Leeuwenhoek’s revelatory observations of microbial life were reported in the journal of the Royal Society of London and became famous in scientific circles throughout Europe, but Charles Bonnet wasn’t interested enough in those “very wee animals” to fit them into his rising scale—not even where they might dismissively have been slotted, somewhere between asbestos and truffles. That omission presages a lasting discomfort with placing microbes on the ladder of life or, harder still, arranging their diverse forms on the tree—and it’s a discomfort to which I’ll return, because it became acute in 1977.
The linear approach to depicting life’s diversity was on the way out, notwithstanding Charles Bonnet’s scale of nature, and being replaced by its more complicated and dimensional successor, the tree. By the late eighteenth century and the start of the nineteenth, natural philosophers (we’d call them scientists, but that word didn’t yet exist) tried to classify and arrange living creatures into distinct groups and subgroups, reflecting their similarities and differences and some sort of organizing schema. The linear alignment, in order of what passed for increasing sublimity, the ladder raised toward God, was no longer satisfactory. There had been a knowledge explosion in Europe since the great age of sailing explorations began—knowledge of diverse animals, plants, and other creatures from all over the world—and scholars wanted to set that explosive abundance of new facts within hierarchical categories so that it could be easily accessed and used.
This wasn’t evolutionary thinking; it was just data management. The knowledge would fill volumes (one man alone, the German naturalist Alexander von Humboldt, published a thirty-volume account of his travels in South America), making all the more necessary an overview, an organizing principle, that could be apprehended at a glance: an illustration. But the illustrators now needed two dimensions, not one, and so the ladder turned into a trunk, and the trunk sprouted limbs, and the limbs diverged into branches. This offered more scope, sideways as well as up and down, for arranging the varied abundance of known creatures.
The tree of life was an old symbol by then, an old phrase, dating back at least to those mentions in Genesis and Revelation. The tree had also served as a model for family histories—the genealogical tree or pedigree of a German duke, for instance. Now the secularized tree became useful for organizing biology. Among the first to embrace this convention was another Frenchman, Augustin Augier, who wrote in 1801 that “a figure like a genealogical tree appears to be the most proper to grasp the order and gradation” of what concerned Augier: the diversity of plants.
Augier was an obscure citizen of the French Republic, living in Lyon, working on botany part-time; his real profession was unknown, his biographical details lost, even to a historian of Lyonnais botanists writing a hundred years later. Augier disappeared. But he left behind a book, a little octavo volume, in which he proposed a new classification of plants, “according to the order that Nature appears to have followed.” That is, a “natural order,” as opposed to an artificial classification system based merely on human whim or convenience. And in the book was a figure representing that system: his arbre botanique (botanical tree). Its trunk and limbs look almost as orderly and stiff as a menorah, but its sideways branching and copious leafing suggest a rife multiplicity of plant forms.
Again, this didn’t imply any heretical ideas about origins. Augier was no evolutionist before his time. His natural order wasn’t meant to suggest that all plants had descended from common ancestors by some sort of material process of transformation. God was their maker, shaping the varied forms individually: “It appears, and one can hardly doubt it, that the Creator, when making flowers, followed certain proportions and progressions in the number of their different parts.” Augier’s contribution, as he saw it, was discovering those proportions and progressions—design principles that had satisfied the Deity’s neat sense of pattern—and using them after the fact to organize botanical knowledge into a tidy system.
Augier wasn’t the first naturalist to hanker for a natural order of nature’s diversity. Aristotle had classified animals as “bloodless” and “blooded.” In the first century of our era a Greek physician named Dioscorides, attached to the Roman army, gathered lore on more than five hundred kinds of plants, arranging them in a compendium mainly on the basis of their medicinal, edible, and perfumatory uses. That book, in various reprints and translations, served as a trusted botany text for the next fifteen hundred years. Toward the end of its run, around the time of the Renaissance, as people traveled more widely and paid closer attention to the empirical details of nature, old Dioscorides gave way to newer illustrated herbals. These were essentially field guides to botany, graced with better illustrations based on improvements in drawing and woodcut techniques, but still organized for convenience of use, not natural order. In the sixteenth century, Leonhart Fuchs produced one of those books, an herbal cataloging hundreds of plants, beautifully illustrated and arranged in alphabetical order. Two centuries later, the great systematizer Carl Linnaeus described a genus of plants with purplish red flowers, naming it Fuchsia in honor of Leonhart Fuchs (and hence we got also the color, fuchsia). Linnaeus himself, a Swede who traveled widely as a young man and then took up a professorial life in Uppsala, emerged from this herbalist tradition but went beyond it.
Augier’s Arbre Botanique, 1801.
Linnaeus’s Systema Naturae, as first published in 1735, was a unique and peculiar thing: a big folio volume of barely more than a dozen pages, like a coffee-table atlas, in which he outlined a classification system for all the members of what he considered the three kingdoms of nature: plants, animals, and minerals. Notwithstanding the inclusion of minerals, what matters to us is how Linnaeus viewed the kingdoms of life.
His treatment of animals, presented on one double-page spread, was organized into six columns, each topped with a name for one of his classes: Quadrupedia, Aves, Amphibia, Pisces, Insecta, Vermes. Quadrupedia was divided into several four-limbed orders, including Anthropomorpha (mainly primates), Ferae (doggish forms such as wolves and foxes, plus cat forms such as lions and leopards, in addition to bears), and others. His Amphibia encompassed reptiles as well as amphibians, and his Vermes was a catchall group containing not just worms, leeches, and flukes but also slugs, sea cucumbers, starfish, barnacles, and other sea animals. He divided each order further into genera (with some recognizable names such as Leo, Ursus, Hippopotamus, and Homo), and each genus into species. Apart from the six classes, Linnaeus also gave a half column to what he called Paradoxa: a wild-card assemblage of mythic chimeras and befuddling but real creatures, including the unicorn, the satyr, the phoenix, the dragon, and a certain giant tadpole (Pseudis paradoxa, under its modern label) that, strangely, paradoxically, shrinks during metamorphosis into a much smaller frog. Across the top of the chart ran large letters: CAROLI LINNAEI REGNUM ANIMALE. His animal kingdom. It was a provisional effort, grand in scope, integrated, but not especially original, to make sense of faunal diversity based on what was known and believed at the time. Then again, animals weren’t Linnaeus’s specialty.
Plants were. His classification of the vegetable kingdom was more innovative, more comprehensive, and more orderly. It became known as the “sexual system” because he recognized that flowers are sexual structures, and he used their male and female organs—their stamens and pistils, those delicate little stems sticking up to present and receive pollen—for characterizing his groups. Linnaeus defined twenty-three classes, into which he placed all the flowering plants, based on the number, size, and arrangement of their stamens. Then he broke each class into orders, based on their pistils. To the classes, he gave names such as Monandria, Diandria, and Triandria (one husband, two husbands, three husbands), and, within each class, ordinal names such as Monogynia, Digynia, and Tryginia