Leonardo’s Mountain of Clams and the Diet of Worms (4 page)

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:

(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.)

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.”)

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

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:

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:

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:

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:

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.”