The hereditary factors of behavior may be studied in animals and humans. Charles Darwin, who originated the theory that natural selection is the basis of biological evolution, was persuaded by Francis Galton that the principles of natural selection applied to behavior as well as physical characteristics. Members of a species vary in the expression of certain behaviors because of variations in their genes, and these behaviors have survival value in some environments. One example of such a behavior is curiosity—some organisms are more curious than others, and in some settings curiosity is advantageous for survival. Therefore, in those environments more of the organisms that express curiosity survive to reproduce than do those that lack this trait.