What is lamarckism?

Lamarckism is the theory that unites the law of use and disuse with the law of the transmission of acquired characteristics, i.e., that asserted that acquired characteristics, for instance, the muscular mass, could be transmitted from a parent to its offspring.

The theory was proposed by the French naturalist Lamarck in the starting of the 19th century. At that time the idea was not so absurd since nobody knew how the transmission of hereditary characteristics occurred. (Lamarck had great merit in introducing an evolutionary theory based in natural law at a time dominated by fixism.)