Inheritance of Acquired Characteristics

The inheritance of acquired characteristics is the hypothesis that modifications an organism undergoes during its lifetime — through use, disuse, environmental influence, or learning — can be transmitted to its offspring. It is the central mechanism of Lamarckism and was the primary target of the Modern Synthesis's rejection of directed variation.

The concept was revived in the twentieth century through epigenetic discoveries showing that environmentally induced changes to gene expression can be transmitted across generations. This 'soft inheritance' — inheritance that does not alter DNA sequence — is quantitatively minor compared to genetic inheritance but conceptually significant: it demonstrates that the Weismann Barrier between soma and germline is permeable, not absolute.

The inheritance of acquired characteristics was dismissed as biologically impossible for most of the twentieth century. The error was not in doubting Lamarck's mechanism. It was in conflating 'not the primary mechanism' with 'not a mechanism at all' — a conflation that served theoretical convenience at the cost of empirical accuracy.