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Zoology, the branch of biology devoted to the study of the animal kingdom (Animalia). This article discusses the history and concerns of that study. For a discussion of animals and a description of animal groups, see Animal.




The study of zoology can be viewed as a series of efforts to analyze and classify animals. Attempts at classification as early as 400 bc are known from documents in the Hippocratic Collection. Aristotle, however, was the first to devise a system of classifying animals that recognized a basic unity of plan among diverse organisms; he arranged groups of animals according to mode of reproduction and habitat. Observing the development of such animals as the dogfish, chick, and octopus, he noted that general structures appear before specialized ones, and he also distinguished between asexual and sexual reproduction. His Historia Animalium contains accurate descriptions of extant animals of Greece and Asia Minor. He was also interested in form and structure and concluded that different animals can have similar embryological origins and that different structures can have similar functions.

In Roman times Pliny the Elder compiled four volumes on zoology in his 37-volume treatise called Historia Naturalis. Although widely read during the Middle Ages, they are little more than a collection of folklore, myth, and superstition. One of the more influential figures in the history of physiology, the Greek physician Galen, dissected farm animals, monkeys, and other mammals and described many features accurately, although some were wrongly applied to the human body. His misconceptions, especially with regard to the movement of blood, remained virtually unchanged for hundreds of years. In the 17th century, the English physician William Harvey established the true mechanism of blood circulation.

Until the Middle Ages, zoology was a conglomeration of folklore, superstition, misconception, and descriptions of animals, but during the 12th century it began to emerge as a science. Perhaps the most important naturalist of the era was the German scholar St. Albertus Magnus, who denied many of the superstitions associated with biology and reintroduced the work of Aristotle. The anatomical studies of Leonardo da Vinci were far in advance of the age. His dissections and comparisons of the structure of humans and other animals led him to important conclusions. He noted, for example, that the arrangement of joints and bones in the leg are similar in both horses and humans, thus grasping the concept of homology (the similarity of corresponding parts in different kinds of animals, suggesting a common grouping). The value of his work in anatomy was not recognized in his time. Instead, the Belgian physician Andreas Vesalius is considered the father of anatomy; he circulated his writings and established the principles of comparative anatomy. See Anatomy.

Classification dominated zoology throughout most of the 17th and 18th centuries. The Swedish botanist Carolus Linnaeus developed a system of nomenclature that is still used today—the binomial system of genus and species (see Classification) —and established taxonomy as a discipline. He followed the work of the English naturalist John Ray in relying upon the form of teeth and toes to differentiate mammals and upon beak shape to classify birds. Another leading systematist of this era was the French biologist Comte Georges Leclerc de Buffon. The study of comparative anatomy was extended by such men as Georges Cuvier, who devised a systematic organization of animals based on specimens sent to him from all over the world.

Although the word cell was introduced in the 17th century by the English scientist Robert Hooke, it was not until 1839 that two Germans, Matthias Schleiden and Theodor Schwann, proved that the cell is the common structural unit of living things. The cell concept provided impetus for progress in embryology, founded by the Estonian scientist Karl Ernst von Baer, and for the development by a Frenchman, Claude Bernard, of the study of animal physiology, including the concept of homeostasis (the stability of the body's internal environment). See Cell; Physiology.

The organization of scientific expeditions in the 18th and 19th centuries gave trained observers the opportunity to study plant and animal life throughout the world. The most famous expedition was the voyage of the Beagle in the early 1830s. During this voyage, Charles Darwin observed the plant and animal life of South America and Australia and developed his theory of evolution by natural selection. Although Darwin recognized the importance of heredity in understanding the evolutionary process, he was unaware of the work of a contemporary, the Austrian monk Gregor Mendel, who first formulated the concept of particulate hereditary factors—later called genes. Mendel's work remained obscure until 1900. See Evolution; Heredity. See also biographical articles on persons mentioned




In the 20th century zoology has become more diversified and less confined to such traditional concerns as classification and anatomy. Broadening its range to include such studies as genetics, ecology, and biochemistry, zoology has become an interdisciplinary field applying a great variety of techniques to obtain knowledge of the animal kingdom.

The current study of zoology has two main focuses: on particular taxonomic groups, and on the structures and processes common to most of them.

Taxonomically oriented studies concentrate on the different divisions of animal life. Invertebrate zoology deals with multicellular animals without backbones; its subdivisions include entomology (the study of insects) and malacology (the study of mollusks). Vertebrate zoology, the study of animals with backbones, is divided into ichthyology (fish), herpetology (amphibians and reptiles), ornithology (birds), and mammalogy (mammals). Paleontology, the study of fossils, is subdivided by taxonomic groups. In each of these fields, researchers investigate the classification, distribution, life cycle, and evolutionary history of the particular animal or group of animals under study. Most zoologists are also specialists in one or more of the process-oriented disciplines described below.

Morphology, the study of structure, includes gross morphology, which examines entire structures or systems, such as muscles or bones; histology, which examines body tissues; and cytology, which focuses on cells and their components. Many great advances made in cytology in recent years are attributable to the electron microscope and the scanning electron microscope. Special staining techniques and radioactive tracers have been used to differentiate structural detail at the molecular level. Methods have been developed for mapping neural connections between parts of the brain and for stimulating and recording impulses from specific brain sites and even individual nerve cells. See Brain.

Physiology, the study of function, is closely associated with morphology. An important subdivision is cellular physiology, which is closely related to molecular biology. Another active field, physiological ecology, studies the physical responses of animals to their environment. Much of this work has been carried out on desert, arctic, and ocean animals that must survive extremes of temperature or pressure.

Animal behavioral studies developed along two lines. The first of these, animal psychology, is primarily concerned with physiological psychology and has traditionally concentrated on laboratory techniques such as conditioning. The second, ethology, had its origins in observations of animals under natural conditions, concentrating on courtship, flocking, and other social contacts. Both subdisciplines have recently merged in large areas of investigation, the same scientists using field and laboratory observations and incorporating many experimental techniques from neurology. Perhaps the most important recent development in the field is the concentration on sociobiology, which is concerned with the behavior, ecology, and evolution of social animals such as bees, ants, schooling fish, flocking birds, and humans. Sociobiology is still in its infancy and is quite controversial, chiefly because it has raised anew the old dispute about whether behavior is genetically determined. See Animal Behavior; Sociobiology.

Embryology, the study of the development of individual animals, has investigated the way in which developing parts interact—for example, the interactions between the eyestalk and the epidermis during development of the lens of the eye. The emerging field of molecular development applies the techniques of molecular biology, including molecular genetics, to the finest and most obscure embryological details. See Development; Embryology.

The study of the interactions between animals and their environment is known as ecology. Primary attention is given to the complex pattern of interactions among the many species constituting a community. Ecology has been central to the development of conservation and environmental control during the past 20 years. It has revealed the deleterious effects of pesticides and industrial pollutants and has provided important insights into wiser management of agriculture, forestry, and fisheries.

Evolutionary zoology, which draws on all of the fields just mentioned, is concerned with the mechanisms of evolutionary change—speciation and adaptation—and with the evolutionary history of animal groups. Particularly relevant to evolutionary studies are systematics, phylogenetics, paleontology, and zoogeography. Systematics deals with the delineation and description of animal species and with their arrangement into a classification. Phylogenetics is the study of the developmental history of groups of animals. Zoogeography, the study of the distribution of animals over the earth, is closely related to ecology and systematics. See Animal Distribution; Paleontology; Species and Speciation.

Contributed By:
Walter J. Bock

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