Heredity and its entities around 1900

This paper aims to give an impression of how biologists, at the turn of the twentieth century, came to conceptualize and define the hidden entities presumed to govern the process of hereditary transmission. With that, the stage was set for the emergence of genetics as a biological discipline that came to dominate the life sciences of the twentieth century. The annus mirabilis of 1900, with its triple re-appreciation of Gregor Mendel’s work by the botanists Hugo de Vries, Carl Correns, and Erich Tschermak, can be seen as the watershed after which theorizing about heredity and experimentation—selecting pure lines and Mendelian crossing—became tightly connected. As to concepts before this, this paper will analyze Carl von Nägeli’s ‘idioplasm’, Hugo de Vries’s ‘pangenes’, and August Weismann’s ‘germ plasm’. Carl Correns’s Anlagen and Wilhelm Johannsen’s ‘genes’ would replace them in the decade after 1900.