In biological classification
, class ( la|classis) is a taxonomic rank
, as well as a taxonomic unit, a taxon
, in that rank. Other well-known ranks in descending order of size are life
, and species
, with class fitting between phylum and order.
The class as a distinct rank of biological classification having its own distinctive name (and not just called a ''top-level genus'' ''(genus summum)'') was first introduced by the French botanist Joseph Pitton de Tournefort
in his classification of plants that appeared in his ''Eléments de botanique'', 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 judgement of taxonomist
s. Often there is no exact agreement, with different taxonomists taking different positions. There are no objective rules for describing a class, but for well-known animals there is likely to be consensus.
In the first edition of his ''Systema Naturae
'' (1735), Carl Linnaeus
divided all three of his kingdoms
of Nature (mineral
s, and animal
s) 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 plant
s up to the level of orders, many sources have preferred to treat ranks higher than orders as informal clade
s. 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
[Collins, A.G., Valentine, J.W. (2001)]
"Defining phyla: evolutionary pathways to metazoan body plans."
''Evol. Dev''. 3: 432-442.
were introduced in the early nineteenth century.
Hierarchy of ranks below and above the level of class
As with the other principal ranks, Classes can be grouped and subdivided. Here are some examples.
* List of animal classes