Lamarckism is the first theory of evolution , which was proposed by Jean Baptiste de Lamarck (1744-1829), a French biologist and was explained in his famous book Philosophie Zoologigue (1809).  It includes four main propositions: Internal vital force, Effect of environment and new needs, use and disuse of organs, inheritance of acquired characters.
Examples explaining Lamarck’s theory
Evolution of long neck in giraff as per use and disuse of organs
A group of scientists has further studied Lamarck’s theory and has supported its modified form, which is known as Neo Lamarkism. A few Neo Lamarckism are Spencer, Cope, Richard, Wells, Lawrence, Naegeli, Gadow, Dali, McDougall etc. They suggested that if not all, some of the acquired characters are inherited to some extent.
Lamarckism  vs  Neo Lamarckism
Neo Lamarckism
It is the original theory given by Lamarck
It is the modification of the original theory of Lamarck in order to  make it suitable to modern knowledge.
The theory lays stress on internal vital force, appetency and use and disuse of organs.
Neo Lamarckism does not give any importance to these factors.
It believes that change in environment brings about a conscious reaction in animals.
The theory stresses on the direct effect of changed environment on the organisms.
According to Lamarckism the acquired characters are pass on to the next generation.
Normally only those modifications are transferred to the next generation which influence germ cells or where somatic cells give rise to germ cells.

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