Lamarckism is a now discredited theory of evolution developed by French biologist Jean-Baptiste Pierre Antoine de Monet, Chevalier de Lamarck in the 19th century. Lamarckism holds that traits acquired (or diminished) during the lifetime of an organism can be passed on to the offspring. Lamarck based his theory on two observations, in his day considered to be generally true:

1. Use and disuse – Individuals lose characteristics they do not require (or use) and develop characteristics that are useful.
2. Inheritance of acquired traits – Individuals inherit the traits of their ancestors.

Examples of Lamarckism would include:

• Giraffes stretching their necks to reach leaves high in trees, strengthen and gradually lengthen their necks. These giraffes have offspring with slightly longer necks.
• A blacksmith, through his work, strengthens the muscles in his arms. His sons will have like muscular development when they mature.

With this in mind, Lamarck developed two laws:

1. In every animal which has not passed the limit of its development, a more frequent and continuous use of any organ gradually strengthens, develops and enlarges that organ, and gives it a power proportional to the length of time it has been so used; while the permanent disuse of any organ imperceptibly weakens and deteriorates it, and progressively diminishes its functional capacity, until it finally disappears.
2. All the acquisitions or losses wrought by nature on individuals, through the influence of the environment in which their race has long been placed, and hence through the influence of the predominant use or permanent disuse of any organ; all these are preserved by reproduction to the new individuals which arise, provided that the acquired modifications are common to both sexes, or at least to the individuals which produce the young.

In essence, a change in the environment brings about change in "needs" (besoins), resulting in change in behavior, bringing change in organ usage and development, bringing change in form over time—and thus the gradual transmutation of the species.

See also: Epigenetic inheritance, Lysenkoism, Orthogenesis.