In the introduction to the 5th edition Darwin gave full credit to Spencer , writing `` I have called this principle , by which each slight variation , if useful , is preserved , by the term Natural Selection , in order to mark its relation to man 's power of selection . In the introduction he gave full credit to Spencer , writing `` I have called this principle , by which each slight variation , if useful , is preserved , by the term Natural Selection , in order to mark its relation to man 's power of selection .