According to Darwin, individuals multiply in geometric ratio whereas space and food remain almost constant. There is an intense competition and three-fold struggle to ensure living, to obtain maximum amount of food and suitable land. The struggle may be intraspecific, interspecific or with the environment. During the struggle for existance only those individuals could survive which exhibit such variations that are proved to be more beneficial in facing the hardships and rigours of environment or which change to adopt them to the changing conditions. Darwin illustrated the survival of the fittest by taking example of giraffe. Lamarck's theory of evolution is popularly known as The inheritance of acquired characters in organisms. According to him, the constant use of an organ increases its efficiency and size and leads to its better development, while the organ which is not used for a long time leads to reduction in efficiency and size. Darwin received a manuscript on The tendency of variations to depart from the original type from Alfred Russel Wallace, in Malaya. Archipelago, who had taken observations almost identical to Darwin. The , paper contained complete details of the theory of origin of species by means of natural selection.