Lamarck theory is called the theory of inheritance of acquired characters. It says that the organisms are exposed to new needs due to a change in the environment. They acquire same characters during their lifetime to adapt themselves to the new needs. These characters are passed on to the next generation. For example, the long neck of the giraffe is due to needing to catch the leaves of long tress. A. Weismann rejected the theory of Lamarck, stating that adaptations based in the somatic cells cannot be inherited.
