ZOOLOGY. - The science that has as its object the study of animals. This very broad definition, which makes the expression "2." synonymous with "animal biology," leads to the inclusion within it of a large number of biological sciences that have today become autonomous, such as anatomy, physiology, and pathology, whether directed solely to the study of man or to that of individual animals or even to the comparison of human phenomena with those occurring in animals. It has therefore come about that zoology has, over time, acquired the more precise meaning of the science that studies the differences among animals, that is, the problems of species. If one considers the fact that animal (and plant) species differ from one another in external form, in internal anatomical constitution, in the structure of their tissues and the cells that compose them, and in chemical composition—which also varies notably from species to species and from one group of species to another—one must conclude that zoology includes, among all the biological sciences directed to the study of animals, that which represents the differential among individual species and groups of species. Since form is the expression of the function that each organ performs to maintain the organism alive, not only anatomy but also comparative physiology are very important parts of zoology; indeed, they have long since achieved didactic autonomy, especially with regard to the study of vertebrates. Moreover, animals live on the earth and within the earth, in the sea and in continental waters; hence more or less intimate correlations are established between the animal organism and the environment that surrounds it, relations that form the subject of a part of zoology, animal ecology, which studies not only the general conditions of existence in each individual environment, such as respiration, locomotion, and reproduction in water and in the atmosphere, but also the correlations existing between organisms and specific biotopes, such as the surface of the sea in comparison to its depths, the littoral or abyssal zones, the virgin forest, the savanna, or the desert, and so on.
The soul, to exercise properly those functions that in their entirety are identified with life, must also perform a series of acts that characterize its behavior. If an animal, whose organs and functions are correlated toward a herbivorous or, respectively, carnivorous diet, did not possess the capacity or means to find, reach, and seize its food—plant or animal prey—it could not survive. The behavior of animals, determined by tropisms, instincts, and intellectual acts that safeguard them from dangers or guide them in performing those acts aimed at the conservation of the individual and the species, constitutes another very important part of zoology, ethology or the science of habits, which also vary from species to species and from group to group of species.
Closely connected with the concept of species (v.) is that of the heredity of characters that fall under our senses and that constitute the phenotype, that is, that complex of morphological and functional manifestations that enable the zoologist to formulate a diagnosis of an individual or a group of individuals. It is now known that the phenotype is the expression of the complex of genes, elementary particles transmissible by heredity, which influence one another and react in various ways to internal stimuli triggered by the organism itself and to external stimuli, climatic, pedological, etc. This study, the special object of genetics (v.), is also a very important part of zoology.
At this point, it is necessary to outline the history of zoology. ARISTOTLE is considered its founder with his treatise *De animalibus*. One may agree with this assertion with the reservation that Aristotle also gathered the knowledge that some of his predecessors, especially DEMOCRITUS, had possessed on the subject. Aristotle can be defined as a general zoologist, who sets forth facts relating to the life of animals that offer particular interest from one or another biological aspect, now morphological, now functional. Although Aristotle affirms the existence of greater genera (γένος) and species (εἶδος), he does not concern himself with the problems of species, which, in his view, is an entity that exists and is not open to question. Aristotle is not the author of that classification that is usually given in treatises on zoology under the name of the Aristotelian classification. This might more precisely be defined as the classification of the greater or lesser groups of animals known to Aristotle, of which mention is made in his works. This conception persists in the zoological works of antiquity and the Middle Ages. It is evident in the *Treatise on Animals* by ALBERTUS MAGNUS, which may be considered a paraphrase of Aristotle and in which, however, the foundations of modern animal ecology are laid. Albertus Magnus does not fail to emphasize in his work the correlations between animal organisms and their environment.
Precisely from the historical development of anatomy and physiology, from the 2nd century of the common era with GALEN, to the 16th with VESALIUS and CAESALPINUS, a substantial contribution to general zoology was made in the 17th century by MARCELLO MALPIGHI with the *Dissertatio de Bombyce*, by SWAMMERDAM with the *Anatomy of the Bee and Other Insects*, by LEEUWENHOEK with the discovery of infusoria, and by FRANCESCO REDI with the application of the experimental method to zoology. Notable monographic works were also written on individual animal species or groups of species, such as that of HANS WORN on the lemming, of GILLE on the elephant, and of SALVIANI, BELON, RONDELET, and others on aquatic animals and especially fish.
While, on the one hand, materials were thus accumulated that served as the basis for the biologists of the late 18th and early 19th centuries in constructing the edifice of comparative anatomy—one of the most notable aspects of zoology, especially with regard to vertebrates, in which science the names of OKEN, GOETHE, and especially CUVIER, and later GEGENBAUR, excel—on the other hand, experimental zoology and physiology had in LAZZARO SPALLANZANI a giant who made a broad contribution to the knowledge of problems of generation, even from a comparative aspect.
From Aristotle to the present day, in general zoology—which thus consists in the study of various problems of life, morphological and physiological, ecological and ethological, for the solution of which this or that species of animal provides, according to the case, the most suitable material—there has been a continuous succession of researches. These have made a vast contribution to the knowledge of the fundamental phenomena of life. Zoology has then reached its greatest development with genetics (plant and animal), a science born, as has been said, at the beginning of this century and which has made gigantic progress, apparently achieving full autonomy from botany and zoology. In substance, however, it is identified with one or the other, inasmuch as it studies the causes of differences in organisms and has transformed descriptive zoology (and botany) into causal zoology (and botany).
Systematic zoology, which through the knowledge of animal species outlines their classification, originated during the Renaissance, when geographical explorations and circumnavigation voyages brought to Europe a great number of natural curiosities, especially preserved animals and their parts, which formed collections to be presented to the public for its instruction. It was the need for order that led zoologists to formulate classifications based on the different characteristics of animals. The first zoological work written with this intent was the *De differentiis animalibus* by the Englishman Edward Wotton, published in 1552, in which a classification is found that differs little from that which modern authors have derived from Aristotle’s work. This was followed by a period that may be called pre-Linnaean, during which, among others, the works of Lonicer (1528–86), Gesner (1516–65), Ulisse Aldrovandi of Bologna (1528–1605), and Johnston (1603–75) excelled; these works have the character of naturalistic encyclopedias. In the final years of his life, Aldrovandi realized the difficulty of teaching zoology effectively through excessively long descriptions, in which the essential differential characteristics were lost, and composed an unpublished *Syntaxis animalium* and *Syntaxis plantarum*, in which, by means of dichotomous keys and brief diagnoses of the most important characteristics, it became possible to arrive at the identification of the species of animal and plant.
John Ray (1628–1705), author in part with the collaboration of Willoughby, of various works, was the first to grasp the concept of species as later defined by Linnaeus, to propose the criterion of fertility as a means of defining species, and to feel the necessity of establishing a rational nomenclature. Among other zoologists of the immediate pre-Linnaean period, mention should be made of Lister, Bonanni, Brein, Bianchi (*Janus Plancus*), and, in the special field of entomology, Vallisneri (1661–1730), Maria Sibylla Merian (1647–1717), and especially J. L. Réaumur (1683–1755), who studied not only morphology but also the life of insects. Luigi Ferdinando Marsili (1658–1730) may be considered the founder of studies in zoology and marine biology.
Carlo Linnaeus, a Swede, is incontestably the founder of systematic zoology and the reformer of the methods used by his predecessors. The Linnaean reform is based on three principles: 1) binomial nomenclature in the Latin language, which, by using two names—one for the genus and one for the species—permits the formal determination of the latter; 2) the formation of frameworks (classes and orders) in which genera and species are grouped; 3) the adoption of differential diagnoses and dichotomous keys, which allow for the distinction of individual groups down to the species. This method is an improvement upon that used by Aldrovandi in his *Syntaxis*. Linnaeus divided the animal kingdom into six classes: mammals, birds, reptiles, fish, insects, and worms. However, in his mind, classification had the precise meaning of a plan of creation, in which the most closely related forms were grouped into subordinate groups. The work of Linnaeus is condensed in his *Systema Naturae*, which, in the edition published in 1758, constitutes the fundamental code of zoological nomenclature. Linnaeus had numerous followers who, applying his rules, described thousands of new species and, by highlighting further categories of differences, increased the number of frameworks by establishing families, subfamilies, tribes, etc.
Already toward the end of the 18th century and at the beginning of the 19th, Lamarck and Cuvier, starting from groups of very different species and studying anatomy and physiology, arrived at conclusions of high biological significance. The former sought to demonstrate that the form of animals is correlated with the environment in which they live and with the use they make of their organs for physiological needs; the latter emphasized that there exists in animals a typical organization, variable from group to group, so that individual animal groupings respond to the criterion of uniting together forms that have the same topographical relationship of parts. The expression “type” is later than Cuvier and belongs to Blainville. Lamarck’s classification (1835–43), which is based first and foremost on the distinction of animals into vertebrates and invertebrates and, of the latter, into apathetic and sensitive (those he studied most), had little success; Cuvier’s classification, on the other hand, based on the typical architecture of the animal body and first and foremost on the general aspect of the nervous system and, subordinately to this, of other organic systems, may be considered the starting point of all modern classifications. Cuvier divided the animal kingdom into four *embranchements* or types: vertebrates, mollusks, articulated animals, and radiates, and his successors increased their number by subdividing and modifying the extent of the types he delineated. Thus, Von Siebold established the type of protozoans or unicellular animals and that of arthropods, separating the former from radiates and the latter from articulated animals. Leuckart further divided radiates into coelenterates and echinoderms. The development of embryology demonstrated the affinities between tunicates and vertebrates, suggesting the establishment of a more comprehensive type of chordates, while a more careful morphological and embryological examination led to the separation of sponges and, respectively, ctenophores, from coelenterates. In all these classifications, the type of worms is always found, which does not correspond to the morphological concept of type but is an unclassified residue of the numerous organizational plans that zoologists have been unable or unwilling to include in the major types. However, platyhelminths and nemathelminths are today considered autonomous types in almost all modern classifications.
In the minds of Linnaeus and Cuvier, classification corresponded to the plan of the creation of species, the work of God; therefore, the effort of the zoologist, in the minds of these two great figures, tended toward the reconstruction of a system, at least in its general outlines, that would be stable, because its elementary units, the species, would be fixed. The work of Charles Darwin, *The Origin of Species*, published in 1859, brought a true revolution to zoology, placing the problem of species at the forefront and attributing to affinities the value of real kinship and to differences the value of constitutional changes due to causes that, according to some authors, are intrinsic and, according to others, extrinsic. In little more than half a century, systematics changed its aspect, becoming from a static science a dynamic one, because the causes of modification of species, although attenuated over the course of geological eras, have by no means ceased, and a new species may be either a form not previously known before its identification or a species of recent formation. With evolutionism, the system was overturned, and classification no longer began with mammals, descending to the simpler classes and types, but started from unicellular animals and ascended, step by step, to groups of increasingly complex organization. The so-called fundamental biogenetic law of Ernst Haeckel—“ontogeny is a recapitulation of phylogeny”—although recognized as erroneous shortly after its enunciation, had a great influence on classification, which ended up assuming the prospective value of a genealogical tree of animal forms, both fossil and living.
Systematics is thus the synthesis of the zoological sciences, and the general classification is its conclusion: the position assigned to a particular animal form or group of animals within the system itself is the result of morphological investigations (anatomical, embryological, histological, cytological, etc.), which determine the relationships of affinity with related forms. However, systematics in the sense of classification is part of zoology and cannot be identified with it: the system of animals is a subjective conception of our mind; it belongs to books and museums, not to nature. System is synonymous with order, expressed in the grouping of the most closely related types of organization, while taking into account the possible and probable descent of each of them. According to the evolutionary concept, which is fully embraced by many of the most recent zoologists, especially American ones, the thread they substitute for the type should express the descent of the groups. The type, however, represents a reality, albeit a static one, which falls under the direct observation of the researcher, whereas the thread represents a dynamic hypothesis, albeit in part causal, for which experimental proof is still lacking.
The system, i.e., classification, leads to diminishing the value of specific differences, to overvaluing theoretical models that, for each group, large or small, do not correspond to the reality of any species. It is true that the system, proceeding from the type or thread to the class, order, family, and genus—groupings that are largely subjective—also reaches the real species, although often indeterminable or undetermined. This has been valorized by modern genetics, since by studying specific and subspecific groups, first in isolation and then turning to populations, it has been able to highlight, through statistical methods, the value and causes of differences and has transformed systematics—which then merges with the entire zoology—from a descriptive science into a causal science.
From a statistical standpoint, the various classes that make up the animal kingdom exhibit enormous, almost incomparable numerical differences. Insects, for example, comprise several hundred thousand species, more than half of the entire animal kingdom. In contrast, ctenophores, which, from a morphological and ecological standpoint and due to their position in the system, are of considerable interest, still number well below a hundred species. Among vertebrates, the most numerous class is that of birds, of which some 25,000 forms are known today, including species and races. The study of these two classes of animals, insects and birds, draws around itself a large number of researchers who have given both groups—rich in practical applications (economic entomology, aviculture)—considerable autonomy. Protozoans have also acquired notable importance and autonomy, to which bacteria are often added in protistology, which studies cells in their complexity as living organisms, whereas sponges and, in part, cnidarians allow us to consider as organisms only those formed by tissues. Platyhelminths and nemathelminths, together with certain groups of protozoans and arthropods (including insects), form an important part of parasitology. The study of fish (ichthyology), along with that of many other aquatic animals, is part of ribobiology. Mollusks have had a certain value in the past as the subject of malacology, which is founded, however, especially on conchology, i.e., on a single organ belonging to a part of mollusks, though this organ is only of particular interest. The complexity of zoology, both general and special, purely scientific and applied, is demonstrated by the number and the names of the sections into which the International Zoological Congresses are divided.