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Sept, 2000Linnaeus's Luck?(Carolus Linnaeus)(includes related article on leaves)
Author/s: Stephen Jay Gould
Why does the great creationist's system of classification work in Darwin's world? And what does the resolution of this paradox teach us about the importance and fascination of taxonomy?
Carolus Linnaeus (1707-78), the founder of modern taxonomy, frequently cited an ancient motto to epitomize his view of life: Natura non facit saltum (Nature does not make leaps). Such unbroken continuity may rule 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 in 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 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 Alvarez had exploded in frustration at the strong biases that initially led most paleontologists to dismiss, 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.
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 clearly preassigned places: pasting stamps into the designated spaces of nature's album, putting hats on the right hooks of the world's objective hat rack, 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, in 1859, he opened Harvard's Museum of Comparative Zoology in his adopted land. 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 will 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 at grasping the otherwise ineffable nature of God himself.)
In framing a modern Goldilockian defense of the importance of taxonomy--far warmer than Rutherford's icy indifference but not quite as 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 express nature's objective realities only 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 simply 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 evolutionarily 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, 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 his polar bears won't asphyxiate in an exhibit on tropical rainforests). But we would label such taxonomic schemes 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, 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 room always available for new occupants in a domicile that can 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, 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 the 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 (increased and more accurate understanding of nature's factuality) now exposes a paradox that defines the central theme 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, in this philatelic view, may have won success by the 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 castings of dice. Perhaps theories do specify the underlying order of any important taxonomic system, and 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. 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 delineated 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 the system for the nature of interrelationships among organisms stand 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.
Linnaeus's taxonomic scheme designates a rigorously nested hierarchy of groups (starting with species as the smallest unit) embedded within successively larger groups (species within genera within families within orders and so forth). Such a nested hierarchy implies a single branching tree with a common trunk that ramifies into ever finer divisions of boughs, limbs, branches, and twigs. This treelike form just happens to express the hypothesis that interrelationships among organisms record a genealogical hierarchy built by evolutionary branching. Linnaeus's system thus embodies the causality of Darwin's world. 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 was, no doubt, both the premier observer and one of the smartest scientists of his (or any) age. But following my central claim that taxonomies must be judged for their intrinsic mixture of accurate observation and fruitful theory, I will 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 mirror--but not by pure accident--the topology of evolutionary systems, even though Linnaeus himself interpreted his 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, as for all inquiries in this speculative domain of human motivation, I suspect 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 species includes two components: the generic designation, given first with an initial uppercase letter (Homo for us, Canis for dogs, and so on), and the so-called trivial name, presented last and in all 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 or so. But in the Waianae Range, only the upper slopes of Kaala are moist enough to support this habitat.
One of the most accessible places to see a bit of the montane rainforest is near Honolulu. The city has spread toward the Koolau mountains, and the suburb of Makiki Heights has penetrated the foothills below Mount Tantalus. This 2,013-foot mountain, which falls within the Honolulu Watershed Forest Reserve, lies northeast of the famous Punchbowl Crater, location of the National Memorial Cemetery of the Pacific.
A map of Mount Tantalus that shows various hiking trails is available from the Hawaii Nature Center on Makiki Heights Drive. I recommend the 3.4-mile-long Manoa Cliff Trail. After leaving the nature center, take Makiki Heights Drive eastward to where it turns into Round Top Drive and proceed on this circuitous route up the mountainside. The trail begins near a parking area, follows the cliff above Manoa Valley, and then continues around Mount Tantalus. It ends at Tantalus Drive, which is the continuation of Round Top Drive. At that point you can retrace your steps or, watching out for traffic, walk along the narrow vehicular road to your starting point. A shorter option is to hike the trail for about one and a half miles to the junction with the Pu`u `Ohi`a Trail (which would take you toward the summit of Mount Tantalus) and then turn back.
At the beginning of Manoa Cliff Trail, numerous nonnative ornamentals grow rampantly. Six-foot-tall ginger plants, with their white and yellow flowers crowded into a club shape, complement the colorful ti plant, whose clusters of long, mottled red-and-green leaves grow from the tops of unbranched stems. Nonnative trees include strawberry guava, common guava, rose apple, kukui (a candlenut), and avocado, while two of the shrubs are lantana and a type of raspberry called ola`a. Farther along the trail are groups of banana plants.
The first native trees encountered are some koas that form a canopy over the trail. Mature koa leaves are sickle shaped, and the tree produces cream-colored flowers in powder-puff clusters and flat pods up to ten inches long. Highly prized, koa wood is used for both traditional and modern products.
Native species abound along much of the rest of the trail--wildflowers, vines, and, above all, trees and shrubs. They are easiest to identify when they are bearing their flowers or fruit, but most of the time the only clues are the leaves. One salient detail is the way the leaves are arranged on twigs or branches. In some species, they are crowded at the tips, a characteristic rarely seen in plants native to the continental United States. One such species is Cyanea grimesiana, a shrubby relative of the lobelia with fernlike leaves. (Lobelias in the continental United States are small wildflowers, but most of their Hawaiian relatives are shrubs or even small trees. Why plants on islands tend to be large and woody while their mainland relatives are not is a phenomenon that has intrigued biologists since the time of Darwin.)
In other species, the leaves may be arranged along the twig in an alternating pattern, in pairs, or in whorls. Additional leaf characteristics that help in identification are size, form, color, vein pattern, thickness, and texture. (The accompanying descriptions in "Telltale Leaves" illustrate some of these distinctions.)
The leaves of three closely related species have tiny chambers called domatia, which commonly serve as shelters for small insects and mites. Domatia were first described more than a century ago by Swedish naturalist Axel Lundstrom, who suggested that the residents they attracted helped protect the plant. This has been confirmed in some species, including these Hawaiian plants.
RELATED ARTICLE: TELLTALE LEAVES
Leaves crowded at the tips of twigs or branches are found in three species related to lobelias. The easiest of these to identify is Cyanea grimesiana, because its leaves are fernlike; the other two have toothed leaves. Another plant in this category, kolea lau nui, has smooth-edged leaves.
Alternating leaves are apparent in koa, readily identified by its sickle-shaped leaves. Others in this category that are easily recognized include Perrottetia sandwicensis, which has red veins in its leaves; maua, which has red leafstalks; and ala`a, a relative of the soapwort, which oozes milky sap from the stalk when a leaf is broken off. Kawa`u (a member of the holly family) has smooth-edged leaves with an intricate network of conspicuous veins. Perhaps the strangest of these plants !s pamakani mahu, a shrub in which the tips of the branches tend to climb like vines.
Leaves arranged in pairs characterize many species, including three with leaf domatia (tiny chambers that house insects and mites). In `ahakea lau nui, the domatia are slightly elevated on the leaf surface, while in kopiko kea and Psychotria mariniana, they are sunken into the surface where the lateral veins join the main vein. `Akoko is a shrub readily identified by its milky sap, and `ohi`a ha leaves have pink or red middle veins. In Dubautia plantagina, the parallel-veined leaves are reminiscent of those of the common North American weed known as plantain. A shrub whose paper-thin leaves emit a foul odor when crushed is au, a member of the coffee family.
Leaves arranged in whorls forming circular patterns at intervals along the twig characterize just one shrub, Coprosma longifolia. The leaves on this slender, branched species are grouped in threes.
For visitor information, contact: Division of Forestry 1151 Punchbowl Street, Room 325 Honolulu, HI 96813 (808) 587-0166 www.state.hi.us/dlrn/dofaw
Robert H. Mohlenbrock, professor emeritus of plant biology at Southern Illinois University, Carbondale, explores the biological and geological highlights of U.S. national forests and other parklands.
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