Classification

CLASSIFICATION.— It is the operation by which a multitude of related objects is arranged in a hierarchical system of groupings, for the purpose of facilitating their study. As a factor of knowledge, classification lies at the basis of the epistemological problem and involves all collective entities whose components present peculiarities, that is, common and differential characters gradable in such a way as to justify ordering in a system. The study of characters is therefore a necessary prerequisite for the establishment of groups (categories or complexes) of different order or hierarchical level. One finds accordingly a classification of ideas, germinated 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 the individual disciplines.

Classification is essentially a problem of method, that is, of criteria in evaluating characters and of procedure in constituting groups. The criteria vary according to the material to be classified, the aims to be pursued and the point of view of the classifier; the procedure is divisive when it descends from the general to the particular on the basis of differential characters, and grouping when it ascends from the particular to the general on the basis of affinities. The choice of method is important when one considers objects of nature which well-founded reasons induce one to believe are already naturally ordered in a system. Then the classification is natural if it faithfully reflects that system and, given its cognitive value, admits of no arbitrary solutions; it is artificial in the contrary case and, given its purely practical aim, may be arbitrary provided it is rational and convenient. Against natural classification stand the gaps in knowledge concerning the objects to be classified, the impossibility of sharply delimiting groups interconnected by common characters, the ease of subjectivism in assessing the discriminative and hierarchical value of the characters assumed for the identification of groups, and finally the doubt in choosing between the divisive and the grouping procedure: the former, which goes back to the Platonic dichotomy

(διαίρεσις) and appeals to the principle of definitions "per genus proximum et differentiam specificam," often appears easier and even more spontaneous as an attitude of thought before the task of sorting a multitude of objects; the latter may be recommended by the greater intuitive graspability of narrow groupings compared with broader ones and hence by the advisability of ascending from the better known to the less known.

In biology classification has two aims: to permit determination, that is, the identification of an animal or a plant, to which a given name has been assigned through nomenclature, and to establish its position in the proposed system; and to adapt them as easily as possible to natural systematics, that is, to the natural distribution of living beings into taxonomic groups or complexes hierarchically subordinated one to another and resulting from the sum of affinities of different order existing among their components. Any classification can suffice for the first aim, but only natural classification can answer the second.

The attempts to discern the natural system, in which a great part of the history of zoology and botany is summed up, often felt the influence of a second aim, which was useful as a stimulus to inquiry but became harmful when it was assumed as an orienting apriorism, namely the search for the plan of organization or indeed of creation of living beings, replaced, with the advent of evolution (v.), by the search for phylogeny: in the one and the other, disputes flared between the supporters of the fundamental and original unity of organisms and the supporters of their multiplicity. To undermine prejudicially the possibility of natural classification there contributed first the conception of a distribution of living beings in a continuous series of increasing complexity, and, later, the evolutionist doctrine: in fact the former, already suggested by Aristotle and later dominant among many systematists of the 18th century, sanctioned a priori the artificiality of the "watertight compartments" of any classification; the latter declared every classification fallacious, inasmuch as its necessarily static frameworks cannot reflect the dynamism of the evolutionary processes acting upon individuals, so that the consistency of the various taxonomic complexes would not be a reality but a transitory appearance. Hence the pernicious dualism between a so-called static systematics and a dynamic systematics, the former the object of the classifier relegated to the rank of librarian or thereabouts, the latter the object of the evolutionist and the general biologist. An attempt is now being made to remedy this situation, on the one hand by considering the slowness of the evolutionary processes pertaining to the great systematic groups (macroevolution) and hence the theoretical possibility of portraying the present system, and on the other by studying methods capable of objectively detecting and adopting as practical limits the natural discontinuities found among the various taxonomic complexes. That, in fact, the definition of not a few groups of animals and plants may now be considered satisfactory is attested by the overall stability that they have long since and often by different paths attained.

To the laborious delineation of natural classification there contributed: the increase in study material consequent upon geographical, paleontological and microscopic discoveries; the deepening of knowledge of characters, determined by the progress of all biological disciplines; and finally the growth of comparisons among the various organisms. Thus the classifications in use were modified as new criteria were added to or replaced the previous ones: the various groups underwent grouping revisions or, more often, splitting revisions and were enriched in the number of their component units; moreover new intermediate levels were introduced among those already established. Of these levels the meaning and the denomination were made precise, while the descriptions of the individual complexes were modeled on uniform patterns: the first task was met by the adoption, largely going back to Linnaeus, of the progressively less comprehensive terms type, class, order, family, genus, species, race, to which were added various intermediate steps (subtype, subgenus, or: superfamily, etc.) and, at times, also other categories of variable rank (tribe, cohort, legion, phalanx, etc.); the second task, also begun by Linnaeus, was aimed at by the adoption of the Latin language in the concise descriptive diagnoses and in the naming of the individual groups, and by the introduction of binomial nomenclature (which sometimes became trinomial and polynomial when determinations descended to subspecific levels), according to which the name of every species must be preceded by the name of the genus and followed by the abbreviation of the first describer. The international rules of nomenclature sought to remedy the drawbacks of homonymies and synonymies derived from the multiplication of determinations: yet it still often happens that identical species and genera are designated by different names by different authors, and this confused instability in terminological formalism is added to that which affects the very substance of classifications, contributing to a certain persistent unease in systematic studies.

Historically, botanical research proceeded in a more homogeneous but less profound manner than zoological research, given the differences of organization and life between plants and animals and also because of the different practical interest in the latter and the former. In fact the classifications of plants were for the most part general, but were based above all on external characters; those of animals, on the other hand, were often specialized to restricted groups that were easier or more attractive to study, but in compensation descended early into anatomical-physiological inquiry.

History, traditions (see among others the second chapter of Genesis) and ethnological and linguistic studies attest that humanity must have grown accustomed from the very earliest times to recognizing and designating 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 Agrigentum is remembered, for zoology the classification of the school of Cos attributed to Erodicus of Selymbria and that contained in Plato's Timaeus are counted. But the first true systematist was Aristotle, who deserves credit 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 characters intrinsic to the organisms, not only morphological but also anatomico-physiological. He divided the species into eight great genera (γένη μέγιστα), four of animals "with blood" (αἴματα), namely mammals (ζωοτοκούντα ἐν αὐτοῖς = viviparous animals), birds (ὄρνιθες), reptiles and amphibians (τετραποδα καὶ ἀποδα ὠοτοκούντα = oviparous quadrupeds and apods) and fishes (ἰχθύες), and four of animals "without blood" (ἐνάμματα), namely mollusks (μαλάκια), crustaceans (μαλακώστρακα), insects (ἔντομα) and testaceans (ὄστρακοβέρματα). Much more artificial is the classification of plants carried out by Aristotle's two disciples, Theophrastus and Phanias: they divided plants into trees (δένδρα), shrubs (θάμνοι), sub-shrubs (σρύγανα), and herbs (πόα) and distinguished plants with flower (= phanerogams) from those without flower (= cryptogams). For many centuries Aristotle's work remained conceptually unsurpassed, even while being 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 the bestiaries and herbals) represent a regression, both because they were mere alphabetical lists or were founded on characters extrinsic to the organisms and often anthropocentric (such as habitat, venomousness, usefulness, etc.), and because they lacked any critical spirit and were often stuffed with fabulous animals and plants. Setting aside the isolated contributions of Albertus Magnus, Leonardo da Vinci and a few other pioneers, one must reach the middle of the 16th century to find the seeds of that renewal in naturalistic studies which then received a powerful impulse from the Galilean experimental method. Among botanists should be noted: Andrea Cesalpino of Arezzo, who in the treatise De plantis (1583) outlined a classification that was artificial indeed, since almost entirely founded on the characters of fructification, but 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 adhering to Aristotelian models are the contemporary works of zoology, among which stand out those of the Bolognese Ulisse Aldrovandi (1522-1605), author of a Historia naturalis celebrated also by Buffon and by Cuvier and of a Syntaxis animalium in which the dichotomous method is specified and applied; such a method is also adopted by Ray in the Synopsis methodica animalium (1693), while minutely descriptive are the Historiae animalium of Conrad Gesner (1551-87). In the 17th century a new field opens to systematic investigation with the invention of the microscope, with which Leeuwenhoek discovers the infusoria (1673) and Marcello Malpighi deepens the studies of anatomy (1671).

With Carl Linnaeus (1707-1778) begins the golden period of systematics, which leads to present-day classifications. It is marked by the interweaving of openly artificial arrangements with others tending toward natural ones: in the former the dichotomous procedure prevails: the various taxonomic entities are defined in a rigorous manner, the characters are examined and graded individually according to absolute scales and with a preference for the most conspicuous ones; in the latter, less exact and complete but more meaningful, the ascending way is preferred: the divisions are less decided, the characters are studied in their relationships and graded according to relative scales on the basis of their biological significance without preconceived preeminences. In the Systema naturae (1735) Linnaeus places at the basis of classification the genera as the first collective entities of the hierarchy and the same author had become aware of its artificiality, so much so that in other works (Classes plantarum, Genera plantarum, Philosophia botanica) he adopted the natural method, based on the detection of the multiple affinities among organisms, in order to portray faithfully the groups most easily identifiable. It was solely to this end 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 enumerate characters, but one must attend to their correlations and "weigh" them, grading and subordinating them on the basis of their constancy, which is an index of their significance and value. Lamarck likewise adhered to the natural method, though in the Flore française (1778) he deliberately wished to give a sample of artificial classification for the sole purpose of determination, and he introduced into botany the dichotomous method with analytical keys, still widely in use today. Other notable botanical classifications are due to A. P. de Candolle, J. Lindley, S. L. Endlicher.

G. L. Leclerc de Buffon (1707-88) declares himself an enemy of the arid and artificial "systems" of Linnaean stamp: he informs his Histoire naturelle (1749-88) with the principle that the only sure criterion for discriminating 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 full of gaps, harks back to evolutionary doctrine and to the idea of the unity of the living world. Étienne Geoffroy Saint-Hilaire (1772-1844) likewise agrees with this idea, and in the Philosophie anatomique (1818-22) and the Principes de philosophie zoologique (1830) he introduces the concept of homology, puts forward the principle of the "balancing" of organs, and gives the greatest prominence to topographic anatomy as revealing the relations among organs and their variations along the animal scale. A follower of Charles Bonnet, G. B. De Monet de Lamarck (1744-1829) upholds the conception of the linear continuity of living beings, bipartite into the two kingdoms, vegetable and animal, and he strives to inform with it the classification which, through his merit, attains the precise definition of many groups, especially of invertebrates. In his numerous works (among which the Philosophie zoologique, the Histoire naturelle des animaux sans vertèbres and the Système analytique des connaissances positives de l'homme stand out), Lamarck refers animal organizations to the human one, considering each of those 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 an increasing perfection, he is led to enunciate his evolutionary theory (v. LAMARCKISMO) as a logical interpretation of the affinities among living beings and of the ascent in their organization: hence the sketch 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 decided opponent of evolutionary and unitary ideas, is Georges Cuvier (1760-1832). The principal merits found in his many works and in particular in the Règne animal (1817) are: the introduction of comparative anatomophysiology and of 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 institution of four embranchements (later called types by De Blainville) representing as many irreducible and distinct fundamental schemes 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 classification is, in its framework, still followed today with the modifications introduced above all by T. Von Siebold and R. Leuckart.

With Cuvier's death there closes the period in which the term naturalist was equivalent to systematist. After him research continues, but becomes fragmented in the works of specialists of individual groups; they revise and enrich, each within his own domain and therefore unevenly, 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 the other biological disciplines that develop rapidly in the 19th and 20th centuries and from which important contributions flow into systematics, modifying criteria and methods: thus to the Linnaean morphological criterion and to Cuvier's anatomophysiological and paleontological ones, there are added with Von Baer the embryological criterion, with Darwin and the evolutionists the phyletic one, with Wallace, Wagner and Turesson the eco-geographical one, with Mendel, De Vries and modern geneticists the genetic one, with Le Dantec, Nilow, Souèges the biochemical one. With the adoption of statistical-biometric methodology, inaugurated by Quetelet, applied by Heincke and framed by Johannsen within genetics, and with the carrying out of experiments intended to establish the capacities for variation of species and to separate their genetic elements from environmental ones, systematics directs itself along the experimental path, particularly fruitful at subspecific levels, while individual analysis is replaced by that of population and pretentiously exact determination tends to be replaced by probabilistic determination.

Although in the practice of classification Linnaean schematism still counts many followers and although evident defects persist in the evaluation of characters (such as their isolated and one-sided analysis, the preeminence attributed to certain categories, the preconceived introduction of phyletic hypotheses), it is certain that the current orientation tends toward natural classification, a goal perhaps not unreachable when one has awareness of the artifices used and has at one's disposal suitable methods for removing them.

Among the most followed classifications one may cite the botanical one of Engler-Diels contained in the Syllabus der Pflanzenfamilien (1936) and the zoological one of Leuckart-Von Siebold with the modifications of Emery-Ghigi. In the first the principal divisions are 15: Schizophyta; Myxomicetes; Flagellatae; Dinoflagellatae; Silicoflagellatae; Heterocontae; Bacillariophyta; Coniugatae; Chlorophyceae; Charophyta; Phaeophyceae; Rhodophyceae; Eunycetes et Lichenes; Archegoniatae subdivided into Bryophyta and Pteridophyta; Embryophyta siphonogama subdivided into Gymnospermae and Angiospermae and the latter into Monocotiledonae and Dicotiledonae.

In the second, 18 types are enumerated (1 of protozoans and 17 of metazoans, of which 16 of invertebrates), namely: Protozoa, Porifera, Coelenterata, Ctenophora, Platyhelminthes, Nemertini, Nemathelminthes, Chaetognatha, 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 Aristoteles bis auf die gegenwärtige Zeit, Norimberga 1811; G. Cuvier-Magdeleine de Saint-Agy, Histoire des sciences naturelles depuis leur origine jusqu'à nos jours chez tous les peuples connus, 5 voll., Parigi 1841-1845; J. Meyer, Aristoteles Thierkand., Berlino 1855; F. A. Pouchet, La biologie aristotélique, Parigi 1855; L. Agassiz, De l'espèce et de la classification en zoologie, trad. F. Vogeli, ivi 1869; J. V. Carus, Geschichte der Zoologie bis auf Joh. Müller und Charl. Darwin, Monaco 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, trad. H. de Vargent, Parigi 1892; E. Durkheim e 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 hoische Tiersystem, eine Vorstufe der zoologischen Systematik des Aristoteles, ibid., 15 (1904), pp. 377-414; id., Das 1er Buch der aristotelischen Tiergeschichte, in Zoologische Annalen, 1 (1905), pp. 1-28; id., Zur Geschichte und Kritik der biologic-historischen Literatur, ibid., pp. 355-375, e 2 (1908), pp. 31-46; id., Aristoteles und Cuvier, ibid., 3 (1908), pp. 69-77; J. Briquet, Règles internationales de la nomenclature adoptées par le Congrès international de botanique de Vienne 1905, Iena 1906; A. Semenov-Tian-Shanski, Die taxonomischen Grenzen der Art und ihrer Unterabteilungen, Berlino 1910; H. De Vries, Gruppenweise Artbildung, ivi 1913; J. P. Lotsy, Que est-ce qu'une espèce?, in Arch. Néerl. des Sciences exactes et naturelles, 3e série, b. 3 (1918), pp. 57-110; O. Hamelin, Le système d'Aristote, Parigi 1920; R. Burckhardt e H. Erhard, Geschichte der Zoologie und ihrer wissenschaftlichen Probleme, 2 voll., Berlino 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 Jussieu, 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 voll., Parigi 1926 (ampia bibl.); O. Kleinschmidt, Die Formenbetriebslehre und das Weltwerden des Lebens, Halle 1926; G. C. Robson, The species problem, Londra 1928 (con bibl.); 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, Berlino 1929 (con bibl.); B. Rensch, Kurze Anweisung für zoologisch-systematische Studien, Lipsia 1934; R. Wettstein, Handbuch der systematischen Botanik, 4e ed., Lipsia 1935; L. Cuénot, L'espèce, Parigi 1936; E. T. Schenk e J. H. MacMasters, Procedure in Taxonomy, Stanford 1936; J. Clausen, D. D. Keck, e W. M. Hiescy, Experimental Taxonomy, in Carnegie Instit. of Washington. Annual Rep. Div. Plant Biology, Years 1935-37; 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, Linnaens and the species concept, in Proceeding of the Linnean Society of London, 150 (1938), pp. 192-219; R. Souèges, L'espèce et les classifications actuelles, Parigi 1938 (con bibl.); J. Huxley (redattore), The New Systematics, Oxford 1940 (raccolta di scritti critici, ciascuno con bibl.), di H. H. Allan, W. J. Arkell, W. T. Calman, M. B. Crane, C. D. Darlinton, G. R. de Boer, 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. Timofeef-Ressovsky, W. B. Turrill, N. J. Vavilos, E. B. Worthington, S. Wright); E. Mayr, Systematics and the origin of species, Nuova York 1942 (con ampia bibl.); R. Ciferri, La sistematica delle piante, in T. Ferraris, Botanica agraria, III, 1, Milano 1946 (con ampia bibl.); A. Sacchetti, Alcuni problemi di sistematica biologica, in Rivista di biologia coloniale, 8 (1947), pp. 79-110; S. Beer, Sistematica e c., in Responsabilità del sapere, 3 (1948), pp. 35-63; A. Ghigi, La specie, in Scientia, 42 (1948), pp. 175-81; v. inoltre International Rules of Zoological Nomenclature, in ogni volume di C. R. Congr. Intern. Zool.

Sergio Beer