Lamarckism is the first theory of evolution, which was proposed by Jean Baptiste de Lamarck. Lamarckism includes four main factors
(i) Internal vital force. All the living things and their component parts are continually increased due to internal vital force.
(ii) Effect of Environment and New Needs. A change in environment brings about changes in organisms. It gives rise to new needs. New needs or desires produce new structures and change habits of the organisms.
(iii) Use and Disuse of Organs. If an organ is constantly used it would be better developed whereas disuse of organ results in its degeneration.
(iv) Inheritance of Acquired Characters. Whatever an individual acquires (to possess) characters in its life time due to internal vital force, effect of environment, new needs and use and disuse of organs, they are inherited (transmitted) to the next generations. The process continues. After several generations, the variations are accumulated upto such extent that they give rise to new species.