Zoology, which is one of two main branches of biology, is the science of living things. Botany, the other main branch of biology, involves the study of plants. Zoology includes the study of any kind of animal, ranging from a 180 ton blue whale to one-celled creatures such as bacteria. Animals include humans and other mammals, birds, insects, fish, reptiles, invertebrates, and even microscopic organisms. Presently, more than a million different species of animals have been identified, and new species continue to be identified every year.

The science of zoology began in the days of Greek scientist Aristotle (384-322B.C.), who developed a system to classify animals. Eighteenth century botanist Carolus Linneaus developed a hierarchical system for classifying plants and animals that is still used today. In his system, the basic unit of classification is called the species.

The science of classification was the primary focus of zoology...