Lamarck's Theory: - Lamarck was the first to establish an analytical theory of expression and recorded the subject in a book entitled Philosophical Zoology in 1809 . Lamarck's theory is called Lamarckism or Lamarckism. Lamarckism is based on a number of key themes. The theoretical aspects of Lamarck's doctrine of expression are discussed: [ 1] Principles of Use and Abuse - According to Lamarck , the emergence of a new organ or the extinction of an old organ may occur in an organism's need. According to him , if an organ of an organism is used continuously , then that organ will gradually become strong and well-organized for the needs of the environment. On the other hand , if an organ of an organism is useless to the environment, then that organ is no longer used — so the organ will become inactive and eventually become extinct as a result of continued misuse. According to Lamarck, the use and disuse of organs signify changes in the organism , which are inherited traits of the organism. [ 2] Influence of environment and the effort of the organism]: - The organism always tries to adapt itself to the ever changing environment. It is an innate instinct of the organism. Various changes are seen in the organism to adapt to the naturally changing environment. According to Lamarck , organisms can change the course of evolution by consciously , in their own efforts, by growing or destroying a particular organ. It is also an acquired trait of an organism. [ 3] Inheritance of acquired traits: - According to Lamarck, all the traits acquired during the life of an organism are transmitted from one gene to another, that is , the inheritance of acquired traits occurs. [ 4] Origin of new species]: - According to Lamarck's theory, due to the inheritance of acquired traits and the acquisition of new traits in each genus, new species gradually evolved from one species to another.