Classification by “natural characters” of Carolus Linnaeus

Linnaeus did not consider the sexual system to be his main contribution toward the “reformation of botany” to which he aspired. His main contribution came in the form of a booklet, Fundamenta Botanica (1736; “The Foundations of Botany”), that framed the principles and rules to be followed in the classification and naming of plants.

In 1735 Linnaeus met Boerhaave, who introduced Linnaeus to George Clifford, a local English merchant and banker who had close connections to the Dutch East India Company. Impressed by Linnaeus’s knowledge, Clifford offered Linnaeus a position as curator of his botanical garden. Linnaeus accepted the position and used this opportunity to expand certain chapters of Fundamenta Botanica in separate publications: Bibliotheca Botanica (1736; “The Library of Botany”); Critica Botanica (1737; “A Critique of Botany”), on botanical nomenclature; and Classes Plantarum (1738; “Classes of Plants”). He applied the theoretical framework laid down in these books in two further publications: Hortus Cliffortianus (1737), a catalogue of the species contained in Clifford’s collection; and Genera Plantarum (1737; “Genera of Plants”), which modified and updated definitions of plant genera first offered by Tournefort.

Genera Plantarum was considered by Linnaeus to be his crowning taxonomic achievement. In contrast to earlier attempts by other botanists at generic definition, which proceeded by a set of arbitrary divisions, Genera Plantarum presented a system based on what Linnaeus called the “natural characters” of genera—morphological descriptions of all the parts of flower and fruit. In contrast to systems based on arbitrary divisions (including his own sexual system), a system based on natural characters could accommodate the growing number of new species—often possessing different morphological features—pouring into Europe from its oversea trading posts and colonies.

Linnaeus’s distinction between artificial and natural classifications of organisms, however, raised the question of the mechanism that allowed organisms to fall into natural hierarchies. He could only answer this question with regard to species: species, according to Linnaeus, were similar in form because they derived from the same parental pair created by God at the beginning of the world. Many of his contemporaries shared a similar species concept. One such notable personage was French naturalist Georges-Louis Leclerc de Buffon, who was engaged in a similar all-encompassing natural history project at the time—though Buffon doubted the existence of natural genera, orders, or classes. Linnaeus tried to explain the existence of these divisions within the context of hybridization; however, the question of natural hierarchies would not receive a satisfying answer until English naturalist Charles Darwin explained similarity by common descent in his Origin of Species (1859).

Binomial nomenclature

Linnaeus returned to Sweden in 1738 and began a medical practice in Stockholm. In 1739 he married Sara Elisabeth. He practiced medicine until the early 1740s but longed to return to his botanical studies. A position became available at Uppsala University, and he received the chair in medicine and botany there in 1742. Linnaeus built his further career upon the foundations he had laid in the Netherlands. He used his international contacts to create a network of correspondents that provided him with seeds and specimens from all over the world. He then incorporated this material into the botanical garden at Uppsala, and these acquisitions helped him develop and refine the empirical basis for revised and enlarged editions of his major taxonomic works. During his lifetime he completed 12 editions of Systema Naturae, 6 editions of Genera Plantarum, 2 editions of Species Plantarum (“Species of Plants,” which succeeded the Hortus Cliffortianus in 1753), and a revised edition of Fundamenta Botanica (which was later renamed Philosophia Botanica [1751; “Philosophy of Botany”]). Furthermore, all these works appeared in countless pirated versions, translations, and popular adaptations in all major European languages.

Linnaeus’s most lasting achievement was the creation of binomial nomenclature, the system of formally classifying and naming organisms according to their genus and species. In contrast to earlier names that were made up of diagnostic phrases, binomial names (or “trivial” names, as Linnaeus himself called them) conferred no bias about the quality or value of plant species named. Rather, they served as labels by which a species could be universally addressed. This naming system was also implicitly hierarchical, as each species is classified within a genus. The first use of binomial nomenclature by Linnaeus occurred within the context of a small project in which students were asked to identify the plants consumed by different kinds of cattle. In this project, binomial names served as a type of shorthand for field observations. Despite the advantages of this naming system, binomial names were used consistently in print by Linnaeus only after the publication of Species Plantarum (1753).

In his own lifetime, Linnaeus became something of an institution in himself, as naturalists everywhere had to address him directly or at least his work in order to determine whether specimens in their collections were indeed new species. The rules of nomenclature that he put forward in his Philosophia Botanica rested on a recognition of the “law of priority,” the rule stating that the first properly published name of a species or genus takes precedence over all other proposed names. These rules became firmly established in the field of natural history and also formed the backbone of international codes of nomenclature—such as the Strickland Code (1842)—created for the fields of botany and zoology in the mid-19th century. The first edition of Species Plantarum and the 10th edition of Systema Naturae (1758) are the agreed starting points for botanical and zoological nomenclature, respectively.