Natural selection is a process in nature by which the organisms best suited to their environment are the ones most likely to leave offspring. This process has been called survival of the fittest, though a more accurate term would be reproduction of the fittest. The theory of natural selection was first explained in detail in the 1850's by the British naturalist Charles R. Darwin. He believed all plants and animals had evolved--that is, developed by changing over many generations--from a few common ancestors by means of natural selection. Plants and animals produce many offspring, but some of the young die before they can become parents. According to Darwin's theory, natural selection determines which members of a species die prematurely and which ones survive and reproduce.