Leonardo’s Mountain of Clams and the Diet of Worms

by Stephen Jay Gould

  For example, the “Introductory Note” in the official catalog for a recent exhibition of the Leicester Codex in New York summarizes the basis of Leonardo’s success in these words: “In it [the codex] we can begin to see how he combined almost superhuman powers of observation with an understanding of the importance of experimentation. The results were inspired insights into the workings of nature that match his artistic achievements.” When such conventional sources acknowledge the persisting medieval character of many Leonardian pronouncements, they almost always view this context as a pure impediment to be overcome by observation and experiment, not as a matrix that might have been useful to Leonardo, or might help us to understand his beliefs and conclusions. For example, the closing passage of the long Encyclopaedia Britannica article on Leonardo states: “Leonardo approached this vast realm of nature to probe its secrets . . . The knowledge thus won was still bound up with medieval Scholastic conceptions, but the results of his research were among the first great achievements of the thinking of the new age because they were based on the principle of experience.”

  I think that this conventional view could not be more wrong in its general approach to the history of knowledge, or more stultifying for our quest to understand this most fascinating man of our intellectual past. Leonardo did make wonderful observations. He did often anticipate conclusions that public science would not reach for another two or three centuries. But he was neither a spaceman nor an angel—and we will never understand him if we insist on reading him as Hank Morgan, a man truly out of time, a modernist among the Medici, a futurist in the court of Francis the First.

  Leonardo operated in the context of his time. He used his basically medieval and Renaissance concept of the universe to pose the great questions, and to organize the subjects and phenomena, that would generate his phenomenal originality. If we do not chronicle, and respect, the medieval sources and character of Leonardo’s thought, we will never understand him or truly appreciate his transforming ideas. All great science, indeed all fruitful thinking, must occur in a social and intellectual context—and contexts are just as likely to promote insight as to constrain thought. History does not unfold along a line of progress, and the past was not just a bad old time to be superseded and rejected for its inevitable antiquity.

  In this essay, I will try to illustrate the centrality of Leonardo’s largely medieval context by analyzing his remarkable paleontological observations in the Leicester Codex. I will begin by acknowledging their truly prescient character, but will then raise two questions that expose the early-sixteenth-century context of Leonardo’s inquiry: first, “What alternative account of fossils was Leonardo trying to disprove by making his observations?” and, second, “What theory of the earth was Leonardo trying to support with his findings?” Leonardo did not make his observations to win the praises of future generations; he studied fossils to probe these two questions of his own time—and his answers could not be more deeply embedded in a “hot topic” of his own century that we would now mock and dismiss as hopelessly antiquated. Thus, we cannot understand Leonardo’s paleontology when we only marvel at his empirical accuracy and ignore the reasons for his inquiry.

  Yes indeed, a thousand times yes, Leonardo’s observations are often stunningly accurate—as experts have always said, and for the reasons conventionally stated. Moreover, their degree of detail, and their centrality to the basic rules of modern paleoecological analysis, only enhance the impression of authorship by a Victorian geologist somehow trapped in the early sixteenth century. But let me stop marveling and start listing a small sample!

  1. Leonardo recognized the temporal and historical nature of horizontal strata by correlating the same layers across the two sides of river valleys:

  How the rivers have all sawn through and divided the members of the great Alps one from another; and this is revealed by the arrangement of the stratified rocks, in which from the summit of the mountain down to the river one sees the strata on the one side of the river corresponding with those on the other.

  (All quotes, unless otherwise stated, come from the Leicester Codex as presented in the MacCurdy translation of Leonardo’s notebooks.)

  2. He observed that rivers deposit large, angular rocks near their sources in high mountains, and that transported blocks are progressively worn down in size, and rounded in shape, until sluggish rivers deposit gravel, and eventually fine clay, near their mouths. (I learned this rule as principle number one on day number one in my college course in beginning geology.)

  When a river flows out from among mountains it deposits a great quantity of large stones . . . And these stones still retain some part of their angles and sides; and as it proceeds on its course it carries with it the lesser stones with angles more worn away, and so the large stones become smaller; and farther on it deposits first coarse and then fine gravel . . . until at last the sand becomes so fine as to seem almost like water . . . and this is the white earth that is used for making jugs.

  3. The presence of fossils in several superposed layers proves their deposition at different and sequential times.

  4. The tracks and trails of marine organisms are often preserved on bedding planes of strata: “How between the various layers of the stone are still to be found the tracks of the worms which crawled about upon them when it was not yet dry.”

  5. If both valves of a clam remain together in a fossil deposit, the animal must have been buried where it lived, for any extensive transport by currents after death must disarticulate the valves, which are not cemented together in life, but only hinged by an organic ligament that quickly decays after death. (This principle of inferring transport by noting whether fossil clams retain both valves persists as a primary rule of thumb for everyday paleoecological analysis. I doubt that any pre-nineteenth-century geologist mentioned this observation in more than a casual manner, while Leonardo regarded the argument as central. This observation first inspired my undergraduate awe for Leonardo, for I had just learned the rule in class and had thought, “How clever; how modern.”)

  And we find the oysters together in very large families, among which some may be seen with their shells still joined together, which serves to indicate that they were left there by the sea and that they were still living.

  At another site, on the other hand, Leonardo inferred extensive transport after death:

  In such a locality there was a sea beach, where the shells were all cast up broken and divided and never in pairs as they are found in the sea when alive, with two valves which form a covering the one to the other.

  6. Leonardo often illustrates the so-called uniformitarian principle of using observations about current processes to infer past events. In a striking example, he notes how far a cockle can move in a day in order to understand the spatial distribution of shells in a layer of fossils:

  It does not swim, but makes a furrow in the sand, and supporting itself by means of the sides of this furrow will travel between three and four braccia in a day. [A braccio, or “arm,” measured about two feet.]

  7. No marine fossils have been found in regions or sediments not formerly covered by the sea.

  8. When we find fossil shells broken in pieces, and heaped one upon the other, we may infer transport by waves and currents before deposition:

  But how could one find, in the shell of a large snail, fragments and bits of many other sorts of shells of different kinds unless they have been thrown into it by the waves of the sea as it lay dead upon the shore like the other light things which the sea casts up upon the land?

  9. The age of a fossil shell can often be inferred from growth rings that record astronomical cycles of months or years. (Sclerochronology, or the analysis of periodicities in growth, has only become a rigorous and important subject in paleobiology during the current generation.) We can, Leonardo writes, “count on the shells of cockles and snails the numbers of months and years of their lives just as one can on the horns of bulls.”

  I have often quoted
a favorite line from Darwin in these essays: “How can anyone not see that all observation must be for or against some view if it is to be of any service?” Leonardo’s keen observations do seem to emit a wondrous whiff of modernity, but when we learn why he made his inquiries, and note how he ordered his facts, we can begin to place him into the proper context of his own world. Leonardo did not observe fossils for pure unbridled curiosity, with no aim in mind and no questions to test. He recorded all his information for a stated and definite purpose—to confute the two major interpretations of fossils current in his day. Both theories had been proposed to resolve a problem that had troubled Western natural history ever since antiquity: If fossil shells are the remains of marine organisms (and some are virtually indistinguishable from modern species), how did they get entombed in strata that now lie within mountains, several thousand feet above current sea level?

  First, Leonardo disproves and ridicules the common idea that all fossils reached the mountains by transport on the high waters and violent currents of Noah’s flood. Observations 3 through 6 of my list refute this theory by noting that many fossils are preserved in their position of life, undisturbed by any movement after death. One flood cannot produce a fossil record in several sequential layers (observation 3). Strata formed by violent currents could not preserve the feeding tracks of worms (observation 4). Noah’s floodwaters would have disarticulated all fossil clams into separate valves (observation 5). As for the cockle, laboriously moving but six to eight feet a day in its furrow, forty days and nights of rain would scarcely provide enough time for a journey 250 miles inland (where fossil cockles now reside) from the nearest modern sea:

  With such a rate of motion it would not have traveled from the Adriatic Sea as far as Monferrato in Lombardy, a distance of 250 miles, in forty clays—as he has said who kept a record of this time.

  Moreover, Leonardo adds, cockle shells are too heavy to be carried at the tops of waves, while they cannot be swept up the mountains along the bottom of the waters because Leonardo believed that bottom currents always move down from higher to lower elevations, even while waves and surface currents sweep inland.

  The explicit refutation of Noah’s flood as a cause of fossils forms a major theme of the Leicester Codex, and occupies several full pages of text—one, for example, titled “of the Flood and of marine shells,” and another, “Refutation of such as say that the shells were carried a distance of many days’ journey from the sea by reason of the Deluge.”

  Second, Leonardo dismisses, even more contemptuously, various Neoplatonic versions of the theory that fossils are not remains of ancient organisms at all, but manifestations of some plastic force within rocks, or some emanation from the stars, capable of precisely mimicking a living creature in order to illustrate the symbolic harmony among realms of nature: animal, vegetable, and mineral. For if fossils really belong to the mineral kingdom, then their position on the tops of mountains ceases to be anomalous, as we need no longer believe that these objects ever inhabited the seas.

  Leonardo made observations 7 through 9 to refute this Neoplatonic theory that fossils “grew” within their entombing rocks and do not represent the remains of organisms. If marine fossils are inorganic, why don’t they “grow” in all strata, rather than only in rocks carrying abundant evidence of an oceanic origin (observation 7)? If fossils belong to the mineral kingdom, why should they so often grow in fragments and jumbles looking exactly like piles of shells on our beaches, or layers deposited by rivers in lakes and ponds (observation 8)? Most convincingly, if fossils grow from inorganic “seeds” in the rocks, how can they expand, year by year, as indicated by growth bands in their shells, without fracturing the surrounding matrix (observation 9)?

  Leonardo reserved his choicest invective for what he regarded as the lingering magical content of this Neoplatonic theory of signs and signatures (although the issue remained alive—and quite lively—within Western science until the late seventeenth century. The Mundus subterraneus [1664] of the great Jesuit scholar Athanasius Kircher represents the last seriously cogent defense of the Neoplatonic position). Leonardo writes:

  And if you should say that these shells have been and still constantly are being created in such places as these by the nature of the locality and through the potency of the heavens in those spots, such an opinion cannot exist in brains possessed of any extensive powers of reasoning because the years of their growth are numbered upon the outer coverings of their shells [observation 9 again]; and both small and large ones may be seen, and these would not have grown without feeding or feed without movement, and here [that is, in solid rock] they would not be able to move . . . Ignoramuses maintain that nature or the heavens have created [fossils] in these places through celestial influences.

  But this demonstration that Leonardo made his paleontological observations to refute the prevailing theories of his time scarcely establishes my argument that he must be evaluated as a thinker immersed in his own premodern context, and not judged for his remarkable foreshadowing of twentieth-century views—for a true spaceman would also have to refute the fallacies of his surroundings in order to introduce superior views from his time warp (just as Hank Morgan had to reject the running messenger service to favor a telephone call for summoning Sir Lancelot’s bicycle corps). I must advance a further claim—one that can be particularly well documented in Leonardo’s case.

  Just as Leonardo made his astute observations to refute prevailing theories of fossils, he also urged his interpretations in support of his own favored theory of the earth. (“All observation must be for or against some view . . .”) And the positive prod for Leonardo’s paleontological observations could not have been more squarely Renaissance or late medieval, more firmly attached to his own time and concerns—and not to ours. Leonardo observed fossils as part of his quest to support a distinctive theory of the earth—a framework that would have been seriously weakened if either the Noah’s flood or the Neoplatonic theory of fossils had been true. If Leonardo had not been so devoted to his “antiquated” theory of the earth, I doubt that he would ever have been inspired to make his wonderfully “modernist” observations about fossils—for the notebooks invariably present his observations as arguments to support his theory.

  Leonardo was so much larger than life, even in the eyes of his contemporaries, that a potent mythology began to envelop him right from the start. Only thirty years after Leonardo’s death, Giorgio Vasari published a first biography full of such touching tall tales as Leonardo’s death in the arms of King Francis the First. (Francis did admire Leonardo greatly, but he and his entire court had decamped to another town on the day of Leonardo’s demise. A. Richard Turner has written an entire, and fascinating, book on the history of the Leonardo legend through the ages: Inventing Leonardo, University of California Press, 1992.) One prominent component of the myth—that Leonardo was an unlettered man who could only work by observation and therefore gained great (if ironic) benefit from not knowing the false traditions of medieval Scholasticism—must be refuted if my case for his medieval impetus has merit. For how could I assert such a controlling context if Leonardo never knew or studied the prevailing traditions of book learning in his time?

  As the illegitimate son of a Florentine notary, Leonardo grew up in comfortable but nonscholarly circles, and received only a limited formal education. Most important, he did not learn Latin, then the nearly universal language of intellectual communication. But Leonardo did study Latin assiduously in later life, even if he never attained more than a halting knowledge. (I love Martin Kemp’s statement in his superb book Leonardo da Vinci: The Marvelous Works of Nature and Man: “It is rather humbling to think of Leonardo in his late thirties secretly schooling himself in the rhythmic rotes of ‘arno, amas, amat . . . ,’ like one of the children of the court.”)

  Moreover, Leonardo studied Latin because he yearned to gain full access to the scholarship of classical and medieval sources. He built a respectable library for the time—Italian
translations whenever possible, but Latin when necessary. He read particularly widely and deeply in this essay’s subject of paleontology and the structure of the earth. Kemp writes: “He was taking up questions which had provided considerable bones of contention in classical and medieval science. An impressive roll call of classical authorities contributed to his education in physical geography . . . There probably is no other field in which Leonardo’s knowledge of classical and medieval sources was so extensive.”

  He read the Greek masters Aristotle and Theophrastus on geology; he owned a copy of Pliny’s encyclopedic Natural History; he studied the views of the great Islamic scholars Avicenna and Averroes (mainly via medieval Christian sources). He listed parts of what he had read and owned on the inside front cover of his Manuscript F—Aristotle’s Meteorologia, Archimedes on the center of gravity, “Albertuccio and Albertus de coelo et mundo.” I found this last comment particularly sweet, as Leonardo follows medieval conventions in distinguishing his sources as “Little Al” (the Italian diminutive Albertuccio) and “Big Al.” Little Al is Albert of Saxony (ca. 1316–1390), the German Scholastic philosopher and physicist. Later scholars frequently confused him with Big Al, or Albertus Magnus (ca. 1200–1280), Albert the Great, the teacher of Saint Thomas Aquinas. Both Als wrote extensively about the form and behavior of the earth, and Leonardo probably learned the views of Jean Buridan (1300–1358) by reading Albert of Saxony’s discussion. Buridan’s concept became the basis for the theory of the earth that Leonardo defended with his observations on fossils.