Zoology is the study of animals. It is a subdivision of biology, which is the study of all living things. Zoology encompasses all aspects, from the molecular level, to the organ, to that of the whole animal, and from unicellular organisms to whales. There are many subdisciplines of zoology (e.g., genetics, biochemistry, parasitology, physiology, ethology, ecology, systematics, ichthyology, ornithology, mammology, and so on), with further subdivisions (e.g., physiological ecology, neurophysiology, respiratory physiology, etc.).

Initial studies of zoology concentrated on describing the anatomy and categorizing animals (systematics), and this goes on today. Not all animals are named and described (especially insects), and there is a continuing argument as to the most appropriate grouping of animals (classification).

More recently, interest has developed into the molecular aspects of zoology, made possible by techniques developed in chemistry and physics but applied to biological systems. There are many similarities at the molecular level between animals and plants, and consequently the subject is usually referred to as Molecular Biology rather than Molecular Zoology or Botany.

The earliest known system of zoological classification is that of Aristotle, who attempted in the 4th century B.C. to group animals according to such criteria as mode of reproduction and possession or lack of red blood. Aristotle's pupil Theophrastus classified plants according to their uses and methods of cultivation. Little interest was shown in classification until the 17th and 18th centuries, when botanists and zoologists began to devise the modern scheme of categories. The designation of groups was based almost entirely on superficial anatomical resemblances.

Before the idea of evolutionĘthere was no push to show more meaningful relationships among species because the species was thought to be uniquely created and fixed in character, the only real, or natural, taxon, while the higher taxa were only regarded as an artificial means of organizing information. Since anatomical resemblance is an important indication of relationship, early classification efforts resulted in a system that often approximated a natural one and that is still the system that is used with considerable modifications. The most extensive work was done in the mid-18th century by Carolus Linnaeus, who devised the presently used system of nomenclature. As biologists came to accept the work of Charles Darwin in the second half of the 19th century, they began to stress the significance of evolutionary relationships for classifications.

Although comparative anatomy remained important, other evidence of relationship was sought as well. Paleontology provided fossil evidence of the common ancestry of various groups; embryology provided comparisons of early development in different species, an important clue to their relationships. In the 20th century, evidence provided by genetics and physiology became increasingly important. Recently there has been much emphasis on the use of molecular genetics in taxonomy, as in the comparison of nucleic acid sequences in the genetic makeup of organisms. Computers are increasingly used to analyze data relevant to taxonomy.

Man, animals, and scientific investigations

From earliest times animals have been vitally important to man; cave art demonstrates the practical and mystical significance animals held for prehistoric man. Early efforts to classify animals were based on physical resemblance, habitat, or economic use.

Although Hippocrates and Aristotle did much toward organizing the scientific thought of their times, systematic investigation declined under the Romans and, after Galen's notable contributions, came to a virtual halt lasting through the Middle Ages (except among the Arab physicians). With the Renaissance, direct observation of nature revived; landmarks were Vesalius' anatomy and Harvey's demonstration of the circulation of blood. The invention of the microscope and the use of experimental techniques expanded zoology as a field and established many of its branches, e.g., cytology and histology. Studies in embryology and morphology revealed much about the nature of growth and the biological relationships of animals.

The system of binomial nomenclature was devised to indicate these relationships; Linnaeus was the first to make it consistent and apply it systematically. Paleontology, the study of fossil organisms, was founded as a science by Cuvier c.1812. Knowledge of physiological processes expanded greatly when physiology was integrated with the chemical and other physical sciences. The establishment of the cell theory in 1839 and the acceptance of protoplasm as the stuff of life 30 years later gave impetus to the development of genetics. Lamarck, Mendel, and Darwin presented concepts that revolutionized scientific thought. Their theories of evolution and of the physical basis of heredity prompted research into all life processes and into the relationships of all organisms. The classic work of Pasteur and Koch opened up bacteriology as a field. Modern zoology has not only concentrated on the cell, its parts and functions, and on expanding the knowledge of cytology, physiology, and biochemistry, but it has also explored such areas as psychology, anthropology, and ecology.

Binomial Nomenclature

The present system of binomial nomenclature identifies each species by a scientific name of two words, Latin in form and usually derived from Greek or Latin roots. The first name (capitalized) is the genus of the organism, the second (not capitalized) is its species. The scientific name of the white oak is Quercus alba, while red oak is Quercus rubra. The first name applies to all species of the genus; Quercus is the name of all oaks, but the entire binomial applies only to a single species. Many scientific names describe some characteristic of the organism (alba=white; rubra=red); many are derived from the name of the discoverer or the geographic location of the organism. Genus and species names are always italicized when printed; the names of other taxa (families, etc.) are not. When a species (or several species of the same genus) is mentioned repeatedly, the genus may be abbreviated after its first mention, as in Q. alba. Subspecies are indicated by a trinomial; for example, the southern bald eagle is Haliaeetus leucocephalus leucocephalus, as distinguished from the northern bald eagle, H. leucocephalus washingtoniensis.

The advantages of scientific over common names are that they are accepted by speakers of all languages, that each name applies only to one species, and that each species has only one name. This avoids the confusion that often arises from the use of a common name to designate different things in different places, or from the existence of several common names for a single species. There are two international organizations for the determination of the rules of nomenclature and the recording of specific names, one for zoology and one for botany. According to the rules they have established, the first name to be published (from the work of Linnaeus on) is the correct name of any organism unless it is reclassified in such a way as to affect that name (for example, if it is moved from one genus to another). In such a case definite rules of priority also apply.