I Have Landed (46 page)

So perhaps Linnaeus enjoyed a little bit of luck in choosing the one logic for a creationist system that would also fit without fuss into a new universe of historical evolution by branching. At least Linnaeus demonstrated exemplary survival skills in passing the test of time as taxonomy's father. But I hesitate to ascribe his remarkable success to pure dumb fortune, and for a primary reason rooted in the key contention of this essay: that taxonomies transcend simple description and always embody particular theories about the causes of order, thus melding preferences of mind with perceptions of nature.

I think that Linnaeus succeeded because, however unconsciously or preconsciously, he made some excellent decisions about both the mental and perceptual aspects of taxonomic systems. On the perceptual side, he must have seen better than any of his colleagues that under the logic of hierarchy and branching, organisms could be arranged into a consistent order that might win general assent without constant bickering among practitioners. Other contemporaries had proposed very different logics for classification, but had never found a way to push them through to an unambiguous and consistent system. In the most telling example, Linnaeus's most famous contemporary and archrival, France's celebrated naturalist the Baron Georges Leclerc Buffon (1707–1788), had struggled through more than forty volumes of his
Histoire naturelle—
in my judgment, the greatest encyclopedia of natural science ever written—to develop, without conspicuous success, a nonhierarchical system
that joined each species to some others by physiology, to a different group by anatomy, and to a still different set by ecology.

But I would, in addition, like to advance the unfamiliar argument that Linnaeus also succeeded because he made a very clever, and probably conscious, choice from the mental side of taxonomic requirements as well. In deciding to erect a hierarchical order based on continuous branching with no subsequent joining of branches, Linnaeus constructed his system according to the most familiar organizing device of Western logic since Aristotle (and, arguably, an expression of our innate and universal mental preferences as well): successive (and exceptionless) dichotomous branching as a system for making ever finer distinctions. In a logical tree of this form—often called a dichotomous key—one may move in either direction to place a particular basic object into ever larger groups by joining successive pairs, or to break down a large category into all component parts by successive twofold division.

One may, for example, interpret the diagram that I presented earlier as a dichotomous key. We can reach dogs by starting with the largest category of all animals, dividing this totality into vertebrates and invertebrates, then splitting the vertebrates into mammals and non-mammals, the mammals into carnivores and non-carnivores, the carnivores into canids and non-canids, and finally the canids into dogs versus others. (We can also work outward in the opposite direction to learn how dogs fit into the hierarchy of all animals.)

In fact, the idea for this essay came to me when I recently purchased an obscure late-sixteenth-century book on Aristotelian logic and noted that its numerous charts for working through the categories of reasoning, and the attributes of human form and behavior, had all been constructed as dichotomous maps bearing an uncanny resemblance to the taxonomic keying devices that I have seen and used in texts and guidebooks for naturalists throughout my career. Thus, Linnaeus gave himself quite a leg up by building his taxonomic system upon a familiar form of logic that scholars had applied to all subjects, scientific and otherwise, from the dawn of Western history—a style of reasoning, moreover, that may track the basic operation of our brains.

I took the accompanying chart from this 1586 treatise, published in Paris by the physician Nicolas Abraham, and entitled
hogogethica ad rationis normam delineata
(an introduction to ethics as delineated by the rule of reason). Abraham first divides the domain of ethical decisions into the dichotomous pair
of mentis
(by the mind) and
moris
(by custom). He then splits the lower domain of custom into the two categories of
privatis
(above) and
publicis
(below). Interestingly, and I suspect consciously, authors of dichotomous keys also seem—at least in my limited study of such devices from pre-Darwinian
times—to order their pairings from the good and most valued on top to the least admirable on the bottom. In this case, reason beats custom, while, within custom, private decisions (presumably made for reasons of personal belief) trump public actions (that may be enforced by social pressure). I am particularly fond of the dichotomous key for birds of prey that the great English naturalist John Ray published in 1678—with a first division of day-flyers on top and night-flyers on the bottom; a second division of the preferred day-flyers into bigger species (above) and smaller (below); and a third division of the big species into “more generous” eagles above and “more cowardly and sluggish” vultures below.

A dichotomous key, as presented by Nicolas Abraham in 1586 to classify ethical decisions
.

But let us follow Abraham's key for the higher category of mental decisions, which then undergoes a further dichotomous split into
sapientia
(done by wisdom) above and
prudentia
(done for reasons of prudence) below. The third and final set of twofold divisions then separates judgments
by sapientia
into
intelligent
(achieved by pure reason) above in preference to
scientia
(achieved by knowledge about material things) below. The lower judgments of
prudentia
then divide into a preferred category of
bona consultatio
above (derived from our seeking a good advice from others) and the less worthy dichotomous alternative
of sagacitas
below (determined only by our own judgment).

I do admire Linnaeus as an intellectually driven and brilliantly complicated, but arrogantly vainglorious, man. If I preferred the hagiographical mode of writing essays, I would stop here with a closing word of praise for Linnaeus's perspicacity in harnessing both the observational and theoretical sides of his mental skills to construct a flexible and enduring taxonomic system that could survive intact in sailing right through the greatest theoretical transformation in the history of biology.

But he who lives by the sword dies by the sword (as Jesus did not exactly say in a common misquotation that remains potent in truth and meaning despite a slight inaccuracy in citation). Linnaeus's consistency and wisdom in developing
and defending the binomial system of hierarchical classification carried him through to intellectual victory. But, like so many originators of grand and innovative systems, he reached too far (whether by arrogance or overexcitement) and became too committed to his procedure as the one true way for classifying any collection of related objects. (I cannot help recalling my experience with a customs official on a small West Indian island who classified my land snails as turtles because his forms only permitted a distinction between warm-blooded and cold-blooded “animals”—and the word “animal,” in his personal understanding, only designated vertebrates. Thus, snails became turtles because both are cold-blooded and move with legendary torpor.)

Once Linnaeus had fully developed the binomial system and its supporting logic of a consistently nested hierarchy, he supposed that he had discovered the proper way to classify any group of natural objects, and he therefore began to apply binomial nomenclature to several classes of inappropriate phenomena, including rocks and even human diseases. Clearly, he had become overen-amored with his own device, and had lost sight of the key principle that hierarchical embedding by dichotomous branching only captures the causal order within
certain kinds
of systems—particularly those that develop historically by successive branching in unbroken genealogical continuity (with no later amalgamation of branches) from a common ancestor. Since Linnaeus tried to apply his binomial system to several groups of objects that, by their own rules of order or development, patently violate the required hierarchical logic, perhaps he never really did grasp the limitations (and therefore the essence) of his binomial system. So maybe Linnaeus did prevail partly by the luck of organic conformity to his general logic, rather than by his correct and conscious reasoning about distinctive causes of relationships among plants and animals.

For example, the accompanying page from the seventh (1748) edition of
Systema Naturae
designates binomial species of the genus
Quartzum
from the classification of rocks that he presented as a third chapter, following his taxonomies for animals and plants. The first “species,”
Quartzum aqueum
(transparent quartz) includes ordinary glasslike quartz; the second,
Quartzum album
(white quartz) encompasses less valued, opaquely water-worn quartz pebbles; the third,
Quartzum tinctum
(colored quartz) gathers together the colored varieties that mimic more-valuable gemstones (Linnaeus calls them false topaz, ruby, and sapphire, for example); and the fourth,
Quartzum opacum
(opaque quartz) describes the even less useful and less transparent flintstones.

But the nature of quartz, and the basis of relationships among minerals in general, defies the required logic of causality for any system legitimately described in Linnaean binomial terms. The members
of Quartzum aqueum
, for
example, do not hang together as a set of mutually closest historical relatives, all physically derived in continuity from a common ancestor that generated no other offspring. Rather, the specimens of this false species look alike because simple rules of chemistry and physics dictate that transparent quartz will form whenever silicon and oxygen ions come together under certain conditions of temperature, pressure, and composition. The members of this “species” can claim no historical or genealogical coherence. One specimen might have originated half a billion years ago from a cooling magma in Africa, and another just fifty years ago in a bomb crater in Nevada. Minerals must be classified according to their own causes of order, a set of rules distinctly different from the evolutionary and genealogical principles that build the interrelationships among organisms.

A page from the mineralogical chapter of the 1748 edition of Linnaeus's
Sy sterna Naturae,
showing that he tried to apply his method of binomial nomenclature to rocks, as well as to organisms
.

Linnaeus clearly overreached in supposing that he had discovered the one true system for all natural objects. In the twelfth and final edition of
Systema Naturae
(1766), he included a section entitled
Imperium Naturae
, dedicated to extolling his hierarchical and binomial method as universally valid. God made all things, Linnaeus argues, and he must have used a single and universal method, now discovered by his most obedient (and successful) servant. Linnaeus writes:
“Omnes res creatae sunt divinae sapientiae et potentiae testes”
(all created things are witnesses of divine knowledge and power). Using a common classical metaphor (the thread of Ariadne that led Perseus out of the labyrinth after he had killed the Minotaur), Linnaeus praises himself as the code cracker of this universal order: “knowledge of nature begins with our understanding of her methods by means of a systematic nomenclature that works like Ariadne's thread, permitting us to follow nature's meanders with accuracy and confidence.”

Ironically, however, Linnaeus had succeeded (in a truly ample, albeit not universal, domain of nature) precisely because he had constructed a logic that correctly followed the causes of order in the organic world, but could not, for the same reasons, be extended to cover inorganic objects
not
built and interrelated by ties of genealogical continuity and evolutionary transformation. The strength of any great system shines most brightly in the light of limits that give sharp and clear definition to the large domain of its non-universal action! By understanding why the Linnaean system works for organisms and
not
for rocks, we gain our best insight into the importance of his achievements in specifying the
varied
nature of
disparate
causes for nature's order among her many realms.