The class as a distinct rank of biological classification having its own distinctive name – and not just called a
top-level genus () – was first introduced by
French botanist Joseph Pitton de Tournefort in the classification of plants that appeared in his
Eléments de botanique of 1694. Insofar as a general definition of a class is available, it has historically been conceived as embracing taxa that combine a distinct
grade of organization—i.e. a 'level of complexity', measured in terms of how differentiated their organ systems are into distinct regions or sub-organs—with a distinct
type of construction, which is to say a particular layout of organ systems. This said, the composition of each class is ultimately determined by the subjective judgment of
taxonomists. In the first edition of his
Systema Naturae (1735),
Carl Linnaeus divided all three of his
kingdoms of nature (
minerals,
plants, and
animals) into classes. Only in the animal kingdom are Linnaeus's classes similar to the classes used today; his classes and orders of plants were never intended to represent natural groups, but rather to provide a
convenient "artificial key" according to his
Systema Sexuale, largely based on the arrangement of flowers. In botany, classes are now rarely discussed. Since the first publication of the
APG system in 1998, which proposed a taxonomy of the
flowering plants up to the level of orders, many sources have preferred to treat ranks higher than orders as informal
clades. Where formal ranks have been assigned, the ranks have been reduced to a very much lower level, e.g. class Equisitopsida for the land plants, with the major divisions within the class assigned to subclasses and superorders. The class was considered the highest level of the taxonomic hierarchy until
George Cuvier's
embranchements, first called
Phyla by
Ernst Haeckel, were introduced in the early nineteenth century. == See also ==