Biology (from Greek βιολογία - βίος, bio, "life"; an λόγος, logos, "speech" lit. "to talk about life"), is a branch of Natural Science, and is the study of living organisms and how they react to their environment. Biology deals with every aspect of life in a living organism. Biology examines the structure, function, growth, origin, evolution, and distribution of living things. It classifies and describes organisms, their functions, how species come into existence, and the interactions they have with each other and with the natural environment. Four unifying principles form the foundation of modern biology: cell theory, evolution, genetics and homeostasis.