Hen’s Teeth and Horse’s Toes

by Stephen Jay Gould

  The quotation is long, but it is, so far as I know, exclusive. Its most remarkable aspect is an extreme weakness, almost speciousness, of argument. Agassiz makes but a single point: many animals of the Galápagos live nowhere else. Yet the islands are so young that a slow process of evolution could not have transformed them from related ancestors in the time available. Thus, they were created where we find them (the obvious bottom line, despite Agassiz’s final disclaimer that we know too little to reach any firm conclusion).

  Two problems: First, although the Galápagos are young (two to five million years for the oldest islands by current reckoning), they are not so pristine as Agassiz indicates. In the letter, Agassiz describes lava flows of the past hundred years or so, and these are virtually devoid of vegetation and so fresh that one can almost see the flow and feel the heat. But Agassiz surely knew that several of the islands (including some on his itinerary) are more densely vegetated and, although not ancient, were surely not formed in a geological yesterday.

  Second, Agassiz leaves out the most important aspect of Darwin’s argument. The point is not that so many species of the Galápagos are unique but rather that their nearest relatives are invariably found on the adjacent South American mainland. If God created the Galápagos species where we find them, why did he imbue them with signs of South American affinity (especially since the temperate climates and lava habitats of the Galápagos are so different from the tropical environments of the ancestral forms). What sense can such a pattern make unless the species of the Galápagos are modified descendants of South American forms that managed to cross the oceanic barrier? Darwin wrote in the Voyage of the Beagle:

  Why, on these small points of land, which within a late geological period must have been covered by the ocean, which are formed by basaltic lava, and therefore differ in geological character from the American continent, and which are placed under a peculiar climate,—why were their aboriginal inhabitats…created on American types of organization.

  And the famous, poetic statement earlier in the chapter: “We seem to be brought somewhat near to the great fact—that mystery of mysteries—the first appearance of new beings on this earth.”

  Agassiz could not have misunderstood, for, like Darwin, he was a professional biogeographer. He had also used arguments of geographical distribution as his primary defense for creationism. Why did he skirt Darwin’s principal argument? Why did he say so little about the Galápagos and argue so poorly?

  I think that we must consider two possibilities as resolutions to the conundrum of Agassiz’s silence (or failure to consider the critical points in his one private statement). Perhaps he knew that his argument to Peirce was hokey and inadequate. Perhaps the Galápagos, and the entire Hassler voyage, had produced the same change of heart that Darwin had experienced in similar circumstances and Agassiz simply couldn’t muster the courage to admit it.

  I cannot accept such a resolution. As I said earlier, we see abundant signs of psychological distress and deep sadness in Agassiz’s last defenses of creationism. No one enjoys being an intellectual pariah, especially when cast in the role of superannuated fuddy-duddy (the part of ignored but prophetic seer at least elicits moral courage). Yet, however weak his arguments (and they deteriorated as the evidence for evolution accumulated), I sense no failure of Agassiz’s resolve. The letter to Peirce seems to represent still another of Agassiz’s flawed, but perfectly sincere, defenses of an increasingly indefensible, yet steadfastly maintained, view of life. (Agassiz’s last article, posthumously published in the Atlantic Monthly in 1874, was a ringing apologia for creationism entitled “Evolution and Permanence of Type.”)

  I think that we must accept the second resolution: Agassiz said so little about the Galápagos because his visit made preciously little impact upon him. The message is familiar but profound nonetheless. Scientific discovery is not a one-way transfer of information from unambiguous nature to minds that are always open. It is a reciprocal interaction between a multifarious and confusing nature and minds sufficiently receptive (as many are not) to extract a weak but sensible pattern from the prevailing noise. There are no signs on the Galápagos that proclaim: Evolution at work. Open your eyes and ye shall see it. Evolution is an inescapable inference, not a raw datum. Darwin, young, restless, and searching, was receptive to the signal. Agassiz, committed and defensive, was not. Had he not already announced in the first letter to Peirce that he knew what he must find? I do not think he was free to reach Darwin’s conclusions, and the Galápagos Islands, therefore, carried no important message for him. Science is a balanced interaction of mind and nature.

  Agassiz lived for little more than a year after the Hassler docked. James Russell Lowell, traveling abroad, learned of his friend’s death from a newspaper and wrote in poetic tribute (quoted from E. Lurie’s fine biography of Agassiz, Louis Agassiz: A Life in Science, University of Chicago Press, 1960):

  …with vague, mechanic eyes,

  I scanned the festering news we half despise

  When suddenly,

  As happens if the brain, from overweight

  Of blood, infect the eye,

  Three tiny words grew lurid as I read,

  And reeled commingling: Agassiz is dead!

  I do not know. Perhaps a bit of his incorporeal self went up to a higher realm, as some religions assert. Perhaps he saw there old Adam Sedgwick, the great British geologist (and reverend), who at age 87 wrote to Agassiz a year before the Hassler sailed:

  It will never be my happiness to see your face again in this world. But let me, as a Christian man, hope that we may meet hereafter in heaven, and see such visions of God’s glory in the moral and material universe, as shall reduce to a mere germ everything which has been elaborated by the skill of man.

  Be that as it may, Agassiz’s ideas had suffered an intellectual death before he ever reached the Galápagos. Life is a series of trades. We have lost the comfort of Agassiz’s belief that a superior intelligence directly regulates every step of life’s history according to a plan that places us above all other creatures. (“If it had been otherwise,” Agassiz wrote to Pedro II in June 1873, “there would be nothing but despair.”) We have found a message in the animals and plants of the Galápagos, and all other places, that enables us to appreciate them, not as disconnected bits of wonder, but as integrated products of a satisfactory and general theory of life’s history. That, to me at least, is a good trade.

  9 | Worm for a Century, and All Seasons

  IN THE PREFACE to his last book, an elderly Charles Darwin wrote: “The subject may appear an insignificant one, but we shall see that it possesses some interest; and the maxim ‘de minimis lex non curat’ [the law is not concerned with trifles] does not apply to science.”

  Trifles may matter in nature, but they are unconventional subjects for last books. Most eminent graybeards sum up their life’s thought and offer a few pompous suggestions for reconstituting the future. Charles Darwin wrote about worms—The Formation of Vegetable Mould, Through the Action of Worms, With Observations on Their Habits (1881).

  This month* marks the one-hundredth anniversary of Darwin’s death—and celebrations are under way throughout the world. Most symposiums and books are taking the usual high road of broad implication—Darwin and modern life, or Darwin and evolutionary thought. For my personal tribute, I shall take an ostensibly minimalist stance and discuss Darwin’s “worm book.” But I do this to argue that Darwin justly reversed the venerable maxim of his legal colleagues.

  Darwin was a crafty man. He liked worms well enough, but his last book, although superficially about nothing else, is (in many ways) a covert summation of the principles of reasoning that he had labored a lifetime to identify and use in the greatest transformation of nature ever wrought by a single man. In analyzing his concern with worms, we may grasp the sources of Darwin’s general success.

  The book has usually been interpreted as a curiosity, a harmless work of little importance by a great naturalist in
his dotage. Some authors have even used it to support a common myth about Darwin that recent scholarship has extinguished. Darwin, his detractors argued, was a man of mediocre ability who became famous by the good fortune of his situation in place and time. His revolution was “in the air” anyway, and Darwin simply had the patience and pertinacity to develop the evident implications. He was, Jacques Barzun once wrote (in perhaps the most inaccurate epitome I have ever read), “a great assembler of facts and a poor joiner of ideas…a man who does not belong with the great thinkers.”

  To argue that Darwin was merely a competent naturalist mired in trivial detail, these detractors pointed out that most of his books are about minutiae or funny little problems—the habits of climbing plants, why flowers of different form are sometimes found on the same plant, how orchids are fertilized by insects, four volumes on the taxonomy of barnacles, and finally, how worms churn the soil. Yet all these books have both a manifest and a deeper or implicit theme—and detractors missed the second (probably because they didn’t read the books and drew conclusions from the titles alone). In each case, the deeper subject is evolution itself or a larger research program for analyzing history in a scientific way.

  Why is it, we may ask at this centenary of his passing, that Darwin is still so central a figure in scientific thought? Why must we continue to read his books and grasp his vision if we are to be competent natural historians? Why do scientists, despite their notorious unconcern with history, continue to ponder and debate his works? Three arguments might be offered for Darwin’s continuing relevance to scientists.

  We might honor him first as the man who “discovered” evolution. Although popular opinion may grant Darwin this status, such an accolade is surely misplaced, for several illustrious predecessors shared his conviction that organisms are linked by ties of physical descent. In nineteenth-century biology, evolution was a common enough heresy.

  As a second attempt, we might locate Darwin’s primary claim upon continued scientific attention in the extraordinarily broad and radical implications of his proffered evolutionary mechanism—natural selection. Indeed, I have pushed this theme relentlessly in my two previous books, focusing upon three arguments: natural selection as a theory of local adaptation, not inexorable progress; the claim that order in nature arises as a coincidental by-product of struggle among individuals; and the materialistic character of Darwin’s theory, particularly his denial of any causal role to spiritual forces, energies, or powers. I do not now abjure this theme, but I have come to realize that it cannot represent the major reason for Darwin’s continued scientific relevance, though it does account for his impact upon the world at large. For it is too grandiose, and working scientists rarely traffic in such abstract generality.

  Everyone appreciates a nifty idea or an abstraction that makes a person sit up, blink hard several times to clear the intellectual cobwebs, and reverse a cherished opinion. But science deals in the workable and soluble, the idea that can be fruitfully embodied in concrete objects suitable for poking, squeezing, manipulating, and extracting. The idea that counts in science must lead to fruitful work, not only to speculation that does not engender empirical test, no matter how much it stretches the mind.

  I therefore wish to emphasize a third argument for Darwin’s continued importance, and to claim that his greatest achievement lay in establishing principles of useful reason for sciences (like evolution) that attempt to reconstruct history. The special problems of historical science (as contrasted, for example, with experimental physics) are many, but one stands out most prominently: Science must identify processes that yield observed results. The results of history lie strewn around us, but we cannot, in principle, directly observe the processes that produced them. How then can we be scientific about the past?

  As a general answer, we must develop criteria for inferring the processes we cannot see from results that have been preserved. This is the quintessential problem of evolutionary theory: How do we use the anatomy, physiology, behavior, variation, and geographic distribution of modern organisms, and the fossil remains in our geological record, to infer the pathways of history?

  Thus, we come to the covert theme of Darwin’s worm book, for it is both a treatise on the habits of earthworms and an exploration of how we can approach history in a scientific way.

  Darwin’s mentor, the great geologist Charles Lyell, had been obsessed with the same problem. He argued, though not with full justice, that his predecessors had failed to construct a science of geology because they had not developed procedures for inferring an unobservable past from a surrounding present and had therefore indulged in unprovable reverie and speculation. “We see,” he wrote in his incomparable prose, “the ancient spirit of speculation revived and a desire manifestly shown to cut, rather than patiently to untie, the Gordian Knot.” His solution, an aspect of the complex world view later called uniformitarianism, was to observe the work of present processes and to extrapolate their rates and effects into the past. Here Lyell faced a problem. Many results of the past—the Grand Canyon for example—are extensive and spectacular, but most of what goes on about us every day doesn’t amount to much—a bit of erosion here or deposition there. Even a Stromboli or a Vesuvius will cause only local devastation. If modern forces do too little, then we must invoke more cataclysmic processes, now expired or dormant, to explain the past. And we are in catch-22: if past processes were effective and different from present processes, we might explain the past in principle, but we could not be scientific about it because we have no modern analogue in what we can observe. If we rely only upon present processes, we lack sufficient oomph to render the past.

  Lyell sought salvation in the great theme of geology: time. He argued that the vast age of our earth provides ample time to render all observed results, however spectacular, by the simple summing of small changes over immense periods. Our failure lay, not with the earth, but with our habits of mind: we had been previously unwilling to recognize how much work the most insignificant processes can accomplish with enough time.

  Darwin approached evolution in the same way. The present becomes relevant, and the past therefore becomes scientific, only if we can sum the small effects of present processes to produce observed results. Creationists did not use this principle and therefore failed to understand the relevance of small-scale variation that pervades the biological world (from breeds of dogs to geographical variation in butterflies). Minor variations are the stuff of evolution (not merely a set of accidental excursions around a created ideal type), but we recognize this only when we are prepared to sum small effects through long periods of time.

  Darwin recognized that this principle, as a basic mode of reasoning in historical science, must extend beyond evolution. Thus, late in his life, he decided to abstract and exemplify his historical method by applying it to a problem apparently quite different from evolution—a project broad enough to cap an illustrious career. He chose earthworms and the soil. Darwin’s refutation of the legal maxim “de minimis lex non curat” was a conscious double-entendre. Worms are both humble and interesting, and a worm’s work, when summed over all worms and long periods of time, can shape our landscape and form our soils.

  Thus, Darwin wrote at the close of his preface, refuting the opinions of a certain Mr. Fish who denied that worms could account for much “considering their weakness and their size”:

  Here we have an instance of that inability to sum up the effects of a continually recurrent cause, which has often retarded the progress of science, as formerly in the case of geology, and more recently in that of the principle of evolution.

  Darwin had chosen well to illustrate his generality. What better than worms: the most ordinary, commonplace, and humble objects of our daily observation and dismissal. If they, working constantly beneath our notice, can form much of our soil and shape our landscape, then what event of magnitude cannot arise from the summation of small effects. Darwin had not abandoned evolution for earthworms; rather, he was u
sing worms to illustrate the general method that had validated evolution as well. Nature’s mills, like God’s, grind both slowly and exceedingly small.

  Darwin made two major claims for worms. First, in shaping the land, their effects are directional. They triturate particles of rock into ever smaller fragments (in passing them through their gut while churning the soil), and they denude the land by loosening and disaggregating the soil as they churn it; gravity and erosive agents then move the soil more easily from high to low ground, thus leveling the landscape. The low, rolling character of topography in areas inhabited by worms is, in large part, a testimony to their slow but persistent work.