Lamarckism \La"marck"ism\, n. [From Lamarck, a distinguished French naturalist.]

(Biol.) The theory that structural variations, characteristic of species and genera, are produced in animals and plants by the direct influence of physical environments, and esp., in the case of animals, by effort, or by use or disuse of certain organs.

Source: Webster's Revised Unabridged Dictionary (1913)