A systematic survey of how organisms act and respond, ranging from cellular and plant reactions to reflexes, instinctive patterns, and learned intelligent acts. The author considers consciousness and mental development, reports experimental and observational evidence from insects, birds, and mammals, and treats social phenomena such as imitation, communication, tradition, play, courtship, and emotion. Physiological, biological, and psychological perspectives are brought together to trace the evolution and interaction of habit, instinct, and intelligence, with comparative examples and experiments used to illustrate continuity in the development of animal life and conduct.