Zoology

ZOOLOGY. - The science whose object is the study of animals. This very broad definition, which makes the expression "z." a synonym of "animal biology," leads one to include within it a great number of biological sciences that have today become autonomous, such as anatomy, physiology, pathology, understood as directed both to the study of man alone and to that of individual animals or even to the comparison of human phenomena with those that occur in animals. It has thus come about that z., in the course of time, has 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 of the cells that compose them, in chemical composition, which also varies notably from species to species, from one group of species to another, one must conclude that z. comprises within itself, of all the biological sciences directed to the study of animals, that portion which represents the differential between individual species and groups of species. Since form is the expression of the function that each organ performs to keep the organism alive, not only anatomy but also comparative physiology are most important parts of z.; indeed, they have long since achieved didactic autonomy, especially as regards the study of vertebrates. Furthermore, animals live on the earth and within the earth, in the sea and in continental waters; more or less intimate correlations are therefore established between the animal organism and the environment surrounding it, relations that form the object of one part of z., animal ecology, which studies not only the general conditions of existence in each individual environment, such as respiration, locomotion, reproduction in water or in the atmosphere, but also the correlations existing between organisms and determinate biotopes, such as the surface of the sea compared with its bottom, littoral or abyssal, the virgin forest, the savanna or the desert, etc.

The animal, in order to exercise conveniently those functions that taken together 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, a carnivorous diet, did not possess the capacity and the means to find, reach and seize its food, plant or animal prey, it could not survive. The behavior of animals, determined by contacts, tropisms, instincts, intellectual facts that safeguard it from dangers or guide it in the performance of those acts that have as their purpose the preservation of the individual and of the species, constitutes another most important part of z., ethology or the science of mores, which likewise vary from species to species, from group to group of species.

Closely connected with the concept of species (v.) is that of the heredity of the characters that fall under our senses and that constitute the phenotype, that is, that complex of morphological and functional manifestations that allow the zoologist to formulate diagnoses of an individual or of a group of individuals. Now it is 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 unleashed by the organism itself and to external stimuli, climatic, pedological, etc. Such study, the special object of genetics (v.), is likewise a most important part of z.

At this point it is necessary to outline the history of z. Aristotle is considered its founder, with his treatise De animalibus. One may agree with this statement with the reservation that Aristotle also gathered the knowledge that some of his precursors, 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 under one or another biological aspect, now morphological, now functional. Although Aristotle affirms the existence of greater genera (γένος) (γένη μέγετα) and of species (εἶδος), he does not concern himself with the problems of species, which is, according to him, an entity that exists and is not discussed. Aristotle is not the author of that classification that customarily goes in treatises on z. under the name of Aristotelian classification. This could be more precisely defined as the classification of the greater or lesser groups of animals known to Aristotle, of which mention is made in his works. Such a conception persists in the zoological works of antiquity and of the Middle Ages. It is evident in the Treatise on Animals of Albertus Magnus, which can be considered as an Aristotelian paraphrase and where, nevertheless, the foundations of modern animal ecology are laid. Albertus Magnus does not miss an occasion, in his work, to bring out the correlations between animal organisms and the environment. Setting aside the historical development of anatomy and physiology, from the 2nd century of the common era with Galen, to the 16th with Vesalius and Cesalpino, a conspicuous contribution to general z. was made in the 17th century by Marcello Malpighi with the Dissertatio de Bombyce, by Swammerdam with the Anatomy of the Bee and of Other Insects, by Leeuwenhoek with the discovery of the infusoria, by Francesco Redi with the application of the experimental method to z. Valuable works of a monographic character 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, of Salviani, of Belon, of Rondelet and others on aquatic animals and especially fishes.

If on the one hand the materials were thus accumulated that served as the basis for the biologists of the end of the 18th century and the beginning of the 19th to construct the edifice of comparative anatomy, one of the most notable aspects of z., especially as regards vertebrates, in which science the names of Oken, of Goethe and especially that of Cuvier and later that of Gegenbaur excel, on the other hand experimental z. and physiology had in Lazzaro Spallanzani a colossus, who brought a large contribution to the knowledge of the problems of generation also under the comparative aspect.

From Aristotle to our own days, in general z., which consists therefore in the study of various problems of life, morphological and physiological, ecological and ethological, for the solution of which this or that species of animals furnishes, according to the case, the most suitable material, a continuous series of researches has followed one another. These have brought a vast contribution to the knowledge of the fundamental phenomena of life. Z. then reached its maximum 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 reaching a full autonomy from botany and from z. Substantially, however, it is identified with the one or with the other, inasmuch as it studies the causes of the differences in organisms and has transformed descriptive z. (and botany) into causal z. (and botany).

Systematic zoology, the branch that through knowledge of animal species outlines their classification, originated during the Renaissance, when geographical explorations and voyages of circumnavigation 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 induced zoologists to formulate classifications based on the differential characters of animals. The first zoological work written with such an intent was the De differentiis animalium of the Englishman Edward Wotton, published in 1552, in which there is found a classification little different from that which modern authors have derived from Aristotle's work. There follows a period that may be called pre-Linnaean, during which there excel, among others, the works of Lonicer (1528-86), of Gessner (1516-65), of Ulisse Aldrovandi, of Bologna (1528-1605), of Johnston (1603-75), which have the character of naturalistic encyclopedias. Aldrovandi, in the last years of his life, became aware of the difficulty of usefully teaching zoology with exceedingly long descriptions, in which the essential differential characters were dispersed, and composed a Syntaxis animalium and a Syntaxis plantarum that remained unpublished, where, by means of dichotomous keys and brief diagnoses of the most important characters, it becomes possible to arrive at the identification of the species of the animal or plant. J. Ray (1628-1705), author, in part with the collaboration of Willoughby, of various works, was the first to intuit the concept of species, as it was later defined by Linnaeus, to propose the criterion of fertility as a means of defining the species, and to feel the necessity of establishing a rational nomenclature. Among the other zoologists of the immediately pre-Linnaean period mention should be made of Lister, Bonanni, Brein, Bianchi (Janus Planus) and, in the special field of entomology, Vallisneri (1661-1730), Maria Sibylla of 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 the studies of marine zoology and biology. Carl Linnaeus, a Swede, is incontestably the founder of systematic zoology and the reformer of the methods used by his predecessors. The Linnaean reform is founded on three principles: 1) binomial nomenclature in the Latin language, which, making use of two names, one for the genus and the other for the species, permits the formal determination of the latter; 2) the formation of frameworks (classes and orders) in which the genera and species are grouped; 3) the adoption of differential diagnoses and dichotomous keys, which permit the distinction of the individual groups, down to the species. This method is a refinement of that used by Aldrovandi in his Syntaxis. Linnaeus divided the animal kingdom into six classes: mammals, birds, reptiles, fishes, insects, worms. But the classification, in his mind, had the precise meaning of a plan of creation, in which the most closely related forms were united into subordinate groups. Linnaeus's work is condensed in his Systema Naturae which, in the edition published in 1750, constitutes the fundamental code of zoological nomenclature. Linnaeus had numerous followers who, applying the Linnaean rules, described thousands of new species and, by bringing to light further categories of differences, increased the number of the frameworks by instituting families, subfamilies, tribes, etc.

Already at 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 their anatomy and physiology, arrived at conclusions of a highly biological character, the former tending 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 bringing to light that there exists in animals a typical organization, variable from group to group, so that the individual animal groupings answer to the criterion of uniting together the forms that have the same topographical relationship of parts. The expression "type" is later than Cuvier and belongs to Blainville. The classification of Lamarck (1835-45), which is founded first of all on the distinction of animals into vertebrates and invertebrates and the latter, with which he was chiefly concerned, into apathics and annellids, had little success; that of Cuvier, on the other hand, founded on the typical architecture of the animal body and first of all on the general aspect of the nervous system and, subordinately to this, of other organic systems, can be considered the starting point of all modern classifications. Cuvier divided the animal kingdom into four embranchements or types: vertebrates, mollusks, articulates and radiates, and his successors increased their number by subdividing and modifying the extent of the types he had outlined. Thus Von Siebold instituted the type of the protozoans or unicellular animals and that of the arthropods, separating the former from the radiates and the latter from the articulates. Leuckart further divided the radiates into coelenterates and echinoderms. The development of embryology demonstrated the affinities existing between the tunicates and the vertebrates, suggesting the institution of a more comprehensive type of the chordates, while a more careful morphological and embryological examination had advised separating the sponges and, respectively, the ctenophores, from the coelenterates. In all these classifications there is always found the type of the worms, which does not answer to the morphological concept of type, but is an unclassified residue of the numerous plans of organization that zoologists have not known how, or been able, to include in the major types. Nevertheless the flatworms and the nematodes are today considered as autonomous types, in almost all modern classifications.

Classification, in the mind of Linnaeus and of Cuvier, corresponds to the plan of creation of the species, the work of God; therefore the effort of the zoologist, in the mind of those two great men, tends toward the reconstruction of a system, at least in its general lines, that is 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 in zoology, placing in the foreground the problem of species and attributing to affinities the value of real kinships and to differences the value of constitutional changes, due to causes that are, according to some authors, only intrinsic, according to others extrinsic. In little more than half a century systematics changed its aspect, becoming from a static a dynamic science, because the causes of modification of species, although attenuated in the course of the geological eras, have by no means ceased, and the new species can be either a form not known before its identification, or a species of recent formation. With evolutionism, the system was overturned and classification no longer began with the mammals, descending to the simpler classes and types, but began with the unicellular animals in order to ascend, step by step, to the groups of ever more complicated organization. The so-called fundamental biogenetic law of Ernst Haeckel: "ontogeny is a recapitulation of phylogeny," although recognized as erroneous shortly after its enunciation, has had great influence on classification, which has ended by assuming the prospective value of a genealogical tree of animal forms, fossil and living.

Systematics is thus the synthesis of the zoological sciences, and general classification is its conclusion: the position assigned to a given 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 kinship with related forms. But systematics in the sense of classification is a 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, which is expressed in the grouping of the most closely related types of organization, while also taking into account the possible and probable descent of each of them, whereas according to the evolutionistic concept, to which several of the most recent zoologists, especially American ones, fully subscribe, the phylum (filo) that they substitute for the type should express the descent of the groups. The type, moreover, represents a reality, albeit a static one, but one that falls under the direct observation of the researcher, whereas the phylum represents a dynamic hypothesis, albeit partly causal, which nevertheless lacks experimental proof.

The system, that is, classification, leads to diminishing the value of specific differences, to overvaluing theoretical models which for each group, larger or smaller, do not correspond to the reality of any species. It is true that the system, leading from the type or phylum, to the class, the order, the family, the genus — groupings that are largely subjective — also arrives at the real species, though often not determinable or determined. The latter has been given its due value by modern genetics, since, by studying specific and subspecific groups, first in isolation and then turning to populations, it has been able to bring out, by the statistical method, the value and the causes of the differences, and has transformed systematics, which then merges with the whole of zoology, from a descriptive science into a causal science.

From the statistical standpoint, the various classes that make up the animal kingdom offer enormous, almost incomparable numerical differences. Insects, for example, comprise several hundred thousand species, far more than half of the entire animal kingdom. On the other hand, the ctenophores, which from the morphological and ecological standpoint, and because of their position in the system, offer conspicuous interest, are still well below a hundred species. Among the vertebrates the most numerous class is that of birds, of which between species and races about 25,000 forms are known today. The study of these two classes of animals, insects and birds, gathers around itself a great number of researchers, who have given to each of the two groups, rich in practical applications (economic entomology, poultry farming), notable autonomy. Notable importance and autonomy has likewise been acquired by the study of protozoa, to which bacteria are also customarily aggregated, in protistology, which studies cells in their complexity as living organisms, whereas sponges and, in part, coelenterates allow only those organisms formed of tissues to be considered. Flatworms and nematodes form, together with certain groups of protozoa and arthropods, insects included, an important part of parasitology. The study of fishes (ichthyology), together with that of several other aquatic animals, is a part of hydrobiology. A certain value in the past was had by cata, is made manifest by the number and the denomination of the sections into which the international Congresses of z. are divided.

Z. is therefore, among the biological sciences, the most vast and multiform; it can be considered according to the different faces of a prism; each of the aspects of animal life is elevated to an autonomous science, whether in specific, descriptive and causal form, or in comparative form; every animal group possesses scientific autonomy because it offers biological problems that are not encountered, or cannot be investigated equally well, in others; every animal group possesses, finally, greater or lesser practical importance, in relation to the value of its medical and economic applications to human life.

Alessandro Ghigi