The study of zoology can be viewed as a series of efforts to analyseand classify animals. Attempts at classification as early as 400 BCare known from documents in the Hippocratic Collection. Aristotle,however, was the first to devise a system of classifying animals thatrecognized a basic unity of plan among diverse organisms; he arrangedgroups of animals according to mode of reproduction and habitat. Observing the development of such animals as the dogfish, chick, andoctopus, he noted that general structures appear before specializedones, and he also distinguished between asexual and sexualreproduction. His Historia Animalium contains accurate descriptions ofextant animals of Greece and Asia Minor. He was also interested inform and structure and concluded that different animals can havesimilar embryological origins and that different structures can havesimilar functions.
In Roman times Pliny the Elder compiled four volumes on zoology in his37-volume treatise called Historia Naturalis. Although widely readduring the Middle Ages, they are little more than a collection offolklore, myth, and superstition. One of the more influential figuresin the history of physiology, the Greek physician Galen, dissectedfarm animals, monkeys, and other mammals and described many featuresaccurately, although some were wrongly applied to the human body. Hismisconceptions, especially with regard to the movement of blood,remained virtually unchanged for hundreds of years. In the 17thcentury, the English doctor William Harvey established the truemechanism of blood circulation. Until the Middle Ages, zoology was a conglomeration of folklore,superstition, misconception, and descriptions of animals, but duringthe 12th century it began to emerge as a science.
Perhaps the mostimportant naturalist of the era was the German scholar St AlbertusMagnus, who denied many of the superstitions associated with biologyand reintroduced the work of Aristotle. The anatomical studies ofLeonardo da Vinci were far in advance of the age. His dissections andcomparisons of the structure of humans and other animals led him toimportant conclusions. He noted, for example, that the arrangement ofjoints and bones in the leg are similar in both horses and humans,thus grasping the concept of homology (the similarity of correspondingparts 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 doctor Andreas Vesalius is considered the fatherof anatomy; he circulated his writings and established the principlesof comparative anatomy. Classification dominated zoology throughout most of the 17th and 18thcenturies. The Swedish botanist Carolus Linnaeus developed a system ofnomenclature and classification that is still used today—the binomialsystem of genus and species—and established taxonomy as a discipline.
He followed the work of the English naturalist John Ray in relyingupon the form of teeth and toes to differentiate mammals and upon beakshape to classify birds. Another leading systematist of this era wasthe French biologist Comte Georges Leclerc de Buffon. The study ofcomparative anatomy was extended by such men as Georges Cuvier, whodevised a systematic organization of animals based on specimens sentto him from all over the world. Although the word cell was introduced in the 17th century by theEnglish scientist Robert Hooke, it was not until 1839 that twoGermans, Matthias Schleiden and Theodor Schwann, proved that the cellis the common structural unit of living things.
The cell conceptprovided impetus for progress in embryology, founded by the Russianscientist Karl von Baer, and for the development by a Frenchman,Claude Bernard, of the study of animal physiology, including theconcept of homeostasis (the stability of the body’s internalenvironment). The organization of scientific expeditions in the 18th and 19thcenturies gave trained observers the opportunity to study plant andanimal life throughout the world. The most famous expedition was thevoyage of the Beagle in the early 1830s. During this voyage, CharlesDarwin observed the plant and animal life of South America andAustralia and developed his theory of evolution by natural selection.
Although Darwin recognized the importance of heredity in understandingthe evolutionary process, he was unaware of the work of acontemporary, the Austrian monk Gregor Mendel, who first formulatedthe concept of particulate hereditary factors—later called genes. Mendel’s work remained obscure until 1900. Current Studies:In the 20th century zoology has become more diversified and lessconfined to such traditional concerns as classification and anatomy. Broadening its range to include such studies as genetics, ecology, andbiochemistry, zoology has become an interdisciplinary field applying agreat variety of techniques to obtain knowledge of the animal kingdom.
The current study of zoology has two main focuses: on particulartaxonomic groups, and on the structures and processes common to mostof them. Taxonomically oriented studies concentrate on the different divisionsof animal life. Invertebrate zoology deals with multicellular animalswithout backbones; its subdivisions include entomology (the study ofinsects) and malacology (the study of molluscs). Vertebrate zoology,the study of animals with backbones, is divided into ichthyology(fish), herpetology (amphibians and reptiles), ornithology (birds),and mammalogy (mammals). Palaeontology, the study of fossils, issubdivided by taxonomic groups. In each of these fields, researchersinvestigate the classification, distribution, life cycle, andevolutionary history of the particular animal or group of animalsunder study.
Most zoologists are also specialists in one or more ofthe process-oriented disciplines described below. Morphology, the study of structure, includes gross morphology, whichexamines entire structures or systems, such as muscles or bones;histology, which examines body tissues; and cytology, which focuses oncells and their components. Many great advances made in cytology inrecent years are attributable to the electron microscope and thescanning electron microscope. Special staining techniques andradioactive isotopic tracers have been used to differentiatestructural detail at the molecular level. Methods have been developedfor mapping neural connections between parts of the brain and forstimulating and recording impulses from specific brain sites and evenindividual nerve cells.
Physiology, the study of function, is closely associated withmorphology. An important subdivision is cellular physiology, which isclosely related to molecular biology. Another active field,physiological ecology, studies the physical responses of animals totheir environment. Much of this work has been carried out on desert,arctic, and ocean animals that must survive extremes of temperature orpressure. Animal behavioural studies developed along two lines.
The first ofthese, animal psychology, is primarily concerned with physiologicalpsychology and has traditionally concentrated on laboratory techniquessuch as conditioning. The second, ethology, had its origins inobservations of animals under natural conditions, concentrating oncourtship, flocking, and other social contacts. Both subdisciplineshave recently merged in large areas of investigation, the samescientists using field and laboratory observations and incorporatingmany experimental techniques from neurology. Perhaps the mostimportant recent development in the field is the concentration onsociobiology, which is concerned with the behaviour, ecology, andevolution of social animals such as bees, ants, schooling fish,flocking birds, and humans. Sociobiology is still in its infancy andis quite controversial, chiefly because it has raised anew the olddispute about whether behaviour is genetically determined. Embryology, the study of the development of individual animals, hasinvestigated the way in which developing parts interact—for example,the interactions between the eyestalk and the epidermis duringdevelopment of the lens of the eye.
The emerging field of moleculardevelopment applies the techniques of molecular biology, includingmolecular genetics, to the finest and most obscure embryologicaldetails. The study of the interactions between animals and their environment isknown as ecology. Primary attention is given to the complex pattern ofinteractions among the many species constituting a community. Ecologyhas been central to the development of conservation and environmentalcontrol during the past 20 years. It has revealed the deleteriouseffects of pesticides and industrial pollutants and has providedimportant insights into wiser management of agriculture, forestry, andfisheries.
Evolutionary zoology, which draws on all of the fields just mentioned,is concerned with the mechanisms of evolutionary change—speciation andadaptation—and with the evolutionary history of animal groups. Particularly relevant to evolutionary studies are systematics,phylogenetics, palaeontology, and zoogeography. Systematics deals withthe delineation and description of animal species and with theirarrangement into a classification. Phylogenetics is the study of thedevelopmental history of groups of animals.
Zoogeography, the study ofthe distribution of animals over the Earth, is closely related toecology and systematics.