Greek philosopher and scientist Aristotle (384–322 B.C.E.) is considered the “father of zoology.” His contributions to the field include vast quantities of information about the variety, structure, and behavior of animals; the analysis of the parts of living organisms; and the beginnings of the science of taxonomy. During Aristotle’s time, only 500 species were known—a list he divided into eight classes. Swiss naturalist Konrad von Gesner is often credited as the “father of modern zoology.” In 1551, he wrote the first volume to his three-volume Historia Animalium (The History of Animals) that served as a standard reference work throughout Europe in the sixteenth and seventeenth centuries. Swiss naturalist and philosopher Abraham Trembley (1710–1784) is considered the founder of experimental zoology. Much of his research involved studying the regeneration of hydras.