this theory was put forth by darwin in 1859 in a work entitled The origin of species by means of natural selection. The author argues that, in the struggle for existence, those plants and creatures best fitted to the requirements of the situation in which they are placed are the ones that will live; in other words, that nature selects those which are survive. This is the theory of natural selection or the survival of the fillest. He also argues that natural selection is capable of modifying and producing organisms fit for their circumstances. See development theory, under development.