CLASSIFICATION. — It is the operation by which a multitude of similar objects is framed into a hierarchical system of groupings, with a view to facilitating their study. As a factor in knowledge, classification is at the root of the problem of cognition and extends to all collective entities whose components display peculiarities, that is, common and differential characteristics gradable in such a way as to justify their ordering in a system. The study of these characteristics is therefore a necessary prerequisite for the establishment of groups (categories or complexes) of varying order or hierarchical level. Hence we find a classification of ideas, originating with Socrates and developed by Plato, Aristotle and later philosophers; a classification of the sciences, outlined by Auguste Comte; and finally classifications pertaining to the objects proper to individual disciplines.
Classification is essentially a problem of method, that is, of criteria in evaluating characteristics and of procedure in forming groups. The criteria vary according to the material to be classified, the aims to be pursued and the classifier’s point of view; the procedure is divisive when it descends from the general to the particular on the basis of differential characteristics, and aggregative when it ascends from the particular to the general on the basis of affinities. The choice of method is important when objects of nature are considered which compelling reasons lead one to believe are already naturally ordered in a system. In that case, the classification is natural if it faithfully reflects this system and, given its cognitive value, admits of no arbitrary solutions; it is artificial otherwise, and, given its purely recognitive purpose, may be arbitrary provided it is rational and convenient. Natural classification is hindered by gaps in knowledge of the objects to be classified, by the impossibility of sharply delimiting groups connected by common characteristics, by the ease with which subjectivism can intrude in assessing the discriminative and hierarchical value of the characteristics chosen for identifying the groups, and finally by the difficulty of choosing between the divisive and the aggregative procedure: the former, which goes back to the Platonic dichotomy
Classification
The principle of definitions "per genus proximum et differentiam specificam" often appears more convenient and even more spontaneous as a mental approach when faced with the task of sorting a multitude of objects; the second principle may be recommended by the greater intuitive manageability of narrower groupings compared to broader ones, and thus by the opportunity to proceed from the better known to the less known.
In biology, classification has two aims: to enable the determination, i.e., the identification of an animal or plant that has been given a specific name through nomenclature, and to establish its position within the proposed system; and to fit it as faithfully as possible into the natural system, i.e., the natural distribution of living beings into hierarchically subordinate groups or taxonomic complexes resulting from the sum of various kinds of affinities among their components. The first aim can be served by any classification, but only natural classification can meet the second.
Attempts to reconstruct the natural system, which constitute much of the history of zoology and botany, were often influenced by a secondary objective that served as a stimulus to research but became harmful when adopted as an a priori guide—namely, the search for the plan of organization or even the creation of living beings. This was later replaced, with the advent of evolution (v.), by the search for phylogeny. In both cases, disputes arose between supporters of the fundamental or original unity of organisms and those advocating their multiplicity. Contributing to the a priori undermining of the possibility of natural classification were, first, the conception of a distribution of living beings in a continuous series of increasing complexity, and later, evolutionary doctrine: the former, already suggested by Aristotle and later dominant among many systematists of the 18th century, a priori sanctioned the artificiality of any "watertight compartments" in classification; the latter declared every classification fallacious, since its necessarily static frameworks cannot reflect the dynamism of the evolutionary processes acting on individuals, so that the consistency of various taxonomic complexes would not be a reality but a transient appearance. Hence the pernicious dualism between a so-called static systematics and a dynamic systematics—the former the province of the classifier relegated to the status of a librarian or thereabouts, the latter the domain of the evolutionist and general biologist. Efforts are now being made to overcome this situation by considering, on the one hand, the slowness of evolutionary processes affecting large systematic groups (macroevolution) and thus the theoretical possibility of depicting the present system, and on the other, by devising methods to objectively detect and adopt as practical limits the natural discontinuities found among various taxonomic complexes. In fact, the definition of not a few groups of animals and plants can now be considered satisfactory, as evidenced by the overall stability they have achieved over time and often through diverse means.
The gradual emergence of natural classification was aided by: the expansion of study material resulting from geographical discoveries, paleontology, and microscopy; the deepening of knowledge of characters due to progress in all biological disciplines; and finally, the increase in comparisons among various organisms. Thus, the classifications in use were modified as new criteria were added or replaced earlier ones: various groups underwent regrouping or, more often, splitting revisions and were enriched in the number of constituent units; moreover, new intermediate levels were introduced between those already established. The meaning and designation of these levels were clarified, while descriptions of individual complexes were standardized: the first task was largely accomplished through the adoption—largely dating back to Linnaeus—of progressively less inclusive terms such as type, class, order, family, genus, species, race, to which various intermediate grades were added (subtype, subgenus, or superfamily, etc.) and sometimes other categories of varying rank (tribe, cohort, legion, phalanx, etc.); the second task, also initiated by Linnaeus, involved the adoption of Latin in concise descriptive diagnoses and in the naming of individual groups, and the introduction of binomial nomenclature (sometimes trinomial or polynomial when determinations reached subspecific levels), according to which the name of each species must be preceded by the name of the genus and followed by the initials of the first descriptor. Attempts were made to address the inconveniences of homonymy and synonymy arising from the multiplication of determinations through international rules of nomenclature; however, it still frequently happens that the same species and genera are designated by different names by different authors, and this confused instability in terminological formalism adds to the instability inherent in the substance of classifications themselves, contributing to a certain persistent unease in systematic studies.
Historically, botanical research proceeded in a more uniform but less profound manner than zoological research, due to differences in organization and life between plants and animals and also because of the differing practical interest in them. Indeed, classifications of plants were mostly general but based primarily on external characters; whereas those of animals were often specialized to narrower, more easily studied or more attractive groups, but in compensation they early delved into anatomical and physiological investigation.
The history, traditions (see, among other things, the second chapter of Genesis) and ethnological and linguistic studies attest that humanity had to become accustomed from the very beginning to recognize and designate with appropriate names the most familiar animals and plants. The first reliable documents of systematic biology date back to the 5th century B.C.: for botany Empedocles of Agrigento is remembered, for zoology the classification of the school of Cos attributed to Herodotus of Selimbria and that contained in Plato’s Timaeus are noted. But the first true systematist was Aristotle, to whom credit is due for having defined in a naturalistic sense the Platonic concepts of species (εἶδος) and genus (γένος), subordinating the former to the latter, and for having based the classification of animals on intrinsic characteristics of organisms, not only morphological but also anatomophysiological. He divided species into eight great genera (γένη μεγάλα), four of animals “with blood” (αἱματοῦντα) — namely mammals (ζῷα ἔναιμα καὶ ζῳοτόκα = viviparous animals), birds (ὄρνιθες), reptiles and amphibians (τετράποδα καὶ ἀπτερά ἀναιμόφυτα = quadrupeds and apodal oviparous animals) and fish (ἰχθύες) — and four of animals “without blood” (ἀναίματοι) — namely molluscs (μαλάκια), crustaceans (μαλακόστρακα), insects (ἕντομα) and testaceans (ὀστρακόδερμα). Much more artificial is the classification of plants carried out by Aristotle’s two disciples, Theophrastus and Phanias:
Classification — Example of a comparative synoptic table on the classification of arthropods according to Linnaeus, De Geer, Geoffroy and Fabricius.
They divided plants into trees (δένδρα), shrubs (θάμνοι), subshrubs (χέρσοι) and herbs (πόαι) and distinguished flowering plants (=phanerogams) from flowerless plants (=cryptogams).
For many centuries Aristotle’s work remained conceptually unsurpassed, even though it was enriched in the number of species. Indeed, many Roman classifications (such as those of Dioscorides and Pliny in the 1st century A.D.) and medieval ones (such as those contained in bestiaries and herbals) represent a regression, both because they were mere alphabetical lists or were based on extrinsic characteristics of organisms and often anthropocentric (such as habitat, edibility, usefulness, etc.) and because they lacked any critical spirit and were often filled with fabulous animals and plants. Apart from the isolated contributions of Albertus Magnus, Leonardo da Vinci and a few other pioneers, it is necessary to reach the mid-16th century to find the germs of that renewal in naturalistic studies which was then given powerful impetus by the Galilean experimental method. Among the botanists, the following deserve mention: Andrea Cesalpino of Arezzo, who in the treatise De plantis (1583) outlined an artificial classification, albeit one almost entirely based on the characteristics of fructification, yet so rational as to serve as a model for Linnaeus; J. Pitton de Tournefort and John Ray, who in the Historia plantarum (1686) distinguished monocotyledons from dicotyledons. More closely aligned with Aristotelian models are the contemporary works of zoology, among which those of the Bolognese Ulisse Aldrovandi (1522–1605) stand out; he authored a Historia naturalis celebrated by Buffon and Cuvier and a Syntaxis animalium in which the dichotomous method is clarified and applied; this method was also adopted by Ray in the Synopsis methodica animalium (1693), while the Historiae animalium of Conrad Gesner (1551–87) are minutely descriptive. In the 17th century a new field opened for systematic inquiry with the invention of the microscope, with which Leeuwenhoek discovered infusoria (1675) and Marcello Malpighi deepened studies of anatomy (1671).
With Carl Linnaeus (1707–1778) the golden age of systematics began, leading to modern classifications. It is marked by the intertwining of classifications that are openly artificial with others that tend toward the natural: in the former, the dichotomous procedure prevails; the various taxonomic entities are defined rigorously, characters are examined and graded individually according to absolute scales and with preference for the most conspicuous ones; in the latter, less exact and complete but more significant, the ascending path is preferred: divisions are less decisive, characters are studied in their relationships and graded according to relative scales based on their biological significance without preconceived precedence. In the Systema naturae (1735) Linnaeus bases classification on genera as the first collective entities of the hierarchy and species as the component units: he considers these fixed and distinct from the beginning (tot enumeramus species quot ab initio creavit infinitum Ens) and disregards minor variations (caristates laevissimas non curat botanicus). About 10,000 species of plants are described with concise precision, divided into 24 classes mainly on the basis of the characteristics of stamens and pistils, and 5,000 species of animals are distributed into 6 classes (mammals, birds, amphibians, fish, insects, worms). The rationality and convenience of Linnaean schemes favoured their prompt acceptance, even though the author himself had warned of their artificiality — so much so that in other works (Classes plantarum, Genera plantarum, Philosophia botanica) he adopted the natural method, based on the detection of multiple affinities among organisms, to faithfully portray the groups most easily identifiable. It was solely for this purpose that A. L. de Jussieu (1748–1836) strove: in Genera plantarum (1789), in proposing a new classification, he observed that to reveal natural groups it is not enough to count characters but it is necessary to consider their correlations and “weigh” them, grading and subordinating them on the basis of their constancy, which is an index of their significance and value. The natural method was also embraced by Lamarck, who in the Flore française (1778), however, deliberately provided an example of artificial classification purely for purposes of determination and introduced into botany the dichotomous method of analytical keys, still widely used today. Other notable botanical classifications are due to A. P. de Candolle, J. Lindley and S. L. Endlicher.
G. L. Leclerc de Buffon (1707–88) declared himself an enemy of the arid and artificial “systems” of Linnaean stamp: he framed his Histoire naturelle (1749–88) on the principle that the only secure criterion for distinguishing species is that of intraspecific fertility and the sterility of hybrids, a criterion that is still among the most accepted today; his classification, imprecise and incomplete, is informed by evolutionary doctrine and the idea of the unity of the living world. This idea is also shared by Étienne Geoffroy Saint-Hilaire (1772–1844), who in Philosophie anatomique (1818–22) and Principes de philosophie zoologique (1830) introduces the concept of homology, advances the principle of the “balance” of organs and gives greatest emphasis to topographic anatomy as revealing the relationships among organs and their variations in the animal scale. A follower of Charles Bonnet, J. B. de Monet de Lamarck (1744–1829) upholds the conception of the linear continuity of living beings, divided into the two kingdoms of plants and animals, and strives to
Inform the reader that, by its merit, it arrives at the precise definition of many groups, especially of invertebrates. In his numerous works (among which stand out the *Philosophie zoologique*, the *Histoire naturelle des animaux sans vertèbres*, and the *Système analytique des connaissances positives de l'homme*), Lamarck relates animal organizations to that of man, considering each of these as a “more or less pronounced degradation of this and, reciprocally, this as a ‘progress’ of those.” And, from the consideration of such progress as an index of the “march of nature toward a growing perfection,” he is led to enunciate his evolutionary theory (v. LAMARCKISM) as a logical interpretation of the affinities among living beings and of the ascent in their organization: hence the outline of those phylogenetic trees later widely developed by evolutionists and culminating with E. Haeckel.
In agreement with Lamarck and Geoffroy on the systematic importance of comparisons and on the concept of degradation, but a determined opponent of evolutionary and unitary ideas, is Georges Cuvier (1769–1832). The principal merits found in his many works and in particular in the *Règne animal* (1817) are: the introduction of comparative physiological anatomy and paleontology among taxonomic criteria, the enunciation and application of the principles of coexistence and correlation among organs and of the subordination of characters, and finally the establishment of four *embranchements* (later called types by De Blainville) representing as many irreducible and distinct fundamental plans of organization: these types are those of the vertebrates (with the classes of mammals, birds, reptiles, and fishes), of the mollusks, of the articulates (with the classes of worms, crustaceans, insects), and of the radiates. Cuvier’s systematics, in its essentials, is still followed today with the modifications introduced especially by T. Von Siebold and by R. Leuckart.
With Cuvier’s death, the period in which the term “naturalist” was equivalent to “systematist” comes to a close. After him, research continues but becomes fragmented in the works of specialists of individual groups; each revises and enriches his own field unevenly at the middle and lower levels of the systematic hierarchy, while general zoologists and botanists touch up the higher levels. The interest of naturalists turns to other biological disciplines that develop rapidly in the 19th and 20th centuries and from which important modifying contributions to criteria and methods flow into systematics: thus to the linear morphological criterion and to Cuvier’s anatomical-physiological and paleontological criteria are added, with Von Baer, the embryological criterion; with Darwin and the evolutionists, the phylogenetic criterion; with Wallace, Wagner, and Turesson, the eco-geographical criterion; with Mendel, De Vries, and modern geneticists, the genetic criterion; with Le Dantec, Nilow, and Souèges, the biochemical criterion. With the adoption of biometric-statistical methodology, framed by Quetelet, applied by Heineke, and framed by Johannsen in genetics, and with experiments intended to establish the capacity for variation in species and to separate genetic elements from environmental ones, systematics is directed onto the experimental path, particularly fruitful at the subspecific levels, while individual analysis is replaced by that of populations and the allegedly exact determination tends to be replaced by the probabilistic.
Although in the practice of systematics the linear schematism still counts many followers and although evident defects persist in the evaluation of characters (such as their isolated and unilateral analysis, the preeminence attributed to certain categories, the preconceived introduction of phylogenetic hypotheses), it is certain that the current orientation tends toward natural systematics, a goal perhaps not unattainable when awareness of the artifices employed is combined with the availability of adequate methods to remove them.
Among the most widely followed classifications, we may cite the botanical one by Engler-Diels contained in the *Syllabus der Pflanzenfamilien* (1936) and the zoological one by Leuckart-von Siebold with modifications by Emery-Ghigi.
In the first, the main divisions are 15: Schizophyta; Myxomycetes; Flagellata; Dinoflagellatae; Silicoflagellatae; Heterocontae; Bacillariophyta; Conjugatae; Chlorophyceae; Charophyta; Phaeophyceae; Rhodophyceae; Eumycetes et Lichenes; Archegoniatae subdivided into Bryophyta and Pteridophyta; Embryophyta Siphonogama subdivided into Gymnospermae and Angiospermae, and the latter into Monocotyledonae and Dicotyledonae.
In the second, 18 phyla are listed (1 of protozoa and 17 of metazoa, of which 16 are invertebrates), namely: Protozoa, Porifera, Coelenterata, Ctenophora, Platyhelminthes, Nemertini, Nemathelminthes, Ctenognatha, Acanthocephala, Rotifera, Gastrotricha, Annelida, Bryozoa, Brachiopoda, Arthropoda, Mollusca, Echinodermata, Chordata with the subtypes of Hemichordata, Urochordata, Cephalochordata, and Craniata.
J. Spix, *Geschichte und Beurtheilung aller Systeme in der Zoologie nach ihrer Entwicklungsfolge von Aristotele bis auf die gegenwärtige Zeit*, Nuremberg 1811; G. Cuvier-Magdeleine de Saint-Agy, *Histoire des sciences naturelles depuis leur origine jusqu’à nos jours chez tous les peuples connus*, 5 vols., Paris 1841-1845; J. Meyer, *Aristoteles Thierkund*, Berlin 1855; F. A. Pouchet, *La biologie aristotéique*, Paris 1855; L. Agassiz, *De l'espèce et de la classification en zoologie*, trans. F. Vogeli, ibid. 1869; J. V. Carus, *Geschichte der Zoologie bis auf Hof, Müller und Charl Darwin*, Munich 1872; T. H. Huxley, *On the Classification of the Animal Kingdom*, in *Nature*, 11 (1875), p. 101; J. Sachs, *Histoire de la botanique du XVIe siècle à 1860*, trans. H. de Varigny, Paris 1892; E. Durkheim and M. Mauss, *De quelques formes primitives de classification*, in *Année sociologique*, 6 (1903), pp. 1-72; R. Burckhardt, *Zur Geschichte der biologischen Systematik*, in *Verhandlungen der Naturforschenden Gesellschaft in Basel*, 16 (1903), pp. 388-440; id., *Das höchste Tiersystem, eine Vorstufe der zoologischen Systematik des Aristoteles*, ibid., 15 (1904), pp. 377-414; id., *Das 1. Buch der aristotelischen Tiergeschichte*, in *Zoologische Annalen*, 1 (1905), pp. 1-28; id., *Zur Geschichte und Kritik der biologie-historischen Literatur*, ibid., pp. 355-375, and 2 (1908), pp. 31-46; id., *Aristoteles und Cuvier*, ibid., 3 (1908), pp. 60-77; J. Briquet, *Règles internationales de la nomenclature adoptées par le Congrès international de botanique de Vienne 1905*, Jena 1906; A. Semenov-Tian-Shanskij, *Die taxonomischen Grenzen der Art und ihrer Unterbeiten*, Berlin 1910; H. De Vries, *Gruppenvieze Artbildung*, ibid. 1913; J. P. Lotay, *Que est-ce qu' une espèce?*, in *Arch. Néerl. des Sciences exactes et naturelles*, 3° série, b. 3 (1918), pp. 57-110; O. Hamelin, *Le système d'Aristote*, Paris 1920; R. Burckhardt and H. Erhard, *Geschichte der Zoologie und ihrer wissenschaftlichen Probleme*, 2 vols., Berlin 1921; G. Turesson, *The genotypical response of the plant-species to the habitat*, in *Hereditas*, 3 (1922), pp. 211-350; H. Daudin, *I. De Linné à Tussieu, Méthodes de la classification et idée de série en botanique et en zoologie, II: Cuvier et Lamarck, Les classes zoologiques et l'idée de série animale*, 3 vols., Paris 1926 (extensive bibliography); O. Kleinschmidt, *Die Formenkreislehre und das Weltwerden des Lebens*, Halle 1926; G. C. Robson, *The species problem*, London 1928 (with bibliography); B. Hayata, *The natural classification of plants according to the dynamic system*, in *Icon Formos*, 10 (1929), pp. 97-234; B. Rensch, *Das Prinzip geographischer Rassenkreise und das Problem der Artbildung*, Berlin 1929 (with bibliography); B. Rensch, *Kurze Anweisung für zoologisch-systematische Studien*, Leipzig 1934; R. Wettstein, *Handbuch der systematischen Botanik*, 4th ed., Leipzig 1935; L. Cuenot, *L'espèce*, Paris 1936; E. T. Schenk and J. H. MacMasters, *Procedure in Taxonomy*, Stanford 1936; J. Clausen, D. D. Keck, and W. M. Hiesey, *Experimental Taxonomy*, in *Carnegie Institution of Washington, Annual Report Div. Plant Biology*, Years 1932-37; T. Dobzhansky, *What is a species?*, in *Scientia*, 31 (1937), pp. 280-286; B. P. G. Hochreutiner, *La valeur relative des groupes systématiques*, in *Boissiera*, 2 (1937), pp. 1-7; J. Ramsbottom, *Linnaeus and the species concept*, in *Proceedings of the Linnean Society of London*, 150 (1938), pp. 192-219; R. Souèges, *L'espèce et les classifications actuelles*, Paris 1938 (with bibliography); J. Huxley (editor), *The New Systematics*, Oxford 1940 (a collection of critical essays, each with bibliography, by H. H. Allan, W. J. Arkell, W. T. Calman, M. B. Crane, C. D. Darlington, G. R. de Beer, C. Diver, E. B. Ford, J. S. L. Gilmour, J. Huxley, L. Hogben, J. A. Moy-Thomas, H. J. Müller, J. Ramsbottom, E. J. Salisbury, J. Smart, T. A. Sprague, W. H. Thorpe, N. V. Timofeeff-Ressovsky, W. B. Turrill, N. I. Vavilov, E. B. Worthington, S. Wright); E. Mayr, *Systematics and the Origin of Species*, New York 1942 (with extensive bibliography); R. Ciferri, *La sistematica delle piante*, in T. Ferraris, *Botanica agraria*, III, 1, Milan 1946 (with extensive bibliography); A. Sacchetti, *Alcuni problemi di sistematica biologica*, in *Rivista di biologia coloniale*, 8 (1947), pp. 79-110; S. Beer, *Sistematica e classificazione*, in *Responsabilità del sapere*, 3 (1948), pp. 35-63; A. Ghigi, *La specie*, in *Scientia*, 42 (1948), pp. 175-81; see also *International Rules of Zoological Nomenclature*, in each volume of *C. R. Congr. Intern. Zool.* Sergio Beer