What is the Lamarckian Theory of Evolution: Acquired Characteristics
The modification an organism acquires during its lifetime can be passed along to its offspring. (ex: giraffes stretched their necks to reach for leaves on higher branches. The offspring were believed to inherit the valuable trait of longer necks b/c of parents use of necks)
however, modern genetics have disproved this theory. Studies have shown that if you cut off a mouse's tail, its offspring still have tails!