How would Darwin’s explanation of long necks in giraffes differ from Jean Baptiste Lamarck’s explanation of the same trait?

According to Darwin's theory, giraffes that happened to have slightly longer necks and limbs would have a better chance of securing food and thus be able to have more offspring -- the "select" who survive. Conversely, in Lamarck's view, a structure or organ would shrink or disappear if used less or not at all.