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:

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 250miles, in forty days-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 `amo, 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 A1 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.

What theory of the earth, then, did Leonardo seek to support with paleontological data? Simply stated, Leonardo was vigorously promoting a common and distinctively premodern view that could not have been more central to all his thought and art: the comparison, and causal union, of the earth as a macrocosm with the human body as a microcosm. We tend to regard such comparisons today as "merely" analogical or "purely" metaphorical-more apt to promote a deluding sense of false unity than any genuine insight about common causality. By contrast, Leonardo's pre-modern world viewed such consonances as deeply meaningful, in part by invoking the same general theory of symbolic correspondence across scales of size and realms of matter that Leonardo (ironically) had rejected so vigorously in denying the Neoplatonic idea that fossils might grow within rocks as products of the mineral kingdom.

No theme recurs so incessantly, and with such central import, both in the Leicester Codex and throughout Leonardo's writing, as the causal and material unity of the body's microcosm and the earth's macrocosm. Leonardo also knew the ancient pedigree of this doctrine, from classical antiquity through medieval Scholasticism. In the A Manuscript (now in the Institut de France), Leonardo stated that he would begin his "Treatise on Water" (never completed or published) with a statement that he later repeats almost verbatim in the Leicester Codex:

Man has been called by the ancients a lesser world, and indeed the term is rightly applied, seeing that if man is compounded of earth, water, air and fire, this body of the earth is the same; and as man has within himself bones as a stay and framework for the flesh, so the world has the rocks which are the supports of the earth; as man has within him a pool of blood wherein the lungs as he breathes expand and contract, so the body of the earth has its ocean, which also rises and falls every six hours with the breathing of the world [the tides]; as from the said pool of blood proceed the veins which spread their branches through the human body, in just the same manner the ocean fills the body of the earth with an infinite number of veins of water.

We need go no further than Leonardo's most celebrated creation-the Mona Lisa-to recognize the analogy of macrocosm and microcosm as the centerpiece of his thought. La Gioconda stands on a balcony overlooking a complex geological background of flowing waters that complete a full hydrological cycle just as blood moves through the human body. Martin Kemp notes:

The processes of living nature are not only mirrored by anatomical implication within the lady's body, but are more obviously echoed in the surface details of her figure and garments, which are animated by myriad motions of ripple and flow. The delicate cascades of her hair beautifully correspond to the movement of water, as Leonardo himself was delighted to observe: "Note the motion of the surface of the water which conforms to that of the hair." . . . The little rivulets of drapery falling from her gathered neckline underscore this analogy, as do the spiral folds of the veil across her left breast.

We now reach the central dilemma that makes the paleontological observations so crucial to the argument of the Leicester Codex. This notebook, as scholars have always recognized, is primarily a treatise on the nature of water in all its properties, manifestations, and uses. Why, then, does Leonardo devote so much apparently subsidiary space to the nature of fossils and the reason for their situation in mountain strata, far above present sea level? The key to this problem lies in his almost heroic struggle to overcome a central difficulty in validating his crucial analogy of the body's microcosm to the earth's macrocosm. Most scholars have missed this theme, and therefore do not grasp the union of the hydrological and paleontological passages of the Leicester Codex.

Leonardo recognizes only too well-for he has struggled with this problem for years, and through several notebooks-that his crucial analogy suffers from a potentially fatal difference between the human body and the earth. Both are constructed from the four elements of antiquity: earth, water, air, and fire. But the human body sustains itself by circulating these elements, particularly by maintaining some mechanism for permitting water (blood) to rise from the legs to the head. The analogy of microcosm and macrocosm can only work if the earth also possesses a comparable device for sustenance by cycling.

But how can such a notion be defended for the planet, especially in the face of an apparently disabling problem: earth and water are heavy elements; their natural motion must produce a downward flow (leading ideally to a planet of four concentric layers with earth at the center, water above, air atop water, and fire at the periphery). If earth and water must move down, these heavy elements will eventually stabilize as two concentric spheres at the center of the planet-and the macrocosm will therefore possess no device for sustenance by circulation. Leonardo knows that he must therefore find a mechanism to make both earth and water move up (against their natural tendency), as well as down, on our planet. This pressing need, so difficult to validate, sets the central struggle that engages Leonardo throughout the Leicester Codex.

Ironically, I wish to argue, he never did solve his problem for the main subject of the codex-water. That is, he tried again and again, but never found a satisfactory mechanism to guarantee the upward motion, hence the cycling, of water. However-and we now come to the crucial point that has usually been missed-Leonardo did succeed (by his standards) in the quest to find a mechanism for upward movement of the other heavy element: earth. Fossils on mountains provide the observational proof that earth can rise, both generally and often, for marine shells once inhabited the sea but now reside in the high mountains. The paleontological observations form a centerpiece in the Leicester Codex not, as has usually been argued, because fossils once lived in water and the codex treats water in all major aspects (an awfully lame reason for devoting so much space to paleontology), but rather because fossils record Leonardo's great success (in contrast to his failure for the central subject of water)-his key evidence for a general mechanism to drive the upward motion of earth, and therefore, his proof for a self-sustaining planet that may legitimately be compared with the human body.

Leonardo knew only too well that he faced a serious problem with the motion of water through the earth, and he virtually obsesses over the issue in notebook after notebook, repeating the conundrum in almost unchanging words, and proposing various solutions, only to abandon them later as untenable. Water, by itself and following its "natural course" (Leonardo's words), can only flow down. But within the earth, water must also move up along internal channels (comparable with blood vessels of the human body) to emerge as springs in the high mountains (and thence, back on track, to flow as rivers to the sea). An earthly force must therefore make water rise through the land against its natural inclination to flow down. The combined action of these two forces will cause water to circulate-and thereby act like the blood in our bodies to sustain a living system.

So does the water which is moved from the deep sea up to the summits of the mountains, and through the burst veins [mountain springs] it falls down again to the shallows of the sea, and so rises again to the height where it burst through, and then returns in the same descent. Thus proceeding alternately upwards and downwards at times it obeys its own desire [to move down] at times that of the body in which it is pent [to move up]. (From the Arundal Codex in the British Museum.)

Leonardo could not have been more explicit in admitting that water can move upward only by running against its natural course and that, if any mechanism can be found at all for this anomalous motion, the analogy between microcosm and macrocosm offers the only reasonable hope: Clearly it would seem that the whole surface of the oceans, when not affected by the tempest, is equally distant from the center of the earth, and that the tops of the mountains are as much farther removed from this center as they rise above the surface of the sea.' Unless therefore the body of the earth resembled that of man it would not be possible that the water of the sea, being so much lower than the mountains, should have power in its nature to rise to the summit of the mountains. We must therefore believe that the same cause that keeps the blood at the top of a man's head keeps water at the summit of the mountains. (From the A Manuscript in the Institut de France.)

But to state a need is not equivalent to finding a mechanism. Throughout the Leicester Codex, Leonardo struggles to discover a physically workable way to raise water within the earth. He tries and rejects several explanations, as Martin Kemp documents in an article entitled "The Body of the Earth." Perhaps, Leonardo first argues, the heat of the sun draws water up through the veins (internal streams) that run through mountains. (Leonardo, in his strongest image of a living earth, had written in the Leicester Codex: "The body of the earth, like the bodies of animals, is interwoven with a network of veins, which are all joined together, and formed for the nutrition and vivification of the earth and of its creatures.") But he then realizes that this explanation cannot work for two reasonsfirst, because, on the highest mountaintops closest to the heating sun, water remains cold, and even icy; and, second, because this mechanism should operate best in summer during maximal solar heat, but mountain rivers often run with lowest waters at this time.

In a second try, Leonardo turns to the earth's internal heat and a process of distillation: perhaps the interior fires boil water in internal caverns, and this water rises as vapor through mountain interiors, where it reverts to liquid form and bursts through as a high spring. But this proposal won't work either because such extensive distillation would require that the roofs of internal caverns be wet with the rising steam-but they are often bone-dry. Leonardo then made a feebler third attempt: perhaps, by analogy to a sponge, mountains somehow suck up water to a point of saturation and subsequent oozing from the top. But Leonardo realizes that he cannot cash out this analogy in mechanical terms: