The now-discredited theory that acquired traits can be inherited is called Lamarckism. Named for French naturalist Jean-Baptiste Lamarck, the idea held that organisms could change during their lifetimes through use and disuse of body parts, and that these acquired changes could be passed to their offspring.
A famous textbook illustration involves giraffes: according to the traditional version, repeated stretching for high leaves lengthened their necks, and that acquired length was inherited by later generations. Lamarck’s broader evolutionary system was more complex than this simplified example, but inheritance of acquired characteristics became its best-known feature.
Mendelian genetics and the discovery of how hereditary information is transmitted through genes undermined classical Lamarckism. Modern evolution explains adaptation primarily through heritable genetic variation and natural selection. Some environmentally influenced epigenetic changes can persist across generations, but these limited phenomena do not restore the original Lamarckian theory or show that acquired organs are inherited as Lamarck proposed.