Darwin's Doubt

by Stephen C. Meyer

  The Cambrian Explosion and the Tree of Life

  The abrupt appearance of the Cambrian fauna also posed a separate but related difficulty for Darwin’s picture of a continuously branching tree of life. To produce truly novel animal forms, the Darwinian mechanism would—by its own internal logic—require not only millions of years, but untold generations of ancestors. Thus, even the discovery of a handful of plausible intermediates allegedly linking a Precambrian ancestor to a Cambrian descendant wouldn’t come close to fully documenting Darwin’s picture of the history of life. If Darwin is right, Agassiz argued, then we should find not just one or a few missing links, but innumerable links shading almost imperceptibly from alleged ancestors to presumed descendants. Geologists, however, had found no such myriad of transitional forms leading to the Cambrian fauna. Instead, the stratigraphic column seemed to document the abrupt appearance of the earliest animals.

  Agassiz thought the evidence of abrupt appearance, and the absence of ancestral forms in the Precambrian, refuted Darwin’s theory.11 Of these earlier forms, Agassiz asked, “Where are their fossilized remains?” He insisted that Darwin’s picture of the history of life “contradict[ed] what the animal forms buried in the rocky strata of our earth tell us of their own introduction and succession upon the surface of the globe. Let us therefore hear them;—for, after all, their testimony is that of the eye-witness and the actor in the scene.”12

  Murchison, Sedgwick, and the Cambrian Fossils of Wales

  Darwin, for his part, responded with more than civility. Far from dismissing Agassiz, he conceded that his objection carried considerable force. Nor was Agassiz alone in pressing these concerns. Other leading naturalists thought the fossil evidence presented a significant obstacle to Darwin’s theory. At the time, perhaps the best place to investigate the lowest known strata of fossils was Wales, and one of its leading experts was Roderick Impey Murchison, who named the earliest geologic period the Silurian after an ancient Welsh tribe. Five years before On the Origin of Species, he called attention to the sudden appearance of complex designs like the compound eyes of the first trilobites, creatures already thriving at the apparent dawn of animal life. For him, this discovery ruled out the idea that these creatures had evolved gradually from some primitive and relatively simple form: “The earliest signs of living things, announcing as they do a high complexity of organization, entirely exclude the hypothesis of a transmutation from lower to higher grades of being.”13

  The other pioneering explorer of Wales’s rich fossil record, Adam Sedgwick, also thought that Darwin had leaped beyond the evidence, as he told him in a letter in the fall of 1859: “You have deserted—after a start in that tram-road of all solid physical truth—the true method of induction.”14 Sedgwick might have had in mind the same evidence the two men had studied together some twenty-eight years before when the Cambridge professor had brought Darwin along as his field assistant to explore, in the Upper Swansea Valley in northwestern Wales, the very strata that seemed to testify so powerfully to the sudden appearance of animal life. It was these strata that Sedgwick named after a Latinized English term for the country of Wales—“Cambria,” a designation that eventually replaced “Silurian” as the name for the earliest strata of animal fossils.

  FIGURE 1.5

  Three organisms that first appear in the Ordivician period: eurypterans (sea scorpions), starfish, and tetracoral.

  Sedgwick emphasized that these Cambrian animal fossils appeared to pop out of nowhere into the geological column. But he also stressed what he viewed as a broader reason to doubt Darwin’s evolutionary model: the sudden appearance of the Cambrian animals was merely the most outstanding instance of a pattern of discontinuity that extends throughout the geologic column. Where in the Ordovician strata, for instance, are many of the families of the trilobites and brachiopods present in the Cambrian just below it?15 These creatures along with numerous other types suddenly disappear. But just as suddenly one finds newcomers in the Ordovician strata like the eurypterans (sea scorpions), starfish, and tetracorals (see Fig. 1.5).16 In a later Paleozoic period called the Devonian, the first amphibians (e.g., Ichthyostega) arise. Much later, many staples of the Paleozoic era (which encompasses the Cambrian, Ordovician, and four subsequent periods) suddenly go extinct in a period called the Permian.17 Then, in the Triassic period that follows, completely novel animals such as turtles and dinosaurs emerge.18 Such discontinuity, Sedgwick argued, is not the exception, but the rule.

  Dating by Discontinuity

  Already by Sedgwick’s time, the various strata of fossils had proved so distinct one from another that geologists had come to use the sharp discontinuities between them as a key means for dating rocks. Originally, the best tool for determining the relative age of various strata was based on the notion of superposition. Put simply, unless there is a reason to believe otherwise, a geologist provisionally assumes that lower rocks were put down before the rocks above them. Now, contrary to a widespread caricature, no respected geologist, then or now, adopts this method uncritically. The most basic training in geology teaches that rock formations can be twisted, upended, even mixed pell-mell by a variety of phenomena. This is why geologists have always looked for other means to estimate the relative age of different strata.

  In 1815, Englishman William Smith had hit upon just such an alternative means.19 While studying the distinct fossil strata exposed during canal construction, Smith noted that so dissimilar are the fossil types among different major periods and so sharp and sudden the break between them, that geologists could use this as one method for determining the relative age of strata. Even when layers of geological strata are twisted and turned, the clear discontinuities between the various strata often allow geologists to discern the order in which they were deposited, particularly when there is a broad enough sampling of rich geological sites from the period under investigation to study and cross-reference. Although not without its pitfalls, this approach has become a standard dating technique, used in conjunction with superposition and other more recent radiometric dating methods.20

  FIGURE 1.6

  The geological timescale.

  Indeed, it’s difficult to overemphasize how central the approach is to modern historical geology. As Harvard paleontologist Stephen Jay Gould explains, it is the phenomenon of fossil succession that dictates the names of the major periods in the geological column (see Fig. 1.6). “We might take the history of modern multicellular life, about 600 million years, and divide this time into even and arbitrary units easily remembered as 1–12 or A-L, at 50 million years per unit,” Gould writes. “But the earth scorns our simplifications, and becomes much more interesting in its derision. The history of life is not a continuum of development, but a record punctuated by brief, sometimes geologically instantaneous, episodes of mass extinction and subsequent diversification.”21 The question that Darwin’s early critics posed was this: How could he reconcile his theory of gradual evolution with a fossil record so discontinuous that it had given rise to the names of the major distinct periods of geological time, particularly when the first animal forms seemed to spring into existence during the Cambrian as if from nowhere?

  A Solution Unseen

  Of course, Darwin was well aware of these problems. As he noted in the Origin, “The abrupt manner in which whole groups of species suddenly appear in certain formations has been urged by several paleontologists—for instance, by Agassiz, Pictet, and Sedgwick—as a fatal objection to the belief in the transmutation of species. If numerous species, belonging to the same genera or families, have really started into life all at once, the fact would be fatal to the theory of descent with slow modification through natural selection.”22 Darwin, however, proposed a possible solution. He suggested that the fossil record may be significantly incomplete: either the ancestral forms of the Cambrian animals were not fossilized or they hadn’t been found yet. “I look at the natural geological record, as a history of the world imperfectly kept, and written in a changin
g dialect,” Darwin wrote. “Of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines… . On this view, the difficulties above discussed are greatly diminished, or even disappear.”23

  Darwin himself was less than satisfied with this explanation.24 Agassiz, for his part, would have none of it. “Both with Darwin and his followers, a great part of the argument is purely negative,” he wrote. They “thus throw off the responsibility of proof… . However broken the geological record may be, there is a complete sequence in many parts of it, from which the character of the succession may be ascertained.” On what basis did he make this claim? “Since the most exquisitely delicate structures, as well as embryonic phases of growth of the most perishable nature, have been preserved from very early deposits, we have no right to infer the disappearance of types because their absence disproves some favorite [i.e., Darwinian] theory.”25

  Though Darwin himself was less than enthusiastic about his response to Agassiz’s objection, it seemed adequate to satisfy the needs of the moment. The overwhelming preponderance of evidence that Darwin had marshaled seemed to support his theory. In any case, many leading naturalists—Joseph Hooker, Thomas Huxley, Ernst Haeckel, and Asa Gray—all younger than Agassiz, quickly aligned themselves with his evolutionary line of thinking. True, some scientists, notably the Scottish engineering professor Fleeming Jenkin and (later) the English geneticist William Bateson, expressed persistent doubts about the efficacy of natural selection. But despite the views of some weighty scientific critics, Darwin’s revolutionary theory won increasingly wide support and soon defined the terms of the debate about the history of life. Those who rejected it wholesale, as Agassiz did, consigned themselves to increasing irrelevance.

  Agassiz Under the Microscope

  So did Agassiz identify a genuine problem for Darwin’s theory, a mystery, at least, waiting to be solved? If so, whatever became of this problem? And if not, how could such a brilliant and knowledgeable scientist, someone so steeped in the evidence, fall so far outside the mainstream of scientific opinion?

  Historians of science in the post-Darwinian era have typically attempted to answer this later question by portraying Agassiz as a brilliant and respected scientist who nevertheless was too ossified to catch the new wave, a figure past his prime and mired in philosophical prejudice.26 Biographer Edward Lurie describes the Harvard naturalist as a “giant of the nineteenth century … a person deeply involved in his surroundings, a man who understood the possibilities of life with an uncommon awareness.”27 Similarly, historian Mabel Robinson says that she long awaited a biography of Agassiz that “would re-create this man of genius and his headlong splendid race through life.” He was, she said, “a man to remember because genius is rare,” “an immortal Pied Piper.”28 These scholars are merely echoing what Agassiz’s contemporaries, even Darwin himself, said. “What a set of men you have at Harvard!” Darwin told the American poet Henry Wadsworth Longfellow. “Both our universities put together cannot furnish the like. Why, there is Agassiz—he counts for three.”29

  Even so, many historians argue that Agassiz was too infected by German idealism to properly assess the factual basis of Darwin’s case. According to idealist philosophers of biology, living forms exemplified transcendent ideas and in their organization provided evidence of purposive design in nature. Comments historian A. Hunter Dupree, “Agassiz’s idealism was of course the basis of his concepts of species and their distribution,” of his insistence that a divine or intellectual cause must stand behind the origin of each type.30 The ship of science was transitioning from idealism to modern empiricism. Agassiz had fallen overboard, since he had imbibed too deeply an outmoded idealism from his teacher, the French anatomist Georges Cuvier, and from philosophers like Friedrich Schelling, who “ran wild in trying to put all nature into a unified and absolute system of ideas.”31 Agassiz wasn’t merely wrong, Dupree explains, but an annoying obscurantist, actively fighting “against the extension of empiricism into natural history.”32

  Edward Lurie offers a similar if somewhat more nuanced assessment: although “quite capable of making the most admirable scientific discoveries reflecting complete devotion to scientific method,” Agassiz “would then interpret the data through the medium of what seemed to be the most absurd metaphysics.”33 The very man who made “the most careful, exact, and precise descriptions” of the natural world would, in his generalizations from those observations, “indulge in flights of idealistic fancy.”34 In short, Lurie thought that “Agassiz’s cosmic philosophy shaped his entire reaction to the evolution idea.”35

  As science advanced in the late nineteenth century, it increasingly excluded appeals to divine action or divine ideas as a way of explaining phenomena in the natural world. This practice came to be codified in a principle known as methodological naturalism. According to this principle, scientists should accept as a working assumption that all features of the natural world can be explained by material causes without recourse to purposive intelligence, mind, or conscious agency.

  Proponents of methodological naturalism argue that science has been so successful precisely because it has assiduously avoided invoking creative intelligence and, instead, searched out strictly material causes for previously mysterious features of the natural world. In the 1840s, the French philosopher August Comte argued that science progresses through three distinct phases. In its theological phase, it invokes the mysterious action of the gods to explain natural phenomena, whether thunderbolts or the spread of disease. In a second, more advanced metaphysical stage, scientific explanations refer to abstract concepts like Plato’s forms or Aristotle’s final causes. Comte taught that science only reaches maturity when it casts aside such abstractions and explains natural phenomena by reference to natural laws or strictly material causes or processes. Only in this third and final stage, he argued, can science achieve “positive” knowledge.

  During the late nineteenth century, scientists increasingly embraced this “positivistic” vision.36 Agassiz, by insisting that the Cambrian fossils pointed to “acts of mind”37 and an “intervention of an intellectual power,” stood firmly against this new vision. For many, his reference to the work of a transcendent mind merely demonstrated that he was unable to abandon an outmoded idealistic approach. The train of scientific progress had left Agassiz behind.

  An Old Fossil Recovered

  Though clearly Agassiz did reject the principle of methodological naturalism, as it is now named, there are problems with portraying him as a fossil of another age. First, Agassiz was unsurpassed in his commitment to the empirical method. It is Agassiz about whom the story is told of the professor instructing one of his students to observe a fish for three arduous days, a story iconic enough that it is reprinted in freshman composition textbooks. In the story, the student, Samuel Scudder, pulls out his hair trying to see anything new about the slimy creature, wondering why Professor Agassiz is torturing him with this “hideous fish.” But in the end Scudder breaks through to new levels of observational depth and precision. Mabel Robinson notes that if such teaching methods seem less revolutionary to contemporary readers than they did to Scudder, that’s because Agassiz trained an army of able young naturalists who took his method to other universities, and they in turn passed them on to their students, themselves future professors.38

  William James, the founder of American pragmatism, extolled Agassiz’s commitment to empirical rigor in a letter he wrote to his father while on an expedition with Agassiz in 1865 to South America. In the letter the young man commented that he felt a “greater feeling of weight and solidity about the presence of this great background of special facts than about the mind of any other man I know,”39 a storehouse of precise data made possible by “a rapidity of observation, and a capacity to recognize them again and remember everything about them.”40 James would eventually enter the field of
psychology, but he took with him the empirical approach to problem solving that Agassiz had modeled so impressively.41

  As Lurie concedes, Agassiz’s stature among American scientists grew out of his unrivaled knowledge of geology, paleontology, ichthyology, comparative anatomy, and taxonomy. So passionate was Agassiz for the particulars of the natural world that he began organizing a system of information-sharing among naturalists, sailors, and missionaries around the world. He collected more than 435 barrels of specimens, among them an extremely rare group of fossil plants.42 In a single year, Agassiz amassed more than 91,000 specimens and identified close to 11,000 new species,43 making Harvard’s natural history museum preeminent among such museums in the world.

  He also appears to have gone to great lengths, literally and figuratively, to assess On the Origin of Species empirically, going so far as to make a research voyage retracing Darwin’s trip to the Galápagos Islands. As he explained to German zoologist Carl Gegenbauer, he “wanted to study the Darwin theory free from all external influences and former prejudices.”44

  The idea that religious or philosophical prejudice compromised Agassiz’s scientific judgment raises other questions. As historian Neal Gillespie explains, Agassiz was “second to no man in his opposition to sectarian religious interference with science.”45 Moreover, Agassiz showed himself perfectly willing to accept natural mechanisms where before supernatural intervention had been the preferred explanation. Since he regarded material forces, and the laws of nature that described them, as the products of an underlying design plan, he saw any creative work they did as deriving ultimately from a creator. For instance, he assumed this was the case with the development of embryos: he attributed their natural evolution from zygote to adult as a natural phenomenon and considered this no threat to his belief in a creator.46 He also readily accepted the notion of a naturally evolving solar system.47 He thought a skillful cosmic architect could work through secondary natural causes every bit as effectively as through direct acts of agency. The marginalia in his copy of On the Origin of Species suggest that he had this same attitude concerning biological evolution. “What is the great difference,” he wrote, “between supposing that God makes variable species or that he makes laws by which species vary?”