Answer: Lamarckism is the theory of evolution given by Jean Baptiste Lamarck. It says that characters acquired during the lifetime of an organism are passed on to its offspring.
The theory has four points: (1) living organisms tend to increase in size, (2) a new need gives rise to a new habit and the organ used more develops while the unused organ degenerates (use and disuse), (3) such changes acquired during life are inherited, and (4) this leads to evolution.
Example 1 — Giraffe: the ancestors of the giraffe had short necks. To reach the leaves on tall trees they had to stretch their necks again and again. The neck became longer during their lifetime, and this acquired character was passed on to the next generation. In this way the long-necked giraffe evolved.
Example 2 — Aquatic birds: the ancestors of ducks and swans lived on land but had to go into water for food. The skin between their toes stretched by constant use and developed into a web; this character was inherited by the next generation.
(Later, August Weismann showed by his experiments on mice that changes in body cells are not inherited, so Lamarckism is not accepted as the main mechanism of evolution.)