I Have Landed

by Stephen Jay Gould

  Stories of differentiation work primarily from the inside out. That is, the sequence begins with all eventual results already preformed, albeit unexpressed, within an initial homogeneity. How, then, can this potential become actualized? In explicit contrast, stories of addition operate primarily from the outside in. That is, the sequence begins with truly unformed stuff, promiscuous potential that could be drawn into any number of pathways by outside forces. How, then, can such a gloppy mass be carved by external agents into such an exquisitely complex final product (an even worse problem for embryology than for evolution, because the carving must follow the same basic path each time for normal embryos within a species, whereas each evolutionary result arises only once).

  In the major weight of difference between the two models, stories of differentiation fit better for determined systems in a predictable world, whereas stories about addition hold the conceptual edge in a contingent world where each historical sequence may follow innumerable (and unpredictable) options, with the actual result conditioned by the particular set of external prods that a rolling ball of promiscuous potential happens to encounter in its trajectory through time. For this primary reason, our modern embryological models tend to be primarily differentiative, and our evolutionary models primarily additive.

  After all, embryology does generally follow an internally prescribed route specified not by the preformed parts of preformationists, but by the programmed instructions of modern genetic understanding. (We should not accuse the eighteenth-century preformationists of stupidity for placing the right idea into the wrong substance. After all, their intellectual world did not include a concept of programmed information, except, perhaps, as embodied in the old trifle of music boxes or the newfangled invention of the Jacquard weaving loom—whereas no sentient person in our age of genes and computers could fail to assimilate such informational models as an intellectual centerpiece.)

  By contrast, the evolution of any lineage wanders along contingent and unpredictable paths of a uniquely complex history. The few lineages, including our own, that do become more complex through time may add their increments of sophistication in a sequence that makes sense after the fact. But even an omniscient observer could never designate, for certain, the next step in an unpredictable future. Therefore, as a description of evolution, additive models that introduce sequential steps from the outside work better than differentiative models that must hypothesize an entire future as already implicit and enfolded within any current form.

  Under this analysis, we should not be surprised that Genesis 1, despite our usual and unconsidered readings, tells a tale of differentiation rather than addition. After all, if God proceeded with the usual care and thought conventionally attributed to his might, he probably had a pretty accurate idea about the finished product even before he began the work. Biological evolution, on the other hand, at least as viewed under the limits of our eminently fallible mental machinery, seems to wander along a wondrously erratic set of specific pathways within its broad predictabilities.

  Our preferred intellectual models do make a difference, and we must therefore be sensitive to the disparate implications of additive and differentiative models as we struggle to understand the history of life. Still, I think that any passionate and sentient person can feel the same emotional thrill that emanates from either intellectual interpretation. We live in one helluva fascinating universe, whatever its modalities of construction. Thus, if I may beg one last indulgence from my readers—this time for ending with the same image that I invoked in essay 19 for a different treatment of Genesis and evolution—I happily embrace the common sentiment behind two maximally different views of organic order: the differentiative model of Genesis 1, with its ending of sublime satisfaction: “And God saw every thing that he had made, and, behold, it was very good.” And the additive model of natural selection, so lovingly described by Charles Darwin in the last paragraph of The Origin of Species: “There is grandeur in this view of life.”

  PARSING AND PROCEEDING

  21

  Linnaeus’s Luck?

  CAROLUS LINNAEUS (1707–1778), THE FOUNDER OF MODern taxonomy and the focus of this essay, frequently cited an ancient motto to epitomize his view of life: natura non facit saltum (nature does not make leaps). Such unbroken continuity may reign in the material world, but our human passion for order and clear distinction leads us to designate certain moments or events as “official” beginnings for something discrete and new. Thus the signatures on a document define the birth of a nation on July 4, 1776, and the easily remembered eleventh hour of the eleventh day of the eleventh month (November 11, 1918) marks the armistice to a horrible war supposedly fought to end all contemplation of future wars. In a small irony of history, our apostle of natural continuity also became the author and guardian of a symbolic leap to novelty—for the modern taxonomy of animals officially began with the publication of the definitive tenth edition of Linnaeus’s Systema Naturae in 1758.

  The current classification of animals may boast such a formally recognized inauguration, but an agreement about beginnings does not guarantee a consensus about importance. In fact, the worth assigned to taxonomy by great scientists has spanned the full range of conceivable evaluations. When Lord Rutherford, the great British physicist (born in New Zealand), discovered that the dates of radioactive decay could establish the true age of the earth (billions rather than millions of years), he scorned the opposition of paleontologists by branding their taxonomic labors in classifying fossils as the lowest form of purely descriptive activity, a style of research barely meriting the name “science.” Taxonomy, he fumed, could claim no more intellectual depth than “stamp collecting”—an old canard that makes me bristle from two sides of my being: as a present paleontologist and a former philatelist!

  Rutherford’s anathema dates to the first decade of the twentieth century. Interestingly, when Luis Alvarez, a physicist of similar distinction, became equally enraged by some paleontologists during the last decade of the twentieth century, he invoked the same image in denigration: “they’re not very good scientists; they’re just stamp collectors.” I continue to reject both the metaphor and the damning of all for the stodginess of a majority—for Luis had exploded in righteous and legitimate frustration at the strong biases that initially led most paleontologists to reject, without fair consideration, his apparently correct conclusion that the impact of a large extraterrestrial body triggered the mass extinction of dinosaurs and about 50 percent of marine animal species 65 million years ago.

  Linnaean taxonomy in one of its geometric portrayals as boxes within boxes for continually finer specifications.

  The phony assumption underlying this debasement of taxonomy to philately holds that the order among organisms stands forth as a simple fact plainly accessible to any half-decent observer. The task of taxonomy may then be equated with the dullest form of cataloging—the allocation of an admittedly large array of objects to their preassigned places: pasting stamps into the designated spaces of nature’s album, putting hats on the right hooks of the world’s objective hatrack, or shoving bundles into the proper pigeonholes in evolution’s storehouse, to cite a standard set of dismissive metaphors.

  In maximal contrast, the great Swiss zoologist Louis Agassiz exalted taxonomy as the highest possible calling of all, when he opened Harvard’s Museum of Comparative Zoology in his adopted land in 1859. Each species, Agassiz argued, represents the material incarnation on earth of a single and discrete idea in the mind of God. The natural order among species—their taxonomy—therefore reflects the structure of divine thought. If we can accurately identify the system of interrelationships among species, Agassiz concluded, we will stand as close as rationality can bring us to the nature of God.

  Notwithstanding their maximally disparate judgments of taxonomy, Rutherford and Agassiz rank as strange bedfellows in their shared premise that a single objective order exists “out there” in the “real world,” and that a proper classification w
ill allocate each organism to its designated spot in the one true system. (For Rutherford, this order represents a basically boring and easily ascertainable aspect of macroscopic nature—too far removed from the atomic world of fundamental laws and causes to generate much scientific interest or insight. For Agassiz, in greatest conceivable contrast, this order represents our best shot for grasping the otherwise arcane and inaccessible intellect of God himself.)

  In framing a modern “Goldilocks” defense for the importance of taxonomy—far warmer than Rutherford’s icy indifference, but not quite so hot as Agassiz’s impassioned embrace—we must begin by refuting their shared assumption that one true order exists “out there,” and that correct classifications may be equated with accurate maps. We can best defend the scientific vitality of taxonomy by asserting the opposite premise, that all systems of classification must express theories about the causes of order, and must therefore feature a complex mixture of concepts and percepts—that is, preferences in human thinking combined with observations of nature’s often cryptic realities. Good taxonomies may be analogized with useful maps, but they reveal (as do all good maps) both our preferred mental schemes and the pieces of external reality that we have chosen to order and depict in our cartographic effort.

  This acknowledgment that taxonomies can only express nature’s objective realities in terms of theories devised by the human mind should not encourage any trendy postmodern pessimism about the relativity of knowledge. All taxonomies do not become equally valid because each must filter nature’s facts through sieves of human thought and perception. Some popular attributions of former centuries may be dismissed as just plain wrong. (Corals, for example, are animals, not plants.) Other common schemes may be rejected as more confusing than helpful in nearly all situations. (We learn more about whales by classifying them genealogically with mammals than by amalgamating them with squids and sharks into an evolutionary heterogeneous group of “things that swim fast in the ocean.”)

  Professional taxonomists have always recognized this inequality among systems of naming by proclaiming the search for a “natural” classification as the goal of their science. Although we may regard the word natural as a peculiar, or even arrogant, description for an optimal scheme of classification, the rationale for this verbal choice seems clear enough. If all taxonomies must express theories about nature’s order, then we may define the most “natural” classification as the scheme that best respects, reveals, and reflects the causes that generated the diversity of organisms (thereby evoking our urge to classify in the first place)!

  A zoo director might, for practical purposes, choose to classify organisms by size (as a convenience for selecting cages) or by climatic preferences (so that his polar bears won’t asphyxiate in an exhibit on tropical rain forests). But we would label such taxonomic schemes as artificial because we know that evolution has generated the interrelationships among organisms by a process of genealogical descent through geological time. The most “natural” classification may therefore be defined as the scheme that best permits us to infer the genealogical connections among organisms—that is, the primary cause of their similarities and differences—from the names and forms of our taxonomies.

  When we recognize all influential classifications as careful descriptions of organisms made in the light of fruitful theories about the causes of order, then we can finally appreciate the fascination of taxonomy as a source of insight about both mind and nature. In particular, the history of changing classifications becomes far more than a dull archive or chronicle of successive purchases from nature’s post office (discoveries of new species), followed by careful sorting and proper pasting into preassigned spaces of a permanent album (taxonomic lists of objectively defined groups, with space always available for new occupants in a domicile that can always grow larger without changing its definitive style or structure). Rather, major taxonomic revisions often require that old mental designs be razed to their foundations, so that new conceptual structures may be raised to accommodate radically different groupings of occupants.

  In the obvious example of this essay, Agassiz’s lovely cathedral of taxonomic structure, conceived as a material incarnation of God’s mentality, did not collapse because new observations disproved his central conviction about the close affinity of jellyfish and starfish (now recognized as members of two genealogically distant phyla, falsely united by Agassiz for their common property of radial symmetry). Instead, the greatest theoretical revolution in the history of biology—Darwin’s triumphant case for evolution—revealed a fundamentally different causal basis for taxonomic order. Evolution fired the old firm and hired a new architect to rebuild the structure of classification, all the better to display the “grandeur” that Darwin had located in “this view of life.” Ironically, Agassiz opened his museum in 1859, in the same year that Darwin published The Origin of Species. Thus, Agassiz’s replica of God’s eternal mind at two degrees of separation (from the structure of divine thought to the taxonomic arrangement of organisms to the ordered display of a museum) became an unintended pageant of history’s genealogical flow and continuity.

  But this argument, that the history of taxonomy wins its fascination, at least in large part, as a dynamic interplay of mind (changing theories about the causes of order) and matter (deeper and more accurate understanding of nature’s factuality), now exposes a paradox that defines the second half of this essay, and leads us back to the official founder of taxonomy, Carolus Linnaeus. Darwinian evolution has set our modern theoretical context for understanding the causes of organic diversity. But if taxonomies always record theories about the causal order that underlies their construction, and if evolution generated the organic resemblances that our taxonomies attempt to express, then how can Linnaeus, a creationist who lived a full century before Darwin discovered the basis of biological order, be the official father of modern—that is, evolutionary—taxonomy? How, in short, can Linnaeus’s system continue to work so well in Darwin’s brave new world?

  Perhaps we should resolve this paradox by demoting the role of theory in taxonomy. Should we embrace Rutherford’s philatelic model after all, and regard organic interrelationships as simple, observable facts of nature, quite impervious to changing winds of theoretical fashion? Linnaeus, on this philatelic view, may have won success by simple virtue of his superior observational skills.

  Or perhaps we should argue, in maximal contrast, that Linnaeus just lucked out in one of history’s most felicitous casting of dice. Perhaps theories do specify the underlying order of any important taxonomic system, and Linnaeus’s creationist account just happened to imply a structure that, by pure good fortune, could be translated without fuss or fracture into the evolutionary terms of Darwin’s new biology.

  I will advocate a position between these two extremes of exemplary observational skill in an objective world and pure good luck in a world structured by theoretical preferences. Linnaeus may, no doubt, be ranked as both the premier observer and one of the smartest scientists of his (or any) age. But, following my central claim that taxonomies should be judged for their intrinsic mixture of accurate observation linked to fruitful theory, I would argue that Linnaeus has endured because he combined the best observational skills of his time with a theoretical conception of organic relationships that happens to conform, but not by pure accident, with the topology of evolutionary systems—even though Linnaeus himself interpreted this organizing principle in creationist terms. (As for the fascinating and largely psychological question of whether Linnaeus devised a system compatible with evolution because he glimpsed “truth” through a glass darkly, or because his biological intuitions subtly, and unconsciously, tweaked his theoretical leanings in an especially fruitful direction—well, I suspect [as for all inquiries in this speculative domain of human motivations] that Linnaeus took this particular issue with his mortal remains to the grave.)

  We refer to Linnaeus’s system as “binomial nomenclature” because the formal name of each spec
ies includes two components: the generic designation, given first with an initial uppercase letter (Homo for us, Canis for dogs, etc.); and the so-called “trivial” name, presented last and in fully lowercase letters (sapiens to designate us within the genus Homo, and familiaris to distinguish dogs from other species within the genus Canis—for example the wolf, Canis lupus). Incidentally, and to correct a common error, the trivial name has no standing by itself, and does not define a species. The name of our species, using both parts of the binomial designation, is Homo sapiens, not sapiens all by itself. We regard the 1758 version of Systema Naturae as the founding document of modern animal taxonomy because, in this edition and for the first time, Linnaeus used the binomial system in complete consistency and without exception. (Previous editions had designated some species binomially and others by a genus name followed by several descriptive words.)

  The binomial system includes several wise and innovative features that have ensured its continuing success. But, for the theme of this essay, the logical implications of this system for the nature of interrelationships among organisms stands out as the keystone of Linnaeus’s uncanny relevance in Darwin’s thoroughly altered evolutionary world. The very structure of a binomial name encodes the essential property that makes Linnaeus’s system consistent with life’s evolutionary topology.