Source : Webster's Revised Unabridged Dictionary (1913)

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 : WordNet®

Lamarckism
     n : a theory of organic evolution claiming that acquired
         characteristics are transmitted to offspring