Weismann's Germ Plasm Theory

Weismann's germ plasm theory holds that, in multicellular organisms, hereditary information is carried only by a distinct line of reproductive cells, which August Weismann called the germ plasm, and that this hereditary material is entirely separate from and unaffected by changes occurring in the organism's other, non-reproductive somatic cells. German biologist August Weismann developed the theory across the 1880s and 1890s, presenting it most fully in his 1892 book Das Keimplasma: eine Theorie der Vererbung, and formulated the related concept of the Weismann barrier, the proposition that hereditary information flows only in one direction, from germ cells to somatic cells, never in reverse. The theory directly contradicted the earlier Lamarckian idea that acquired characteristics could be inherited, and Weismann supported his position experimentally by cutting off the tails of mice across many successive generations and showing their offspring continued to be born with tails of normal, unreduced length. Formulated before the rediscovery of Gregor Mendel's work and before the chromosomal basis of heredity was understood, Weismann's germ plasm theory anticipated the modern distinction between an organism's genotype and its phenotype and became a foundational pillar of the twentieth century Modern Synthesis of evolutionary biology.