Previous Period

Three giants of nineteenth-century life sciences set the stage for many of the biological investigations undertaken in the first half of the twentieth century. Charles Darwin's (1809-1882) On the Origin of Species (1859) established the theory of evolution by natural selection and focused attention on questions of how organisms are related to each other and how life on earth has changed over time. Claude Bernard's (1813-1878) research in physiology, particularly on the chemical functioning of animal tissue, and his writings advocating the experimental method helped make biology less of a purely observational science in which most emphasis was placed on classification and on describing the anatomy of organisms. Because of Bernard's influence, biology became more experimental in the twentieth century. Louis Pasteur's (1822-1895) research on infectious disease and on the chemistry of microbes made careful microscopic and biochemical studies more crucial...