Beginning with Darwin's work in the 1870s, this text selects the important works from the discipline's first hundred years (44 papers) and presents them in facsimile, tracing the development of the field. These papers are considered classics because they either founded a line of investigation, established a basic method, or provided a new approach to an important research question. The papers are divided into six sections, each introduced by prominent researchers. Sections one and two cover the origins and history of the field and the emergence of basic methods and approaches. They provide a background for sections three to six, which focus on development and learning; neural and hormonal mechanisms of behaviour; sensory processes, orientation, and communication.