JÄGER, GUSTAV (1832– ), German naturalist and hygienist, was born at Bürg in Württemberg on the 23rd of June 1832. After studying medicine at Tübingen he became a teacher of zoology at Vienna. In 1868 he was appointed professor of zoology at the academy of Hohenheim, and subsequently he became teacher of zoology and anthropology at Stuttgart polytechnic and professor of physiology at the veterinary school. In 1884 he abandoned teaching and started practice as a physician in Stuttgart. He wrote various works on biological subjects, including Die Darwinsche Theorie und ihre Stellung zu Moral und Religion (1869), Lehrbuch der allgemeinen Zoologie (1871–1878), and Die Entdeckung der Seele (1878). In 1876 he suggested an hypothesis in explanation of heredity, resembling the germ-plasm theory subsequently elaborated by August Weismann, to the effect that the germinal protoplasm retains its specific properties from generation to generation, dividing in each reproduction into an ontogenetic portion, out of which the individual is built up, and a phylogenetic portion, which is reserved to form the reproductive material of the mature offspring. In Die Normalkleidung als Gesundheitsschutz (1880) he advocated the system of clothing associated with his name, objecting especially to the use of any kind of vegetable fibre for clothes.