Class (taxonomy)

History

The concept of class as a formal taxonomic rank with its own distinct name originated with French botanist Joseph Pitton de Tournefort in his 1694 work on plant classification. Historically, a class has been understood to group together taxa sharing both a specific level of organizational complexity—based on how differentiated their organ systems are—and a particular structural arrangement of those systems. In practice, however, taxonomists ultimately decide the membership of each class based on their own judgment. When Carl Linnaeus published the first edition of his Systema Naturae in 1735, he applied classes to all three of his kingdoms: minerals, plants, and animals. Only his animal classes resemble modern usage; his plant classes and orders were designed as an artificial key, relying heavily on flower structure under his Systema Sexuale. Today, classes are seldom used in botany. Following the APG system introduced in 1998, which classifies flowering plants only down to the order level, many references now treat ranks above orders as informal clades. When formal ranks are still applied, they have been lowered considerably, as seen with class Equisitopsida for land plants, where major subdivisions are handled as subclasses and superorders. The class was regarded as the highest taxonomic rank until the early nineteenth century, when George Cuvier introduced embranchements, later termed phyla by Ernst Haeckel.