Richard Woltereck was a German zoologist and hydrobiologist who studied aquatic animals and extended the concept of Reaktionsnorm (norm of reaction) to the study of genetics. He also provided some of the first experimental evidence for the early twentieth-century embryological theory of heredity known as cytoplasmic inheritance. Through experiments on the water flea, Daphnia, Woltereck investigated whether variation produced by environmental impacts on development could play a role in heredity and evolution. Woltereck's research emphasized the importance of environment and development in Wilhelm Johannsen's concepts of genotype and phenotype. Biologists throughout the twentieth century used Woltereck's concept of Reaktionsnorm to develop theories and experiments to explain the evolution of adaptive developmental responses to environmental conditions. Later in his career, Woltereck developed a theory of heredity that sought to reconcile embryological concepts, such as regulation and body plans, with Mendelian heredity and Darwinian evolution by natural selection.