August Weismann

August Weismann was a German evolutionary biologist who proposed the germ plasm theory in the late nineteenth century, arguing that within a multicellular organism heredity is transmitted only through a distinct line of specialized reproductive, or germ, cells, and that the ordinary body cells, called somatic cells, form a separate line that cannot pass on to offspring any changes it experiences during the organism's lifetime. This theory directly contradicted the older idea, associated particularly with Jean-Baptiste Lamarck, that traits acquired by an organism during its life, such as a muscle strengthened through exercise, could be passed on to its offspring, and Weismann's germ plasm theory became an influential early argument against Lamarckian inheritance, helping shift biological thinking toward the view that heredity depends on a distinct, sequestered set of hereditary material passed largely unchanged between generations. In support of his theory, Weismann carried out experiments cutting the tails off mice over many successive generations and observing that the offspring continued to be born with full-length tails, which he offered as evidence against the inheritance of an acquired physical change. Weismann's central distinction between the heritable germ line and the non-heritable body, developed decades before the physical basis of heredity in chromosomes and DNA was understood, anticipated important elements of the modern understanding of genetics and evolutionary biology.