved; to a searcher for an evolutionary mechanism, which (to judge from his first passage on natural selection in his Notebook D) came to him in a maelstrom of excited detail, later to be coolly, analytically parsed in three simple sentences; to a synthesis of his perceptions of patterns with his later-found theory of evolutionary process (where, according to me at least, he begins to go a bit astray); to the clever, yet still thoroughly methodical, deviser of experimental protocols to test the myriad separate portions of his theory.
The Darwin Industry has done a fine job with much of these phases of Darwin’s scientific sojourn (and I speak here solely of his evolutionary odyssey; his strictly geological, botanical, and zoological work, prodigious and generally excellent as it all was, is nonetheless not what makes someone like me a “Darwinist”). But one brings to the table what one is already equipped with, and I soon found, while reading at least the more recent literature on Darwin, that the focus has been more on mechanism—variation and natural selection—than on what it was that Darwin saw—and how he saw it—that led him to the idea of evolution in the first place.
What a briar patch for me! We paleontologists (as George Simpson put it so well) are consigned to studying the dead hand of history—the “completed demonstration” that life has evolved. But Simpson’s further point in 1944 was that whatever we say about how evolution happens, it had better match up with those facts of evolutionary history. And—better yet—recurrent patterns in the history of life should suggest to the evolutionary theorist just what processes and mechanisms—in modern terms, mutation rate, natural selection, the role of isolation, etc.—combine to give the basic elements of the evolutionary process. In other words, you can extrapolate from “flies disporting themselves in bottles,” but you had better check to see if your long-term extrapolations bear at least a passable resemblance to the facts of history.
That was how I had originally stumbled on what later came to be known as “punctuated equilibria.” Stasis is a common (I would say the dominant) pattern of anatomical (non)change in the evolutionary history of species; geographic isolation is a common (again, the dominant) pattern underlying the development of new species from old. I know about patterns and what they can tell us as a sort of resonating feedback against ideas of evolutionary process developed from and for short-term observations in the wild and in the laboratory. And so, it turns out, did Charles Robert Darwin.
Darwin tells us in his Autobiography (actually, it wasn’t written for “us,” but rather just for his immediate family) that he was as much a Baconian inductivist as he was “eager to test” his ideas in what we would these days call good hypothetico-deductive style. More recent Darwin scholars have overwhelmingly championed Darwin as one of the earliest exponents of modern hypothetico-deductive science—tending to treat his protestations of fealty to Bacon’s inductivism as a polite sop to his aged mentors, exponents of a by then discarded and discredited approach to science.
But Darwin, I am convinced, meant it when he said he was at least in some measure an inductivist. Though my early training in anthropology taught me to be leery about taking what “informants” say at face value, I cannot help but feel that Charles Darwin was an extremely honest person—above all to himself. He wanted to learn as much as possible about anything—a fossil, an idea, a breeding procedure. He looked at everything from as many sides as presented themselves to him—and devised ways of peering at sides otherwise not immediately evident. Darwin scholars, like all historians, and anthropologists with their informants tend to look askance at the recollections of old folks (Darwin was sixty-seven when he penned his Autobiography in 1876).
And perhaps I am being disingenuous to take Darwin’s words so literally to heart when he tells us what it was that he actually did. But fortunately there is the contemporary record of his experiences on the Beagle and the immediate aftermath—when, in a breathtaking interval of only 2½ years, in his “Red” and “Transmutation” notebooks, he synthesized the lessons already beginning to dawn on him inductively on the Beagle, then derived a subsidiary set of patterns as expected outcomes if evolution were true (purely deductive) and went on to derive inductively (as a flash of insight), then parse deductively, the principle of natural selection. We can see him in action in these notebooks and in his only slightly later first two essays on evolution. And I do think that my own experiences grasping patterns almost intuitively allows me to recognize the very same behavior that first brought Darwin to the idea of evolution—just as he himself said it did.
Darwin writes briefly of the patterns that first brought him to evolution in four passages sprinkled fairly evenly through his lifetime. The first two are in a diary (1837) and a letter (the famous 1844 letter to Joseph Hooker, just before Darwin announces his suspicion that species are “not immutable”—and says “it is like confessing a murder”). These passages, of course, are best known to Darwin scholars. But the third is hiding out in plain sight in the opening sentences of On the Origin of Species:
When on board H.M.S. “Beagle,” as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species—that mystery of mysteries, as it has been called by one of our greatest philosophers.
The philosopher in question was John Herschel, who had openly wondered, in a letter written to Charles Lyell in 1836 and published in 1838, when a young naturalist would appear who would put forth a credible, scientific explanation for the replacement of extinct species by later ones—the “mystery of mysteries.” Darwin met with Herschel in Cape Town in June 1836, after Herschel had written Lyell, and one is tempted to wonder (along with historian David Kohn in a footnote of his edition of Notebook E) whether they discussed the “mystery of mysteries” along with the geological portions of the letter.
Darwin’s topic sentence paints a bare-bones, nearly abstract picture of his crucial patterns. He does not discuss them further in The Origin—as these were simply the “facts” that got him started as a naïve ship’s naturalist. Long before 1859, Darwin had come to see as “my theory” not just the simple idea of evolution, but “evolution through natural selection.” Indeed, by the 1850s, evolution was spoken about openly in medical schools and other institutions; it was natural selection that Darwin saw as his special property—and his major contribution to science. That’s why Alfred Russel Wallace’s missive, arriving in 1858, made Darwin so frantic. For Wallace had discovered natural selection. The morals to that story, of course, are: if there is any truth to an idea, someone else will surely discover it; and if you do have a good idea, by all means publish it before someone else does!
The Wallace event—and the consequent hurried writing of The Origin—ended Darwin’s agonizing twenty-two-year period of keeping his evolutionary ideas almost completely to himself. Thus freed, by the time he wrote his Autobiography, he was able to say that his greatest contribution had been, in fact, the convincing demonstration simply that life has evolved—rather than the narrower (though monumentally important) formulation of natural selection. So it is no surprise that Darwin is far more expansive in his Autobiography than in any earlier statement when describing exactly what it was that he saw that took him to evolution in the first place:
During the voyage of the Beagle I had been deeply impressed by discovering in the Pampean formation great fossil animals covered with armour like that of the existing armadillos;secondly, by the manner in which closely allied animals replace one another in proceeding southwards over the Continent; and thirdly, by the South American character of most of the productions of the Galapagos archipelago, and more especially by the manner in which they differ slightly on each island of the group; none of the islands appearing to be very old geologically. (Autobiography, pp. 52–53)
The key words here are “closely allied” and “replace”; critical too is the concept of “distribution”—found in the preceding quote from The Origin but also implied here.
Darwin had three patterns in mind—all of them involving the replacement of one species by a similar one. The word “allied” virtually begs the question of evolutionary relationship—but Linnaeus, long before Darwin’s birth, had been recognizing “natural” groups without the internal evolutionary connections so offensive to the creationist mind-set. Edentate mammals (present-day armadillos and sloths) were already known by the time Darwin reached Argentina and began finding the fossilized bones of large mammals. The common denominator in all three sets of Darwin’s original patterns was the replacement—in space or in time—of similar species of organisms known only in that part of the world—meaning South America plus the Galápagos archipelago lying 600 miles to the west of Ecuador.
Modern Darwin scholarship tends to emphasize the importance of taxonomic experts back in England, whose analyses of Darwin’s specimens (including ones he sent home while still on the voyage) for the most part were rendered after Darwin returned. The classic example is, of course, “Darwin’s finches”: it was the ornithologist John Gould who figured out that there are thirteen species of a single related group of little brown, greenish, and black birds displaying an interesting array of beak sizes and shapes that, taken with their distribution patterns on the various islands, make a compelling case for evolution. This came long after Darwin sailed away from the Galápagos, having paid these birds hardly any heed. Indeed, he learned of Gould’s results only when he reached home.
But all the stress on the importance of homegrown taxonomic expertise tends to obscure what Darwin actually did see and what he made of it while he was still on the voyage. Darwin’s reference to “great fossil animals covered with armour like that of the existing armadillos” is a case in point. It is true that Darwin did not learn of Richard Owen’s conclusions concerning the large extinct armadillo Glyptodon until his arrival home. And it is also the case that there had been some speculation, known to Darwin when he was collecting these fossils in the fall of 1832, that giant ground sloths (already known to Western science) may have had “coats of armor”—raising questions about just what form of extinct edentate mammal sported such a bony coat. Yet Darwin wrote to his mentor John Stevens Henslow, upon finding “osseous polygonal plates” among his fossils: “Immediately I saw them I thought they must belong to an enormous Armadillo, living species of which genus are so abundant here.”
This is not a statement about evolution. But it is a statement about one extinct form succeeded by other, still living species: replacement in time.
Darwin soon thereafter encountered another pattern of replacement—spatial, or geographic—embodied most tellingly in the existence of two distinct, if basically similar, species of the ostrich-like rheas. Rheas, like armadillos and sloths, are endemic to South America—so here again he is struck by replacement patterns among similar species found nowhere else on earth. The two species in question occupy separate territories—the larger (“greater”) rhea living on the Argentine pampas, the somewhat smaller (“lesser,” for a while known as “Darwin’s”) rhea living to the south in Patagonia (and curving up northward as far as Peru in the Andes). Late in the voyage, Darwin wrote of these species replacing one another—the main (though not the sole) source of his comment about allied species replacing one another “in proceeding southwards over the continent.”
Again, this is simply replacement—no explicit mention of evolution. But by the time Darwin had written up the rheas in his “Ornithological Notes,” he had already written up his third replacement pattern—for the first (and apparently only) time on the voyage seeing, and explicitly commenting on, a possible evolutionary implication. This is the famous passage, also in the “Ornithological Notes,” on the Galápagos mockingbirds—with the Galápagos tortoises thrown in for good measure:
These birds are closely allied to the Thenca of Chile. . . . I have specimens from four of the larger islands. . . . The specimens from Chatham and Albemarle Isd appear to be the same; but the other two are different. In each Isld. each kind is exclusively found; habits of all are indistinguishable. When I recollect, the fact that the form of the body, shape of scales & general size, the Spaniards can at once pronounce, from which Island any Tortoise may be brought. When I see these Islands in sight of each other, & possessed of but a scanty stock of animals, tenanted by these birds, but slightly differing in structure and filling the same place in Nature, I must suspect they are only varieties. The only fact of a similar kind of which I am aware, is the constant/asserted difference—between the wolf-like Fox of East and West Falkland Islds.—If there is the slightest foundation for these remarks to zoology of Archipelagoes—will be well worth examining; for such facts (would) undermine the stability of Species. . . .
Here is replacement on a more microgeographic scale (“Islands in sight of each other”), once again involving closely similar forms “closely allied” with mainland South American mockingbirds—the group as a whole being confined, once again, to the Americas.
Darwin did not need John Gould to tell him there were distinctly different mockingbirds on some of the islands (the older ones, in the southeastern part of the chain). Gould did later describe the distinct forms as separate species, thereby resolving Darwin’s indecision on their status. But I do agree with Kohn et al. (2005), who have convincingly argued that “only varieties” is Darwin’s first statement toward the view that became so central to his later evolutionary arguments: that varieties, rather than simply reflecting variation within created kinds, are actually incipient species.
Hence Darwin’s ringing conclusion: “for such facts (would) undermine the stability of species.” The verb in this quote is usually rendered “would undermine”—as “would” is inserted into the sentence as an afterthought. Others disagree, but even if Darwin wrote “would” on top of the line only thirty seconds after writing out his sentence, we nonetheless see his mind at work. For such facts do undermine the stability of species—and that’s what Darwin initially wrote.
Darwin, as the journey ended, was no longer eager to become a quiet curate in the English countryside; rather, he was anxious to take his place among the emerging ranks of scientific professionals, or so he says in his Autobiography. Coupled with his replacement patterns and that key passage on the Galápagos mockingbirds and tortoises, it is difficult for me to believe that he did not have evolution firmly in mind as the Beagle finally turned from its second trip to the coast of Brazil—and finally headed for home.
Granted there is no unequivocal Darwin Beagle paper trail to back this supposition up. Rather, after a few months at home, Darwin completes his “Red Notebook,” begun on the Beagle but containing no explicit reference to evolution in its initial pages. But then, as he works on the book, more or less confining his thoughts to the things he saw and collected while on the voyage, we do find, in disconnected snippets, Darwin’s earliest speculations on evolution. By the time we pick up on his active mind, he is already convinced of evolution. He has already let the patterns take him there—and he is already hard at work imagining how—not if—evolution occurs.
He starts by letting the patterns set the terms and conditions of the evolutionary process. On page 130, he sees the parallels of replacement patterns in space and time:
The same kind of relation that common ostrich bears to Petisse . . . ; extinct Guanaco to recent: in former case position, in latter time (or changes consequent on lapse) being the relation—As in first cases distinct species inosculate, so we must believe ancient ones, therefore not gradual change or degeneration from circumstances: if one species does change into another it must be per saltum—or species may perish. This inosculation (representation) of species important, each its own limit and represented.
Well! I had grown up believing that Darwin was the quintessential gradualist—so I was flabbergasted to read in his very first evolutionary thoughts he was a “saltationist.” Darwin let the patterns of abrupt replacement in time and space speak for themselves. Small wonder that when we resurrected stasis as a pattern for all to contemplate, Gould and I were tarnished with that same saltationist brush—this time hurled as an insult. With Darwin, it was something more pure and innocent—he was letting nature’s patterns that led him to evolution guide him too in his initial explorations of how the evolutionary process actually works. His initial ideas along these lines saw species—much like individual organisms—to have finite lifespans, a sort of built-in survival limit. New species had to be created to keep life going. But just how new species were created from others in one blow Darwin never really answered.
Darwin knew about stasis—and was troubled by it from the outset of his fervent evolutionary explorations in 1837–39. He opened his “Transmutation Notebooks” (Notebooks B–E devoted to evolution) not long after filling the “Red Notebook.” The “Red Notebook” had been looking primarily backward to his Beagle experiences—melded now with the results of study of his specimens by the likes of John Gould and Richard Owen. He was truly an honest man, not least to himself—and he wanted to explore not only all avenues that corroborated his evolutionary views, but also those that seemed opposed. On stasis, he wrote (Notebook E, 1838):
My very theory requires each form to have lasted for its time: but we ought in same bed if very thick to find some change in upper & lower layers.—good objection to my theory: a modern bed at present might be very thick & yet have same fossils. (p. 6)
Darwin, by this time, had discovered natural selection—and even before that had abandoned saltationism in favor of a more Lyellian-infused gradualism. But he was still honest enough not simply to deny stasis, but actually to acknowledge it as a “good objection to my theory.” Indeed, at one point, Darwin seemed to have the reconciliation of stasis with his emerging notions of evolutionary process in hand—only to let it go. Instead he writes (Notebook E):
Species not being observed to change is very great difficulty in thick strata, can only be explained, by such strata being merely leaf, if one river did pour sediment in one spot, for many epochs—such changes would be observed. (p. 18)
Darwin went on to expand this argument—that the fossil record is too incomplete to allow us to trust its patterns of apparent change—and especially nonchange. He developed this argument in increasing detail in his 1842 “Sketch” and 1844 “Essay”—and in The Origin devoted an entire chapter to the “imperfections of the geological record.” How ironic that Darwin felt himself forced to turn his back on one of the main signals in nature that took him to evolution in the first place!
But in the main, in his four “Transmutation Notebooks,” Darwin had other fish to fry. Instead of patiently compiling case after case from the literature—inductively piling on further examples of the three sorts of patterns that struck him on the Beagle and which formed the basis of most of his theoretical explorations in the “Red Notebook”—Darwin pursued his work on two main fronts: addition of other patterns pointing to evolution and a search through what was known of the principles of inheritance, in large part through the work of breeders, for a mechanism of evolutionary change.
There was a distinct mental shift in Darwin’s search for additional evolutionary patterns to bolster his argument. For in a flash of insight (itself intuitive and therefore inductive) he realized that Linnaeus’s pattern of nested sets of taxa linking up all forms of life is precisely what you would expect if all species had indeed descended from a single common ancestor. He wrote “I think” on page 36 of Notebook B and proceeded to draw the first evolutionary tree—a diagram I feel is the equivalent of Einstein’s E=MC2 (our exhibition “Einstein,” including the manuscript of this most famous of equations, was still on display at the American Museum when I first accepted the challenge of mounting the Darwin exhibition).
Thus in a trice Darwin brought the very idea of evolution into the realm of hypothetico-deductive science. Neatly hierarchically structured classification schemes are what you would expect—i.e., predict—if evolution had occurred. The physical resemblances that led Linnaeus in the pre-evolutionary era to link humans with the great apes would, now, in an evolutionary context, lead one to predict greater genetic resemblances between humans and apes than, say, humans and chickens; and we would predict that the human fossil record, when explored down deep enough, would reveal our ancestors to have had smaller, ape-sized brains and not to have been, at the outset, fully upright and bipedal. That’s what we find.
Darwin (in the Autobiography) took special delight in claiming that it was he who had first pointed out that the earlier stages of the embryos of “allied” forms would resemble each other more closely than the adult stages—again, what you would expect if these species had descended from a common ancestor. And he also saw that the “unity of type”—the close structural similarity of, say, the arms of bats, birds, horses, and humans—is also a natural expectation if these species shared an evolutionary ancestor in the remote geological past.
As to mechanism—natural selection—Darwin seemed close to finding it throughout the notebooks. He had long since known that evolution must be a matter of change in the hereditary materials—whatever those might be. After all, that’s essentially how breeders achieve their results—knowing that organisms tend to resemble their parents. The variation in the barnyard lets the breeder select just those animals (or plants in the greenhouse) that have the best development of the traits to be enhanced in the lineage.
But it was that famous encounter with Malthus’s 1798 Essay on the Principle of Population in which Darwin learned that more organisms are produced each generation than can possibly survive and themselves reproduce—else the world would be “standing room only” (as he put it in The Origin) in a single species. As David Kohn points out, Darwin, in an impassioned passage full of imagery but little explicit detail (Notebook D, p. 134), records his initial—and again largely intuitive—impression of natural selection as “a force of a hundred thousand wedges” shaping the adaptations of species. It is only after some time goes by that a clear-eyed Darwin analytically and economically parses natural selection into three simple “principles” that “will account for all”:
- Grandchildren like grandfathers
- Tendency to small variation . . . especially with physical change
- Great fertility in proportion to support of parents
(Notebook E, p. 58)
In other words, inheritance, variation, and overproduction of offspring—then as now the main ingredients of the idea of natural selection.
But then Darwin takes a further, fateful step: he cannot resist rethinking his original patterns as outcomes, now, of evolution through natural selection. The results are interesting—as they have dictated in large measure the subsequent history of evolutionary biology. On page 118 of Notebook E, Darwin writes:
Has nature any process analogous—if so she can produce great ends—But how.—Make the difficulty apparent by cross-questioning—even if placed on Island—if &c &c.—Then give my theory—excellently true theory.
Darwin was coaching himself on the structure of the argument to be made when he wrote his theory up in essay form (not to happen until 1842). “Process analogous” meant natural selection (analogous to artificial selection, the subject of the preceding sentences). He was telling himself to pose hypothetical situations, such as species on an island, and then produce an explanation of that case based on his idea of natural selection.
Indeed, this was to become his argumentative structure throughout all his expositions of evolution: start with chapters on variation, inheritance, and natural selection; then go on to explain various patterns in nature as expected outcomes of the evolutionary process.
Darwin had already dismissed the incongruity between his vision of gradual evolution through natural selection with the typical, recalcitrant refusal of species to demonstrate much if any change even through “thick strata.” That left evolution on island archipelagos and the evolution of new species from older ones on continents to reconsider.
And here is a further irony, considering the importance of the Galápagos patterns in bringing Darwin to evolution in the first place: Darwin in the end downplays the significance of islands as a general model of evolutionary change. Island archipelagos afford the clearest examples of the importance of physical isolation in promoting evolutionary change—and isolation in general played an important role in Darwin’s thinking clear through his (unpublished) “Essay”of 1844. But in The Origin Darwin writes:
Although I do not doubt that isolation is of considerable importance in the production of new species, on the whole I am inclined to believe that largeness of area is of more importance, more especially in the production of species, which will prove capable of enduring for a long period, and of spreading widely. (p. 105)
Probably just because there are far more species living on continents than on island archipelagoes, in search of a truly general evolutionary theory, Darwin looked to continental patterns: the rheas over the mockingbirds. And though he had discussed isolation on continents in his notebooks and earlier manuscripts, by the time he wrote The Origin, Darwin was having a difficult time imagining the action of physical isolation disrupting species and promoting the evolution of new ones on continents.
Thus the picture of evolution that I grew up with—the one Steve Gould and I reprinted in our original paper on punctuated equilibria. The original shows the gradual divergence of two lineages of snails (redrawn here as scallops); what we are really looking at, though, is Darwin’s decision regarding which was the most important of his three patterns—and a version, at least, of his explanation of it through natural selection. In truth, these snails and scallops are all, at base, Darwin’s rheas.
But it is nonetheless true that some of the most talked-about issues in the years since the 1859 publication of The Origin have in fact been hard-won restitutions of some of the very patterns that Darwin saw but later abandoned. I have already mentioned two. The first is the reemergence of the importance of geographic isolation in the production of new species in a movement led by Dobzhansky and Mayr in the 1930s and 1940s. Isolation had always had its champions, but the theme was definitely muted in the decades between Darwin’s On the Origin of Species and Dobzhansky’s Genetics and the Origin of Species, published nearly eighty years later.
With the importance of geographic isolation reestablished, my rediscovery of stasis (the second restitution), combined with the theory of geographic speciation, led to punctuated equilibria. Paleontologists had occasionally mentioned stasis over the intervening years, but most were content to accept Darwin’s condemnation of the fossil record as too faulty to be trusted. Stasis is now seen to be common—and the expected outcome of the fates of geographically disjunct populations within a species adapted to different local ecological conditions: the probability of natural selection being able to modify the features of all the populations of a species spread out over wide areas in a single direction is now seen to be rather small.
Which brings us to the most dramatic and superficially un-Darwinian pattern of them all. It is a pattern I had looked for in vain as I reread The Origin a few years back, and I supposed Darwin knew nothing of it—or at least thought so little of it that it didn’t warrant discussion. It is a pattern that goes back to the great paleontologist and anatomist Baron Georges Cuvier, who wrote about it in 1812—three years after Darwin’s birth. And it is the pattern that, if it gets the attention and survives the debate it merits, promises to produce a significant change in our picture of the evolution of life on Earth.
It turns out Darwin did know this pattern—and I will let him say what it is in his own words. These words were never published in his lifetime. I found them as a footnote in tiny type at the bottom of a page of Darwin’s son Francis Darwin’s 1909 transcription of his father’s 1844 “Essay.” Francis found a note, in Darwin’s hand, in the fair copy of this manuscript. Darwin wrote: “Better begin with this: If species really, after catastrophes, created in showers world over, my theory false.” Wow! Darwin, of course, never began with this—he never mentioned mass extinction anywhere in his published works. It was just too far beyond the pale—too far outside his expectations of what evolution under natural selection should look like in geological time.
But Cuvier was right. There are numerous events that paleontologists call “turnovers”—episodes of extinction followed by many examples of the same sort of replacement that Darwin saw with his extinct and modern armadillos. Cuvier was given short shrift because his explanation of those replacements—separate creations by the deity—hardly solved the problem of the “mystery of mysteries.” And, of course, given a concatenation of many separate “punctuated equilibria” events all happening more or less at the same time, Darwin, armed with his particular conception of evolution through natural selection, must have felt totally helpless in the face of such a monstrous pattern. He could blame stasis on a poor geological record, but faced with paleontologists talking up Cuvierian patterns, Darwin simply cut and ran. It’s the only time I have ever found him doing that.
Turnovers have been actively discussed in evolutionary terms by paleontologists for over twenty years now—ever since the Alvarez group, beginning in 1980, restored mass extinctions to enough respectability to be taken seriously. A few years ago I developed a general theory I called the “sloshing bucket”—an attempt to meld Cuvier with modern work on turnovers, folding in Darwin’s evolution through natural selection, speciation, and the aperçus of punctuated equilibria. It is a view of evolution that sees the physical environment as the fundamental driving force—a theme Darwin would not throw out of bed a priori, even though he himself preferred notions of interspecies interactions to explain such important phenomena as extinction. It says that the degree of evolutionary innovation is roughly proportional to the degree of severity of extinction.
The Sloshing Bucket theory is really very simple. It says that we know the details of two separate kinds of phenomena that turn out to be extremes of a spectrum. At the lower level—short-term, localized—we have the environmental degradation of local ecosystems: wildfires, volcanic eruptions, and suchlike damage, and sometimes obliterate, local ecosystems. But their effects are not so widespread, typically, as to cause the actual extinction of entire species. Or at least not many species (some species have very limited ranges—more typically in the tropics than in the higher latitudes, but this scenario is possible anywhere). In time, when the cause of the degradation has died down, the local ecosystem is usually rebuilt to something that looks very like what it had been: ecological succession may start with a few hardy “pioneer” species, but over time recruits from all the surviving populations of the various species formerly there will show up—and the ecosystem will be restored more or less to its former self.
At the other extreme we have mass extinctions—events that have happened five or six times since the advent of complex life (and a decent fossil record thereof) some 535 million years ago. Though it is not always clear what causes these mass extinctions, physical events are always implicated—such as the collision between the earth and one or more “extraterrestrial bolides” (read comets or meteors). Entire groups that had been around for 100 million years, sometimes longer, disappear. Dinosaurs and ammonites at the end of the Mesozoic era, or trilobites and rugose corals at the end of the Paleozoic. The divisions of geological time, largely worked out in the creationist world of the 1830s and 1840s while Darwin was developing and then honing his evolutionary ideas, reflect these larger-scale turnovers in the history of life. That’s how geologists could tell time and order their rocks: the sequence of turnovers great and small in the rock record. It was Darwin’s mentor Adam Sedgwick whose training in the field, just before the letter inviting him to join the Beagle arrived, equipped Darwin to do all the “geologising” he did on the voyage. (Darwin was grateful and wrote Henslow asking him to convey his thanks to Sedgwick—a letter we have included in the American Museum exhibition.) And it was Adam Sedgwick who named the Cambrian System—the lowermost division of the Paleozoic.
Mass extinctions are turnovers on the grandest scale. Mammals had evolved when the dinosaurs first appeared, back in the Triassic. They stayed, ecologically speaking, pretty much in the deep background until the dinosaurs finally disappeared, 65 million years or so ago, in the mass extinction that drew the Mesozoic to a close. Then, after what seems to be a customary lag, mammals rapidly differentiated into a panoply of large and small herbivores, carnivores, and scavengers. They were then cut back in a succession of smaller turnovers, which pretty soon produced the main groups of mammals—your carnivores, perissodactyls, artiodactyls, etc.—familiar to us (albeit in earlier, more primitive guise) today.
Well, if there are small-scale effects leading to ecological reconstitution rather than palpable evolution, and if there are global mass extinction events, in which large-scale groups become extinct, to be replaced by other large-scale groups—wouldn’t we predict (“expect,” Darwin would have said) that there would be intermediate-scale events sufficiently large to knock out entire species, but smaller in scale nonetheless than truly horrendous mass extinction events on a global scale? We would—and that is what we find. They are the basis of all the smaller-scale, finer subdivisions of geological time empirically deciphered by geologists and paleontologists since the 1840s. These turnovers tend not to match up too well across entire continents—or for that matter between continents—because they are more localized, less extreme versions of the far less numerous global mass extinction events.
There have been at least hundreds—and probably a few thousand—of these events since complex life appeared on earth just over half a billion years ago. These events are where the vast majority of speciation has occurred. That means most evolution in the history of life is tied up in such episodes. And that is a very different picture of evolution than the cartoon version derived from Darwin’s thinking later in his life.
Yet this new picture is not un-Darwinian, let alone anti-Darwinian. Selection is there—keeping species stable instead of inexorably changing them. And selection is there as the opportunity for change comes—for the most part, if not exclusively, when the physical environment changes the conditions of life.
Darwin would have been fascinated by all the new discoveries in genetics, which have essentially left his theory of evolution through natural selection unscathed. But he would also, I feel sure, welcome with relief the validation of natural patterns he felt compelled to reject because they seemed to present incongruities with his vision of how evolution through natural selection should play out through geological time.
It takes a true Darwinist to tweak the great man’s vision, bringing it into line with the facts of the matter. I confess that I am a true Darwinist.
 Many of Darwin’s writings are to be found online at the Darwin Digital Library (D. Kohn, editor): http://darwinlibrary.amnh.org.
 C. Darwin, “Red Notebook,” ed. S. Herbert, and “Transmutation Notebooks,” ed. D. Kohn, in Charles Darwin’s Notebooks, 1836–1844, ed. P. H. Barrett et al. (Ithaca: Cornell University Press, 1987); “Sketch” (1842) and “Essay” (1844), in The Foundations of the Origin of Species, ed. F. Darwin (Cambridge: Cambridge University Press, 1909).