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classification, in biology, the systematic categorization of organisms into a coherent scheme. The original purpose of biological classification, or systematics, was to organize the vast number of known plants and animals into categories that could be named, remembered, and discussed. Modern classification also attempts to show the evolutionary relationships among organisms (see the table entitled Examples of Systematic Classification). A system based on categories that show such relationships is called a natural system of classification; one based on categories assigned only for convenience (e.g., a classification of flowers by color) is an artificial system.

Modern classification is part of the broader science of taxonomy, the study of the relationships of organisms, which includes collection, preservation, and study of specimens, and analysis of data provided by various areas of biological research. Nomenclature is the assigning of names to organisms and to the categories in which they are classified.

A modern branch of taxonomy, called numerical taxonomy, uses computers to compare very large numbers of traits without weighting any type of trait—in contrast to the traditional view that certain characteristics are more significant than others in showing relationships. For example, the structure of flower parts is considered more significant than the shape of the leaves in flowering plants because leaf shape appears to evolve much more quickly. Much of the science of taxonomy has been concerned with judging which traits are most significant. If new evidence reveals a better basis for subdividing a taxon than that previously used, the classification of the group in question may be revised. A considerable number of classification changes as well as insights in recent years have been the result of comparisons of nucleic acid (genetic material) sequences of organisms.

See also cladistics.

The Kingdoms

The broadest division of organisms has been into kingdoms. Traditionally there were two kingdoms, Animalia and Plantae, but many unicellular and simple multicellular organisms are not easily classified as either plants or animals. In 1866 the zoologist Ernst Heinrich Haeckel proposed a third kingdom, the Protista, to include all protozoans, algae, fungi, and bacteria. In the 20th cent. his proposal was refined, and a grouping became widely accepted that was made up of five kingdoms: animals; plants; Protista, including protozoans and some algae; Monera, comprising the prokaryotic bacteria and cyanobacteria (blue-green algae); and Fungi. Other groupings have been proposed from time to time.

Analysis of genetic sequences in various organisms has recently suggested placement of the Archaebacteria into a separate major group called the archaea. In this system, the second and third major groups are the other bacteria and the eukarya (or eukaryotes), organisms that have cell nuclei and include the fungi, plants, and animals.

The Lower Taxa

Kingdoms are divided into a hierarchical system of categories called taxa (sing. taxon). The taxa are, from most to least inclusive: phylum (usually called division in botany), class, order, family, genus, and species. Intermediate divisions, such as suborder and superfamily, are sometimes added to make needed distinctions. The lower a taxon is in the hierarchy, the more closely related are its members.

The species, the fundamental unit of classification, consists of populations of genetically similar interbreeding or potentially interbreeding individuals. If two populations of a species are completely isolated geographically and therefore evolve separately, they will be considered two species once they are no longer capable of mixing genetically if brought together. In a few cases interbreeding is possible between members of closely related species—for example, horses, asses, and zebras can all interbreed. The offspring of such crosses, however, are usually sterile, so the two groups are nonetheless kept separate by their genetic incompatibility. Populations within a species that show recognizable, inherited differences but are capable of interbreeding freely are called subspecies, races, or varieties.

The genus (pl. genera) is a grouping of similar, closely related species. For example, the domestic cat and the jungle cat are species of the genus Felis; dogs, wolves, and jackals belong to the genus Canis. Often the genus is an easily recognized grouping with a popular name; for example, the various oak species, such as black oak and live oak, form the oak genus (Quercus). Similarly, genera are grouped into families, families into orders, orders into classes, and classes into phyla or divisions.

Binomial Nomenclature

The present system of binomial nomenclature identifies each species by a scientific name of two words, Latin in form and usually derived from Greek or Latin roots. The first name (capitalized) is the genus of the organism, the second (not capitalized) is its species. The scientific name of the white oak is Quercus alba, while red oak is Quercus rubra. The first name applies to all species of the genus—Quercus is the name of all oaks—but the entire binomial applies only to a single species. Many scientific names describe some characteristic of the organism (alba=white; rubra=red); many are derived from the name of the discoverer or the geographic location of the organism. Genus and species names are always italicized when printed; the names of other taxa (families, etc.) are not. When a species (or several species of the same genus) is mentioned repeatedly, the genus may be abbreviated after its first mention, as in Q. alba. Subspecies are indicated by a trinomial; for example, the southern bald eagle is Haliaeetus leucocephalus leucocephalus, as distinguished from the northern bald eagle, H. leucocephalus washingtoniensis.

The advantages of scientific over common names are that they are accepted by speakers of all languages, that each name applies only to one species, and that each species has only one name. This avoids the confusion that often arises from the use of a common name to designate different things in different places (for example, see elk), or from the existence of several common names for a single species. There are two international organizations for the determination of the rules of nomenclature and the recording of specific names, one for zoology and one for botany. According to the rules they have established, the first name to be published (from the work of Linnaeus on) is the correct name of any organism unless it is reclassified in such a way as to affect that name (for example, if it is moved from one genus to another). In such a case definite rules of priority also apply.


The earliest known system of classification is that of Aristotle, who attempted in the 4th cent. B.C. to group animals according to such criteria as mode of reproduction and possession or lack of red blood. Aristotle's pupil Theophrastus classified plants according to their uses and methods of cultivation. Little interest was shown in classification until the 17th and 18th cent., when botanists and zoologists began to devise the modern scheme of categories. The designation of groups was based almost entirely on superficial anatomical resemblances.

Before the idea of evolution there was no impetus to show more meaningful relationships among species; the species was thought to be uniquely created and fixed in character, the only real, or natural, taxon, while the higher taxa were regarded as artificial means of organizing information. However, since anatomical resemblance is an important indication of relationship, early classification efforts resulted in a system that often approximated a natural one and that—with much modification—is still used. The most extensive work was done in the mid-18th cent. by Carolus Linnaeus, who devised the presently used system of nomenclature. As biologists came to accept the work of Charles Darwin in the second half of the 19th cent., they began to stress the significance of evolutionary relationships for classification.

Although comparative anatomy remained of foremost importance, other evidence of relationship was sought as well. Paleontology provided fossil evidence of the common ancestry of various groups; embryology provided comparisons of early development in different species, an important clue to their relationships. In the 20th cent., evidence provided by genetics and physiology became increasingly important. Recently there has been much emphasis on the use of molecular genetics in taxonomy, as in the comparison of nucleic acid sequences in the genetic makeup of organisms. Computers are increasingly used to analyze data relevant to taxonomy.


See E. Mayr, Principles of Systematic Zoology (1969); T. Savory, Animal Taxonomy (1972); H. M. Hoenigswald and L. F. Wiener, eds., Biological Metaphor and Cladistic Classification (1987); F. A. Stafleu and R. S. Cown, Taxonomic Literature: A Selective Guide to Botanical Publications and Collections (1988); N. Eldredge, Fossils: The Evolution and Extinction of Species (1991).

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  1. any attempt to identify regularly occurring types of social structure, e.g. types of society, types of organization, types of social relationship.

    In biology, the classification of animals and plants, which has sometimes been used as a model for sociological classification, has operated according to two main principles:

    1. Linnaean classification (synchronic) of mutually exclusive possibilities;
    2. the arrangement of types as an evolutionary (diachronic) sequence, representing evolutionary relationships.

    While purists may claim that all particular phenomena are different and never absolutely identical, the aim of any classification is to place together all instances of a phenomenon whose similarities, and differences from other types of phenomena, are such as to justify the classification for particular theoretical purposes. See also TAXONOMY.

  2. (SOCIOLOGY OF EDUCATION) the identification of the boundaries between different forms of human knowledge. In the formal educational process this relates to the organization of knowledge into curricula, or the various domains of educational activity The term is a key concept in BERNSTEIN's theory of knowledge codes.
Collins Dictionary of Sociology, 3rd ed. © HarperCollins Publishers 2000
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a system of coordinated concepts (classes of objects) in any area of human knowledge or activity, frequently presented in the form of diagrams (tables) and used as a means for establishing relationships between these concepts or classes of objects, as well as for an exact orientation amid the diversity of concepts or the corresponding objects.

A classification should establish the natural relationships between the classes of objects, with the goal of defining an object’s place in the system; the definition should also indicate the object’s properties. This aspect of classification serves as a means for the storage and retrieval of the information in the classification itself—for example, biological taxonomies, the classification of chemical elements (as in D. I. Mendeleev’s periodic system of elements, the classification of sciences, and the classification of metallurgical processes. Another objective of classification is to provide effective retrieval of information or items kept in special storage (data files, archives, and warehouses); examples include library classification systems, information retrieval languages, and product classifiers.

A truly scientific classification should express the system of laws, inherent to the aspect of reality depicted in it, that determines the properties and relationships of the objects fixed in the classification. The systematization of these laws must take into account that in nature there are no strict delimitations and that transitions from one class to another are an inseparable property of reality. This demand upon classification is reflected in such special procedures as the use in library classifications of cross-references (“see” and “see also”) and the location of the same concept in different places of the classification.

Classification contributes to the advancement of science and the branches of technology from the level of the empirical accumulation of knowledge to the level of theoretical synthesis and the systems approach. Such a transition is possible only with a theoretical analysis of the multiplicity of facts. The practical need for classification encourages the development of the theoretical aspects of science or technology, while the creation of a classificatory system represents a qualitative leap in the development of knowledge. Classification based upon profound scientific principles not only represents a full picture of the state of the science (or technology) or one of its aspects but also makes it possible to draw up valid predictions of still unknown facts or laws. An example is the prediction of properties of still undiscovered elements by the Mendeleev system.

When a classification is a system of coordinated concepts, its structure can sometimes be depicted in the form of an inverted tree. The most general concept corresponds to the node that is the root, the most particular concepts are the leaves, and the names of the remaining classes correspond to the nodes of the branches. The segments that connect all these points express the relationship of subordination that governs the general concepts of varying degrees of abstraction. Lines running from the root to the leaves are called the vertical series of classification; levels equidistant from the general subordinating concept form the horizontal series. Thus, in the Universal Decimal Classification for printed works, the concept of the entire aggregate of printed works corresponds to the root, and the concept is then divided into ten main classes, and so forth.

There are two ways to form the tables of a classification, deductive and inductive. The deductive approach consists in establishing the initial general concepts and the criteria for subdivision. The elucidation of subordinate concepts occurs in the division of the subordinating concept, and the unity of the criteria for subdivision and the stability of classification are guaranteed by the very method of its construction. The inductive approach is based on the concepts of individual objects or aggregates of objects and unites them into classes. The achievement of logical unity and stability of classification becomes more difficult than with the former method. Usually classifications are constructed using both approaches. The more general classes, as a rule, are formed deductively, while the less general ones are formed inductively. Preference is given to deduction in systematizing areas of knowledge and to induction in elaborating the factual material and formulating it in diagrams and tables.

A distinction is made between natural and artificial classifications depending on the degree of essentiality of the criteria for subdivision. If essential features are used as the criteria and if the maximum of derivative features can be derived from these features, so that the classification can serve as the source of knowledge of the objects being classified, then such classification is termed natural (for example, the periodic system of chemical elements). If unessential features are used in the classification, the classification is considered artificial. Artificial classifications include auxiliary classifications (alphabetical subject indexes and name catalogs in libraries). Depending on their breadth, classifications can be encyclopedic (universal), specialized (for one branch of knowledge), or narrow (covering a limited range of uniform phenomena).

At times the term “classification” is used to designate the process of putting objects in classes. Here it would be more correct to use the term “classifying.” The basic principle of this process is the comparing of the designated items with set models or standard representatives of the classes. This principle is used, for example, in biological taxonomies and also lies at the basis of the algorithms for the automatic classification of documents or figures (image recognition).

The problem of the construction and use of classifications has become particularly acute during the modern scientific and technical revolution that has led to the information explosion. The abundance and difficulty of systematization of new concepts and terms, as well as printed and unpublished materials, impede the retrieval and use of the required data, causing an information scarcity that impedes social progress. For this reason the elaboration of an optimal classification becomes not only a scientific but also an economically important problem.




(in beneficiation of minerals), the separation of fine materials into individual size classes using the differences in the size, shape, and other specific characteristics of the particles being separated. Large pieces (up to 2–4 mm) are divided into classes by screening or sifting. The use of screens for dividing fine materials into individual size classes is difficult, since the fine mesh of the screens becomes clogged, and sifting has low output. The difference in the rate of motion of particles in water or air under the influence of gravity or centrifugal force is most commonly used in classification. In such cases, the speed depends mainly on the size of the particle, but the density and shape are also essential. For example, the rate of fall of a large particle with lower density and a small particle with greater density may be the same. Flat particles fall more slowly than round particles. Therefore, the clearest division of particles by size occurs when the density and shape characteristics are close.

The difficulty of classification increases with decreasing particle size. Very fine particles (less than 10 microns) adhere strongly to each other—that is, they coagulate or flocculate, disrupting the clarity of separation. For the clear separation of fine particles, they must be separated or peptized by adding special peptizing agents, which prevent the cohesion of fine particles. Fine particles descend very slowly in water under the force of gravity, and the classification process becomes unproductive. Devices must be used in which the force of gravity is replaced by centrifugal force, which is hundreds of times stronger than gravity.

Depending on the medium in which the separation of particles by size occurs, a distinction is made between wet (hydraulic) and dry (pneumatic) classification. The advantage of the former is the possibility of classification of wet materials and suspensions, as well as the better separation of cohering particles using peptizing agents. The advantage of dry classification is the elimination of complex drying processes, which increase the cost of classification and sometimes impair the properties of powders.

The theory of classification examines the movement of solid particles in a liquid or gaseous medium on the basis of Stokes’ law, according to which the small particles settle at a speed directly proportional to the square of the cross section and density of the particles and inversely proportional to the viscosity of the medium. When the density of the suspension is relatively high, the rate of fall of the particles is retarded to such a degree that even large particles do not settle out over the time they are in the classifying equipment.

Classification methods have also been developed in which the particles are given a certain electric charge (electric separators); such methods are used for the classification of small quantities of valuable products. Classification is performed in special devices called classifiers. The choice of the classification method depends on the characteristics of the material and the required output, as well as the tasks of the process. Wet classification usually provides optimum sizing of the material before gravity concentration and flotation. Classification sometimes makes possible the output of finished products whose grade is determined by their size (for example, in concentrating kaolin and asbestos and in producing abrasive powders); in such cases various classification methods are possible, including pneumatic classification (in the concentration of asbestos). A special use of classification is to characterize the fineness of powders using sedimentation analysis, which can be performed only under laboratory conditions.


Liashchenko, P. V. Gravitatsionnye metody obogashcheniia [2nd ed.]. Moscow, 1940.
Eigeles, M. A. Obogashchenie nemetallicheskikh poleznykh iskopaemykh. Moscow, 1952.
Pol’kin, S. I. Obogashchenie rud. Moscow, 1953.
Olofinskii, N. F. Elektricheskie metody obogashcheniia, 3rd ed. Moscow, 1970.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


Sorting out or categorizing of particles or objects by established criteria, such as size, function, or color.
Stratification of a mixture of various-sized particles (that is, sand and gravel), with the larger particles migrating to the bottom.
(industrial engineering)
Placing of military documents in special groups for safeguarding defense information.
A systematic arrangement of plants and animals into categories based on a definite plan, considering evolutionary, physiologic, cytogenetic, and other relationships.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


1. Biology the placing of animals and plants in a series of increasingly specialized groups because of similarities in structure, origin, molecular composition, etc., that indicate a common relationship. The major groups are domain or superkingdom, kingdom, phylum (in animals) or division (in plants), class, order, family, genus, and species
2. Government the designation of an item of information as being secret and not available to people outside a restricted group
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005