I Have Landed (45 page)

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 species 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.

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, etc.). Such a nested hierarchy implies the organizing geometry of a single branching tree, with a common trunk that then ramifies into ever-finer divisions of boughs, limbs, branches, and twigs. This treelike form also happens to express the hypothesis that interrelationships among organisms record a genealogical hierarchy built by evolutionary branching. Linnaeus's system thus embodies, quite apart from Linnaeus's own intentions or theoretical connections, the causality of Darwin's world.

This correspondence between the Linnaean hierarchy and life's evolutionary tree achieves its clearest expression in pictorial form. The accompanying drawings show a Linnaean ordering of box within box, as well as the alternate expression of this logical order in a branching diagram of sequential genealogical splitting—in this case, a successive carving of the kingdom of all animals into, first, chordates contrasted with all other animals; then vertebrates contrasted with all invertebrates; mammals contrasted with all other vertebrates; the order Carnivora contrasted with all other mammals; the family Canidae contrasted with all other carnivores; the genus
Canis
contrasted with all other doglike carnivores; and domestic dogs contrasted with all other members of the genus
Canis
. I stated that binomial nomenclature expresses the first step of this hierarchical ordering—and thus presents a microcosm of the entire scheme—because the two parts of a species's name record the first act of embedding smaller units within more inclusive groups of relatives. The name
Canis familiaris
states that the smallest unit, the dog species, ranks as one member of the next most inclusive group, the genus
Canis
, which unites all other species (e.g., the wolf
Canis lupus
and the coyote
Canis latrans
) that originated from a common ancestor shared by no other species in any other group.

Linnaeus thought that his chosen scheme of mapping biological relationships as smaller boxes within successively larger boxes, until all units nested within the
most inclusive box of life itself, represented the best human device for expressing the eternal order that God had chosen when he populated the universe. I doubt that he ever explicitly said to himself (for I suspect that his mental mansion included no room for such a thought): “But if, quite to the contrary, life evolved by a process of ever-ramified branching from a single ancestor over a long period of time, then the hierarchical order of the binomial system will capture the topology of organic relationships just as well, because the logic of my system translates pictorially into a tree with a single trunk at the base, and subsequent division into branches that never coalesce thereafter. And such a topology might represent either God's permanent order, preconceived from the first, or the happenstances of historical change and development on an evolutionary tree growing from a single starting point under the constraint of unbroken continuity (although branches may die and fall from the tree as lineages become extinct), and continuous bifurcation without subsequent joining of lineages.”

Linnaean taxonomy in another of its geometric portrayals as a branching system with all divisions beginning from a common trunk and no fusion of separated branches allowed
.

I emphasize this property of irrevocable branching without subsequent amalgamation because the Linnaean logic of placing small boxes into larger boxes—which just happens to conform to the historical reality of Darwin's system—establishes such a map of organic relationships as its primary and inevitable consequence. One can't, after all, cram big boxes into smaller boxes. Therefore, for example, two species in the same genus can't reside in different families, and two orders in the same class can't be placed in different phyla. If lions and tigers rank as two species in the same genus
(Panthera)
, they cannot then be allocated to different families of higher rank (lions to the Felidae and tigers to the Canidae, for example)—for the two larger family boxes would then have to fit within the smaller box of the genus
Panthera
, and both the rules of Linnaean logic, and the requirements of Darwinian evolutionary history, would then be fractured. I can only be a monkey's uncle or a horse's ass in a metaphorical sense—for my species fits into the small box of the genus
Homo
, which must nest within the larger box of the family Hominidae; and one member of my species can't opt out of our box to join the Cercopithecidae or the Equidae, thus splitting a coherent lower group into two higher groups, and violating both Linnaean logic and Darwinian reality.