zoology, branch of biology concerned with the study of animal life. 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 (see classification) 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.