We human beings are brilliant at ignoring uncomfortable truths, one of the most durable of which is that the world clearly contains vastly more species of living organisms than Noah could ever have fit into his ark. A recent estimate is that our planet currently hosts just shy of eight million species of animals alone; the mind boggles at the scale of Noah’s problem. Yet before the mid-nineteenth century few in the Christian world saw fit to question this or indeed any of the many other anomalies in the biblical account of how the world had got to be the way it is. Earth and its denizens were simply what the Creator had wanted them to be, and it was pretty universally acknowledged that his desire had been to sit at the top of the “Great Chain of Being,” a hierarchy of increasing “perfection” among his many creations. At the bottom of the chain were the inert minerals; above them, in ascending order of perfection, were the plants, the nonhuman animals, and human beings. Only the angels came between humans and the deity. The concept actually went back two millennia to Aristotle’s “Ladder of Nature,” although the notion that organisms could be viewed as links in a chain was a medieval embellishment.

As Europe emerged from the Middle Ages, the main task facing early botanists and zoologists was to properly identify those links, the basic components of life’s edifice. This, of course, required names everyone could agree on—a requirement that hugely variable vernacular labels could not fulfill (not everyone would know that the Indian ichneumon and the gray mongoose were the same thing). Characterizing and naming the basic units of nature duly became the order of the day, leaving larger matters of process unexamined. So it was not until late in the seventeenth century that the English botanist John Ray suggested that species were more usefully defined by reproductive continuity among their members than by the physical attributes arbitrarily bestowed on them by God. By Ray’s time examples of strange animals and plants from far-flung lands had been trickling into Europe for some while, and naturalists were scrambling both to describe and to make sense of the variety these unexpected new arrivals represented. But it would nevertheless be many years before anyone thought it advisable to investigate whether there was any overarching order in that variety, and longer still before the mechanisms behind it would be brought to light.

Jason Roberts’s engaging book Every Living Thing picks up the quest to fathom the riotous diversity of nature early in the eighteenth century, by which time such seventeenth-century pioneers as Ray and his French colleague Joseph Pitton de Tournefort had already begun to clarify not only the notions of species and genera but also the problems inherent in assembling genera into larger units. As for naming those units, or taxa, Tournefort and the German botanist Augustus Quirinus Rivinus had previously experimented with the “binominal,” or two-name (Roberts regrettably prefers the terms “binomial” and “binary”), system of nomenclature that Carl Linnaeus would later promote. By themselves, however, these were modest advances, and at the beginning of the eighteenth century the complexity of the natural world was still begging for some kind of synthesis and usable general classification.

Into this space strode two magisterial figures, the joint subjects of Roberts’s book. The Swedish botanist Linnaeus and the French polymath Georges-Louis Leclerc, comte de Buffon, were exact contemporaries (both were born in 1707), and each is still revered in his own and very different way. More people nowadays have heard of Linnaeus than of Buffon because it was he who promoted the binominal nomenclature and the hierarchical method of classifying living things (groups within groups) that we still use today. Whether you like the idea or not, you belong to the species Homo sapiens (“man the wise”) of the genus Homo (“man”). In turn, Homo belongs to the family Hominidae, which lies within the order Primates of the class Mammalia of the kingdom Animalia; blame it all on Linnaeus. Given the frequently untidy ways in which opportunistic nature works, it is inevitable that regular complaints should be heard about the rigid Linnaean system of classifying the biota. But nothing has yet replaced it, not simply because it is by now thoroughly entrenched but because nobody has come up with an alternative that is even remotely as useful.

As an academic, Buffon cut a more traditional figure than Linnaeus, at least in the sense that he belonged to the leisured class. But he stood out in the depth and breadth of his erudition, and his multivolume masterwork, Histoire naturelle, continues to astonish by the span of its author’s curiosity and by its penetrating observations on a huge range of subjects. While Buffon’s fame has dimmed, the spirits of both naturalists, if for markedly different reasons, live on vigorously in contemporary evolutionary biology even though both were long dead by the time evolutionary thought finally came along.

The two could hardly have been more different in background, personality, or scientific agenda. The son of an obscure country pastor, Linnaeus was unkempt, calculating, and self-aggrandizing, and he appears to have cheated his way up the academic ladder even as he was producing one of the greatest scholarly monuments ever created: the first comprehensive hierarchical classification of known living things. In sharp contrast, Buffon was beautifully mannered, graceful in appearance, and careless both of fame and of the importance of classifying organisms. Almost all the two had in common was a consuming interest in the natural world and a lifelong devotion to a single large project. Roberts makes clear where his sympathies lie: he wastes no time in painting Linnaeus as a “doctor with a diploma-mill medical degree and a flair for self-promotion,” while Buffon was “as renowned for his elegance as for his brilliance.” There is truth in both characterizations, but it is nonetheless unfortunate that Roberts elects to take sides so nakedly, because it makes it hard for him to discern the true extent of Linnaeus’s accomplishment. The man may have been a driven monomaniac who never exhibited the nuance of thought or the appreciation of complexity in natural processes that Buffon so abundantly displayed, but in the end this hardly detracts from the practical and historical importance of the system he created.

As befits what is in essence a double biography, Roberts digs deep into the lives of his subjects. Carl (“Carolus” when he published in Latin) Linnaeus was born to a cash-strapped lineage of Lutheran clergymen in rural southern Sweden. Fascinated by plants from early youth, he was a mediocre student whose search for a medical education was continually frustrated by limited finances. At the age of twenty-one he contrived to enroll in the rather undistinguished local Lund University; frustratingly, when he transferred to the better-reputed university at Uppsala a year later its medical school proved an almost equal disappointment. Nonetheless, while in Uppsala Linnaeus wrote a thesis on plant reproduction that became something of an underground hit, and he eventually returned as a professor. With amazing persistence, during a long sojourn in the Netherlands from 1735 to 1738 Linnaeus finally obtained a medical doctorate from the University of Harderwijk. (It was undeniably a degree mill; he obtained his diploma in a matter of days.)

Before that, however, in 1732, he had completed an incredibly formative six-month odyssey through remote Lapland, by horse and on foot. While in Lapland Linnaeus experienced a flash of insight that would serve him well when he began to compile his multi-edition opus Systema Naturae, which ultimately became enshrined as the starting point of the modern taxonomy of plants and animals (plus minerals, now long forgotten). Having examined a horse’s jaw that he found at the side of a muddy track, the young naturalist wrote in his journal:

If only I knew how many teeth and of what kind every animal had, how many teats and where they were placed, I should perhaps be able to work out a perfectly natural system for the arrangement of all quadrupeds.

Although an observant Christian and a strict creationist who believed that organisms simply had the form God wanted, and who equated the number of existing species with “the number of different forms that were created in the beginning,” Linnaeus had clearly intuited the intimate linkage between morphology and relationship that we now know can be explained only by common descent.

The first edition of the Systema Naturae appeared in Holland in 1735, having apparently arrived there in manuscript with its author. Far from the comprehensive classification of living things it later became, this initial edition was a modest pamphlet of a mere twelve pages. Its main innovation was to consider animals, plants, and minerals as separate but equal groups, divorced from the Great Chain of Being. But it was the start of something much bigger, for Linnaeus subsequently devoted his career to enlarging and revising the Systema, which greatly evolved as the decades passed. By the time its definitive tenth edition appeared in 1758 it had expanded to contain names, diagnoses, and distributions for some 4,400 species of animals and 7,700 species of plants. From the beginning, all names were in Latin, and all were also strictly binominal, with rules to determine precedence, contrasting with the much more casual use of binomina by Linnaeus’s predecessors. Most importantly, perhaps, the Systema organized its subjects into nested sets: species were grouped into genera, genera into families, families into orders, orders into classes, and classes into kingdoms, reflecting the patterns of similarity perceived at the relevant levels.

The number of ranks in the inclusive Linnaean hierarchy has swelled considerably since the eighteenth century, but the hierarchical principle itself has stood the test of time. And the secret of the Linnaean classification system’s utility and longevity is that it maps very neatly onto the pattern of forking branches produced by evolution as species split to produce new lineages (a point Roberts neglects to emphasize). Of course, as a conventional creationist living a century before Charles Darwin, Linnaeus had no idea that this was what his new system of classification was actually doing. He might indeed have been shocked to learn it. (He once wrote, “There are no new species. Like always gives birth to like.”) But his system fit extremely well with what could already be observed out there in nature, and once evolutionary thought had come along, there was no turning back.

The life trajectory of Georges-Louis Leclerc, comte de Buffon, was utterly unlike Linnaeus’s. Born in Burgundy to a striving rural family that had achieved the status of minor officialdom, he was soon the beneficiary of a windfall inheritance that permitted the acquisition of extensive properties, among them the hamlet of Buffon, which allowed him to add “de Buffon” to his name. (The “comte” came much later, in recognition of his achievements as a naturalist.) He attended the University of Angers, where he was best known for fighting duels, a proclivity that eventually shortened his time there. In 1730 he joined an intellectual English aristocrat and his tutor on their European Grand Tour and by the time the trio fetched up in Paris a year and a half later he had been awakened to the life of the mind. Buffon soon left the capital to take care of family affairs and the development of his estates, including the construction of a mansion in his home village of Montbard and the establishment of an experimental forest there. (He was the first thinker to see forests as integrated ecosystems rather than as mere collections of trees.)

He also maintained his Paris connections coming under the wing of the comte de Maurepas, who as minister for the navy shared his interest in the trees that shipbuilders were cutting down all over France. Maurepas also oversaw the Académie des Sciences, to which Buffon was admitted after giving a celebrated lecture on probability theory. It was 1734, and he had joined France’s intellectual elite through his mathematical prowess. Five years later Maurepas somewhat improbably engineered Buffon’s appointment as intendant of the Jardin du Roi, a marvelously eccentric institution of inquiry and informal education that had grown up around the garden in southwest Paris where the king’s supply of medicinal plants was grown and processed.

At that point Buffon was living primarily at Montbard, where he had established a rigorous daily schedule of early rising followed by work that soon became overwhelmingly a matter of producing, with the help of some illustrious collaborators, his great series Histoire naturelle, générale et particulière. The first three quarto volumes appeared in 1749 and dealt with a raft of different themes, including geology and the origin of Earth, the inadequacy of Linnaeus’s taxonomy, animal reproduction, and humankind itself. They were instant best sellers and boosted Buffon’s public reputation, but they also earned their author the censure of the theologians at the Sorbonne for departing from biblical views of creation. Buffon duly issued a mild retraction, but the only concession he made in later volumes was to periodically insert statements designed to forestall such criticism—a well-known ploy that Linnaeus also used when he felt he was steering too close to the theological wind. Sadly, it is too infrequently recognized by modern readers that those comments were tongue-in-cheek.

Thirty-six splendidly illustrated volumes of Histoire naturelle were published in his lifetime (in addition to the first three, there were twelve on quadrupeds, nine on birds, and five on minerals, as well as seven supplementary volumes), with several more organized later by Bernard Germain Etienne de la Ville-sur-Illon, comte de Lacépède, but the series never came near to its author’s ambition of covering all of life. Still, it more than amply showed not only dazzling scholarship expressed in a persuasive if rather demanding writing style but also unmatched intellectual curiosity and insight, and the effect of a new volume appearing every year or two for several decades before Buffon’s death in 1788 was to make its author a national institution. Those volumes remain quite astonishing. Buffon was not just interested in documenting nature; he saw how dynamic and integrated it was, and he wanted to understand the interactions involved. He acknowledged the great age of Earth, he recognized the significance of biogeography, he accepted both that species could become extinct and that they could change, and he even entertained the idea that humans and apes, so structurally similar, might have had a common ancestry. What he did not accept was the need to classify his subjects, believing that to do so would be to impose an artificial order on nature—which, of course, made Linnaeus a prime target for his disdain.

Linnaeus, meanwhile, had been struggling to earn a living as a physician. His financial difficulties diminished only in 1739 with his marriage to an heiress and then, after some complicated maneuverings, his assumption of a professorship at Uppsala. Work resumed on the Systema, which underwent constant revision. Roberts seems to be a little alarmed by the multiple changes of mind the succession of new editions reflected, but in fact they provide evidence of the flexibility of the underlying classification system. Science proceeds largely by the correction of error; and while Linnaeus never had a hope of nailing a definitive classification of life, his system itself proved robust enough to accommodate changes in knowledge and interpretation even as major revolutions swept through biological thought. At the time, however, the conversion of existing names to Linnaean binomina unsurprisingly raised the hackles of his naturalist colleagues, as they saw Linnaeus discarding their own names wholesale and replacing them with his own.

Something else that worries Roberts is Linnaeus’s attempt to classify humans, whom the Swede bravely brought within his system in the ninth edition of the Systema. When he discussed Homo sapiens in the definitive tenth edition of 1758 (abandoning Homo diurnus, or “day-dwelling man,” the name he had previously used), he recognized (in addition to some fantastical ones) four human geographic varieties (subspecies): europaeus, americanus, asiaticus, and afer (African). Each was given a capsule characterization; for example, asiaticus was “sallow, melancholic, strict. Hair blackening. Eyes dark. Severe, haughty, greedy. Covered by loose vestments. Governed byopinions.” To Roberts those stereotypes are “profoundly prejudiced,” while to others they just make Linnaeus a man of his time. After all, racism as we are now familiar with it is an end product of the slave trade and the colonial period, while in Linnaeus’s day attitudes had not yet hardened, and it’s worth noting that Linnaeus did not rank his varieties, as was later done by the German naturalist Johann Blumenbach. It is true that, as Roberts points out, Linnaeus believed a lot of things that we find absurd today (e.g., that epilepsy is caused by washing one’s hair). But cognitive dissonance is baked into the human condition, and such beliefs (far from his alone) hardly discredit Linnaeus’s distinctive contribution as a systematist. That said, by spurning classification Buffon unquestionably came out much better on this issue: instead of categories of humans he saw geographical variation, which he astutely attributed to adaptation to local conditions.

Some readers may be intrigued by the presence of the word “deadly” in the book’s title, for what could be deadly about a purely scientific quest to enumerate species? Roberts appears to be referring to Linnaeus’s practice, once he was ensconced at Uppsala, of dispatching a series of “apostles” to far-flung corners of the planet in search of the new and the undescribed. Most of them operated on a shoestring budget, as Linnaeus himself had in Lapland, and many of them met lonely deaths far from home. In most cases the professor appears to have done as much as he could to help them, but absent much international infrastructure there was for the most part little he could do. Indeed, in the mid-eighteenth century, disease and death typically awaited those Europeans who ventured as far afield as the apostles tended to do.

Linnaeus died in 1778, and Buffon followed him ten years later. Early on in the French Revolution, a radical faction installed a bust of Linnaeus in the former Jardin du Roi (renamed Jardin des Plantes), while Buffon’s tomb was despoiled by activists seeking lead for bullets. Roberts fascinatingly recounts how a new cohort of savants then took over both the Jardin and the newly established Muséum National d’Histoire Naturelle adjacent to it. The most underappreciated but ultimately most consequential member of this distinguished group, all variously influenced by Buffon, was Jean-Baptiste Pierre Antoine de Monet, chevalier de Lamarck, whose studies of mollusk fossils early in the nineteenth century convinced him that lineages of organisms could change over time. With the Italian Giambattista Brocchi’s 1814 notion that species gave rise to other species, this finding provided the basic ingredients for Darwin and Alfred Russel Wallace’s idea of evolution by natural selection, which was presented at the Linnean Society of London in 1858.

Roberts to the contrary, the exact role of natural selection in evolution is still debated. But Darwin’s core concept, that all life on Earth is related by common ancestry, nonetheless gives us the only rational explanation we have for the pattern of physical (and nowadays genomic) resemblances we see in the diversity of the living world. What’s more, the repeated splitting of lineages that generates diversity just happens to produce precisely the nesting of sets (species within genera within families, etc.) that underpins the creationist Linnaeus’s system for classifying living things. The inherent messiness of the natural world aside (particularly among bacteria), the two complement each other perfectly, which is why the flexible Linnaean system remains actively and indispensably at the center of modern biology. Our understanding of particular relationships in nature may change, but the framework within which they are expressed endures.

For all his brilliance and insight, Buffon offered no comparable overarching organization or explanatory scheme. On the other hand, it is true both that Linnaeus’s contribution to modern biology is as narrow as it is vital and that the story of life on Earth is a rich and deep one that echoes far beyond nested sets of species names. As Buffon so clearly understood, it is the interaction among the players in the ecological play that has produced the drama of life through time, and no modern evolutionary account is complete without considering ecology and function and environment and time and myriad other factors that Buffon brought into biology and that still enliven the evolutionary sciences today. The count may not have articulated a theory of evolution, but many of the elements were there. As Darwin himself wrote to a colleague, “I have read Buffon—whole pages are laughably like mine.”

Roberts wraps up his absorbing book with a brisk canter through later developments in taxonomy and evolution. He covers the arrival of evolutionary thought and genetics (and even genomics), along the way exposing some of the practical difficulties that have been encountered in the application of Linnaean principles of classification as the number of known organisms has skyrocketed and the knowledge of individual taxa has expanded. Complications of this kind will only grow as the relationships among taxa are reanalyzed and identifications are revised—after all, that is the daily bread of taxonomists, and it’s what science is all about. Nonetheless, after almost three centuries the Linnaean system itself continues to march along more or less intact.