Hen’s Teeth and Horse’s Toes

by Stephen Jay Gould

  Steno uses two criteria for his subdivisions. (They are blessedly obvious once you state the problem, but Steno’s revolution is the statement itself.) First, in what might be called the principle of molding, Steno argues that when one solid lies within another, we can tell which hardened first by noting the impress of one object upon the other. Thus, fossil shells were solid before the strata that entomb them because shells press their form into surrounding sediments just as we make footprints in wet sand. But surrounding rocks were solid before the calcite veins that run through them because the calcite fills preexisting channelways just as Jello matches the flutes of a mold. The principle of molding allows us to establish the temporal order of formation for two objects in contact. In a world still regarded by many of Steno’s contemporaries as formed all at once by divine fiat, this criterion of history struck a jarring chord and eventually forced a transposition in thought.

  Early in the Prodromus, Steno stated the problem that he wished to solve with his second criterion: “Given a substance possessed of a certain figure, and produced according to the laws of nature, to find in the substance itself evidences disclosing the place and manner of its production.” His solution, the basic principle for any historical reconstruction, holds:

  If a solid substance is in every way like another solid substance, not only as regards the conditions of its surface, but also as regards the inner arrangement of parts and particles, it will also be like it as regards the manner and place of production.

  Past processes cannot be observed in principle; only their results remain. If we wish to infer the processes that formed any geological object, we must find clues in the object itself. The surest clue is detailed similarity—part by internal part—with modern objects formed by processes we can observe directly. Similarity can be misleading—and great mistakes have been made in applying Steno’s principle—but our confidence in common origin mounts as we catalog more and more detailed similarities involving internal structure and chemical composition as well as external form.

  Thus, Steno argues, sedimentary rocks must be the deposits of rivers, lakes, and oceans because they “agree with those strata which turbid water deposits.” Fossil shells once belonged to animals, and crystals precipitated from fluids as we make salt or rock candy today.

  With these two principles—molding and sufficient similarity—Steno established both prerequisites for geological, or any historical, reconstruction: he could determine how and where objects formed, and he could order events in time. Steno’s genius, to say it one more time, lay in establishing this new conceptual framework for observation, not in the acuity of the subsequent observations themselves. Steno’s break with older traditions stands out most clearly in his complete failure, save in one sheepish passage, to consider the primary subject that obsessed his colleagues: the identification of goals and purposes for all things, including what we now regard as purely physical processes of uplift, erosion, and crystallization. In one fleeting passage, Steno cites dissimilarity of function as a reason subservient to his usual argument about internal resemblance for stating that rocks and bones form differently. But he quickly adds the disclaimer, “if one may be permitted to affirm aught about a subject otherwise so little known as are the functions of things.” As Foucault also argues, the subjects you leave out of your taxonomies are as significant as the ones you put in.

  The four-part organization of the Prodromus has generally been viewed as disjointed or even incoherent—thrown together by a man itching to leave Florence but forced to justify the grand duke’s patronage. I view it instead as a comprehensive and tightly reasoned brief for a science of geology based upon the two principles of molding and sufficient similarity.

  Part one is a teaser, a specific example to demonstrate the power of the general method. The glossipetrae, Steno argues, must be sharks’ teeth because they are identical in form and internal arrangement with the objects he had plucked from the mouth of his quarry from Leghorn. They solidified before the rocks that enclose them because they impress their form upon the surrounding sediment. Therefore—and now the argument begins to move toward revolutionary generality—sedimentary rocks were not created with the earth, but have formed as the deposits of turbid waters in rivers, lakes, or oceans. Moreover, similar marine fossils are often found high in mountains and far from the sea; these fossils also solidified before the strata enclosing them. Thus, the earth has an extensive history: seas and lands have changed places, and mountains have emerged from the waters.

  In the second part, Steno argues that glossipetrae are but one example of the general problem of solids within solids, and that the principles of molding and sufficient similarity can establish proper taxonomic subdivisions based on common modes of origin. The third part treats the major classes of solids within solids and establishes two basic categories for objects within rocks: fossils that harden before the enclosing strata, and crystals and veins that form within solid rocks.

  The fourth part, a reconstruction of the geological history of Tuscany, has been problematic or even embarrassing to geologists who wish to view Steno as their founding saint. (The Catholic Church, by the way, is also considering Steno for sainthood, and he may eventually attain an unprecedented double distinction.) Steno constructs his history to match biblical chronology, with two cycles of deposition—from the original void and from Noah’s universal ocean. The essence of this part, however, is not his continued loyalty to Moses—Steno was, after all, not a man of our century—but rather his demonstration that the principles of molding and sufficient similarity can be used not only to classify objects (part three), but also to reconstruct the history of the earth from these objects (part four). The last part of the Prodromus demonstrates by specific example, drawn from the local terrain, that the proper classification of solids within solids can establish a science of geology.

  In 1678, Athanasius Kircher published a figure showing all letters of the alphabet, including the contraction etched in veins of calcite. Today, we chuckle and dismiss the well-formed letters as accidents. But to Kircher they were no less significant than the shells of clams also found in rocks. One might argue that clams are more complex than letters, but a Venetian work of 1708 depicted an agate that seemed to show, in its bands of color, Christ on the cross with all proper accouterments, including a sun on the favored right side and a moon on the despised left. The caption proclaimed in German doggerel: Solche wunderbarliche Gestalt, hat die Natur in ein Agat gemahlt—“Nature herself has painted this wonderful figure in an agate.” Why was a clam in a rock different from a letter or a crucifixion? Since alphabets and religious scenes cannot be preexisting objects buried in strata, they must be made by a plastic power in the rocks themselves. As long as “odd things in rocks” formed a single category, clams and sharks’ teeth would also be manifestations of the plastic force and no science of paleontology or of historical geology would be possible. But Steno’s classification recognized the basic distinction between fossils that hardened before the rocks that enclosed them and intruding veins that might by accident resemble some abstract form or design.

  Steno changed the world in the simplest and yet most profound way. He classified its objects differently.

  6 | Hutton’s Purpose

  IN HIS TRIBUTE to Lucretius, Virgil wrote: “Happy is he who could learn the causes of things” (Felix qui potuit rerum cognoscere causas). A noble and uncomplicated sentiment to be sure, but an even more illustrious predecessor had shown that causality is no simple matter. Aristotle, in the Posterior Analytics of the Organon, stated: “We only think that we have knowledge of a thing when we know its cause.” He then proceeded to give a complex analysis of the concept of causality itself.

  Each event, Aristotle argued, has four distinct kinds of causes. Consider the so-called parable of the house, the standard example, probably in continual use for more than two thousand years, for illustrating Aristotle’s schema. What is the cause of my house? What are the sine quibus no
n, the various factors whose absence would lead to no house at all or to a house of markedly different design?

  First, Aristotle argues, we must have the straw, sticks, or bricks—the material cause. It obviously matters, as the three little pigs discovered, what material you choose. Next, someone must do the actual work, thatch the roof or lay the bricks—the effector, or efficient cause. The blueprint that the mason follows doesn’t do anything actively, and it is not building material. But it is a cause of sorts, since different blueprints yield different houses and no plan at all leaves you with a pile of bricks. These preconceived marching orders are formal causes in Aristotle’s lexicon. Finally, if the house served no purpose as an abode for its inhabitants, no one would bother to build it. Purposes are final causes.

  We do not follow Aristotle’s analysis in our linguistic habits today; our entire notion of “cause” is now pretty much restricted to Aristotle’s efficient causes. We do not deny the material and formal aspects, but we no longer call them causes. When I identify the motion of my pool cue as the cause of a ball’s errant trajectory (though only an efficient cause to Aristotle), I do not regard the composition of the ball or the blueprint of the table as irrelevant, but I no longer call them causes.

  The elimination of purpose, or final cause, tells a more important story and represents a major change in style for Western science. Aristotle saw nothing absurd in granting each event both an efficient cause (a mechanism, in our terminology) and a final cause (a purpose). He writes, for example,

  Light shines through a lantern. Being composed of particles smaller than the pores of the lantern, it cannot help passing through them (assuming that this is how the light is propagated); but it also shines for a purpose, so that we may not stumble [Posterior Analytics, 94b, 1. 28].

  We can follow Aristotle for devices constructed by humans for definite purposes. We did put holes in lanterns to let the light through. Final cause also remains a legitimate concept for the adaptations of organisms, even though these features arise by natural processes and not by any conscious activity of the animals involved. It remains good vernacular English to say that bats and birds have wings “for” flight, and the wolf legitimately invoked final cause in replying to Red Riding Hood’s inquiry about the sharpness of his teeth, “All the better to eat you with, my dear.”

  But we balk at ascribing final causes to the physical workings of inanimate objects, although Aristotle did not. Aristotle was comfortable with the idea that “it thunders both because there must be a hissing and roaring as the fire is extinguished, and also (as the Pythagoreans hold) to threaten the souls in Tartarus and make them fear” [Ibid., 94b, 1. 34]. We chuckle at Aristotle here, and that chuckling represents perhaps the greatest change that science has undergone in modern times. We no longer view the universe as explicitly designed in all its minor and multifarious parts to serve some human purpose. We have replaced this cosmic hubris with a more mechanical view of nature. God may have wound the clock and established the laws of ticking at the outset, but he surely does not spend his precious time fashioning each blade of grass and grain of sand to provide explicit instruction or sustenance for his favored species on earth. The mechanical view, based on the primacy of efficient causation, has properly banished final cause from the domain of natural, physical objects.

  So absolute is this proscription of final cause, and so essential to a modern definition of science, that old passages about the final causes of physical objects are unsurpassed as targets of ridicule when, in our arrogant approach to history, we choose to flay the past, all the better to bask in our current wisdom (a legitimate final cause in human psychology). It would, indeed, be hard to deny that many of these passages are, well, simply funny.

  Louis Agassiz, for example, seriously argued in the 1860s that ice ages could be understood both by the physics of glacial motion (efficient cause) and as a dispensation of divine benevolence designed to churn and enrich the soil:

  One naturally asks, What was the use of this great engine set at work ages ago to grind, furrow, and knead over, as it were, the surface of the earth? We have our answer in the fertile soil which spreads over the temperate regions of the globe. The glacier was God’s great plough.

  In 1836, William Buckland, Oxford’s first academic geologist, claimed that abundant coal, the fuel of England’s glory, was so cleverly distributed in the bowels of the earth that God himself must have placed it there, many million years ago, in loving preparation for its future use. We might rightly suspect that as old rock, coal should now be buried under so many miles of younger strata that it would lie beyond (or, rather, beneath) our reach. But God saw fit to ordain its deposition not in vast horizontal sheets but in discontinuous bowl-shaped basins whose edges still intersect or lie just a bit below the earth’s surface. Moreover, the strata that were buried at inaccessible depths have often been extensively faulted and uplifted to the surface. These faults are a further boon to miners because the streams that often run along their fractured boundaries can guide us to the precious substance underneath, and because destructive fires can no longer ravage an entire field when faults break an extensive stratum into several discontinuous segments separated by rock that will not burn. Buckland wrote:

  However remote may have been the periods, at which these materials of future beneficial dispensations were laid up in store, we may fairly assume that…an ulterior prospective view of the future uses of Man formed part of the design, with which they were, ages ago, disposed in a manner so admirably adapted to the benefit of the Human Race.

  Alexander Winchell, prominent American geologist and first chancellor of Syracuse University, could scarcely contain himself in paying lyrical tribute (in Sketches of Creation, 1870) to the faults that bring coal within our orbit:

  Buried ten thousand feet from view, man would never have learned of its existence, much less would he have known how to raise it to the surface. See the provision of Nature in breaking up the coal-bearing strata and tilting them on edge, as much as to say, “Lo! here is your desire; search not in vain; dig, and be satisfied with warmth; drive forth the hidden energy…and bid the servants furnished to your hands execute all the behests of your convenience.”

  I have, needless to say, no desire to resurrect this tradition of argument about final causes. But I would condemn, for two reasons, any attempt to parade old passages about final cause as a source for ready laughter and self-congratulation in transcending past ineptitude. First, it subverts any effort to understand the past and use it as a guide for interpreting the present. When such fine intellects as Agassiz and Buckland (not to mention Aristotle) seriously advance these arguments, we must view them as markers of a fundamentally different conception of the world, not as signs of personal stupidity or general naïveté. Second, even failed views of the world, when they have both grandeur and depth, can serve as wonderfully fruitful sources of insight. To recycle a favorite quote used in my previous volume, The Panda’s Thumb, “Give me a fruitful error any time, full of seeds, bursting with its own corrections. You can keep your sterile truth for yourself” (Pareto’s comment on Kepler).

  Final cause served as such a fruitful error at a crucial moment in my own profession. The greatest reconstruction of geology—James Hutton’s theory of the earth—rested squarely on an argument about final causes. And few geologists have the slightest inkling of this “antiquated” well-spring of insight because it has been subverted by a comfortable (and comforting) myth that locates the source of Hutton’s success in his pursuit of modern tactics—fieldwork and a mechanical concept of physical causality (see G. L. Davies, The Earth in Decay [American Elsevier, 1969], for a fine account of this myth and its correction).

  Born in Edinburgh in 1726, James Hutton hobnobbed with the likes of Adam Smith and James Watt in the great Scottish intellectual circle that so influenced the life of eighteenth-century Europe. After abandoning an apprenticeship in law, he studied medicine. As a man of means, he felt no need to practice a
nd opted instead for farming (on land inherited from his father and after bolstering his economic security by inventing and marketing a process for producing sal ammoniac from chimney soot). No rustic he, but no gentleman farmer either, he studied the latest methods in husbandry and ran a profitable, modern, model farm. In his early forties, Hutton gave up country life, returned to Edinburgh and spent the remaining three decades of his life as a full-time unemployed, intellectual gentleman.

  In 1788, Hutton published his reconstruction of geology in the first volume of the Transactions of the Royal Society of Edinburgh (expanded in 1795 to a multivolumed work entitled Theory of the Earth, following a blistering attack for supposed atheism and other improprieties by the Irish chemist and mineralogist Richard Kirwan). Hutton, although usually cast as a modern empiricist, really belongs to the great tradition of comprehensive (and at least partly speculative) system building that dominated most of eighteenth-century “geology” (the term had not yet been invented, and no profession, with formally recognized procedures, then existed).

  Hutton’s system, his “world machine,” embodied a cyclical notion of history—dynamic and endlessly recurring, but moving nowhere, as the Preacher of Ecclesiastes proclaimed:

  All the rivers run into the sea; yet the sea is not full. Unto the place from whence the rivers come, thither they return again [1:7]. The thing that hath been, it is that which shall be; and that which is done, is that which shall be done; and there is no new thing under the sun [1:9].

  This view of history contrasted sharply with the more familiar Christian concept of a linear and directional sequence moving ever onward from creation to resurrection. (Hutton, who was decidedly not an atheist, did not deny that God had ordained a beginning and would decree an end. But these miraculous events lie outside the purview of science. In between these singularities, God rested and permitted the world to run by the natural laws that he had established. Only this period could be studied by science, and here Hutton discerned no direction, but only endless cycling.)