Before 1753, the name of a plant was a description. The common tomato was known in one catalogue as Solanum caule inermi herbaceo, foliis pinnatis incisis, racemis simplicibus, which distinguishes it from other nightshades by listing its features in Latin. The names were accurate, they were unmemorable, and they changed whenever someone found a similar plant that required the description to be more specific.
Carl Linnaeus replaced that with two words. The tomato became Solanum lycopersicum, and every other known organism received a comparable pair. The system he established is still in use, still governed by rules descended from his, and still expanding at a rate of around eighteen thousand new species a year.
The Problem He Solved
European naturalists in the early eighteenth century were drowning. Ships were returning from the Americas, Africa, Asia and the Pacific with specimens by the thousand, and there was no agreed way to record them.
The polynomial names in use had two fatal defects. They were unstable, since adding a newly discovered relative forced the description to be rewritten, and they were unusable in conversation and in indexes. Two botanists in different countries could easily be discussing the same plant under different names, or different plants under the same one.
Linnaeus, born in 1707 in southern Sweden and trained in medicine at Lund, Uppsala and Harderwijk, was a compulsive organiser by temperament. The first edition of Systema Naturae, published in 1735 when he was twenty eight, was eleven pages: a set of large folding tables covering the three kingdoms of nature. By the thirteenth edition it ran past three thousand pages.
Two Words
The binomial system gives each species a name in two parts. The first is the genus, shared with close relatives. The second is the specific epithet, which distinguishes this species within the genus.
The name is not a description and does not have to be. It is a label, and this is the crucial design decision. Homo sapiens does not tell you much about human beings. What it does is remain valid regardless of how many further species are discovered, and provide a unique key that anyone can look up.
Linnaeus first used the method systematically in Species Plantarum in 1753, which is why that date is the formal starting point for botanical nomenclature. The tenth edition of Systema Naturae, published in 1758, serves the same purpose for animals. Names published before those dates have no standing, which gives the whole system a clean beginning.
Above the genus he arranged a hierarchy of ranks: kingdom, class, order, genus, species. Family and phylum were inserted later, but the principle of nested groups within groups is his.
The Sexual System
To sort the plants, Linnaeus used a classification based on their reproductive parts, counting the number and arrangement of stamens to determine the class and pistils to determine the order.
He was entirely aware that this was artificial. Grouping plants by stamen count does not reflect their real affinities, and he said so, describing it as a temporary arrangement pending a natural system that nobody yet knew how to construct. Its merit was that anyone with a lens and the ability to count could identify a plant in the field without training.
It was also, in the eighteenth century, faintly scandalous. Linnaeus described the arrangements in frankly marital language, referring to husbands and wives sharing a bed, and to classes where several husbands share one wife. The Prussian botanist Johann Siegesbeck denounced the system as loathsome harlotry and asked how such immorality could be taught to the young. Linnaeus named a small, unremarkable and rather ugly weed Sigesbeckia in his honour.
Putting Humans in the System
One decision stands out for its nerve. Linnaeus placed human beings in the same order as monkeys and apes, first as Anthropomorpha and then as Primates, on straightforward anatomical grounds.
This was a century before evolution provided a reason to expect the resemblance, and Linnaeus held to a fixed view of species: God created, Linnaeus organised, as his own motto had it. He defended the placement by challenging his critics to name a single anatomical character that separates humans from apes, and observed that as a naturalist he could not find one.
The same section of his work contains its worst legacy. He subdivided Homo sapiens into geographical varieties and attached descriptions of temperament and character to each. Those passages fed directly into the racial classification that European science pursued through the nineteenth century, with consequences that are well documented and grim. It is part of the record and it belongs in any honest account of what the taxonomy was used for.
The Apostles
Linnaeus stayed in Sweden, but he needed specimens from everywhere. His solution was to send his students.
Seventeen of them, whom he called his apostles, travelled on trading and naval expeditions to Japan, China, South America, Africa, North America, the Middle East and the Pacific, collecting and describing as they went. Daniel Solander sailed with Cook on the Endeavour. Carl Peter Thunberg spent years in Japan and produced the first modern flora of the country.
Seven of them died on their journeys, from fever, shipwreck or violence. The material they sent back is a substantial part of the collection on which the system was built.
What Happened to the Collection
Linnaeus died in 1778. His herbarium, library and correspondence were sold by his widow in 1784 to a young English naturalist, James Edward Smith, and shipped to London.
There is a persistent story that the King of Sweden dispatched a warship to intercept the vessel and recover the national treasure, arriving too late. It appears to be a nineteenth century embellishment. Smith founded the Linnean Society of London in 1788, and the collection remains there in a purpose built vault.
The Society’s rooms have a second claim on scientific history. It was at a meeting there on 1 July 1858 that the joint papers of Charles Darwin and Alfred Russel Wallace on natural selection were read aloud, to almost no reaction from those present.
The Hierarchy Turned Out to Mean Something
Linnaeus believed species were fixed, created in their present form, though late in life his work on hybridisation led him to soften the position. His nested hierarchy was intended as a convenient filing structure imposed on creation.
A century later it turned out to be something else. If species descend from common ancestors, then a branching tree is exactly what you would expect the pattern of similarities to form, and groups within groups is what a genealogy looks like when you cannot see the ancestors.
Darwin’s theory did not replace the Linnaean hierarchy. It explained why the hierarchy exists. Classification stopped being a matter of convenience and became a hypothesis about descent, testable against anatomy, the fossil record and, later, genetics.
The taxonomists who followed inherited a working structure. Humboldt’s expeditions catalogued thousands of species in Linnaean form while developing an entirely new way of thinking about how species distribute across climate and altitude, and Haeckel drew the tree explicitly as a tree.
The illustrated natural history that accompanied all of this is collected in Illustrating Nature, which brings together the plates through which the expanding catalogue of the living world was actually communicated.
Why It Survived
Linnaeus was not the first to propose a hierarchy or to use two part names occasionally. What he did was apply a single consistent method to every organism he could get his hands on, publish the result in a form other people could extend, and do it at a moment when the volume of new material made the old approach untenable.
The system has since absorbed evolution, the discovery of microorganisms, the reclassification of fungi, the splitting of prokaryotes into two domains, and molecular phylogenetics. The stamen counting is long gone. The two word name and the nested ranks are still standing, which for an eighteenth century filing system is a remarkable record.
Around 1.2 million species have been formally described. Estimates of the true number run to nearly nine million, most of them insects and fungi that nobody has looked at yet. Every one of them will get two words.
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