I was reading in Chapter I on “Domestic Dogs and Cats,” in Darwin’s Volume I of his “Variations book”, 1897, that greyhounds who had “failed in courage” had been successfully bred with a bulldog to improve their “perseverance.”  Apparently, after 6 or 7 generations, the bulldog external traits disappeared in the greyhounds, but the vestiges of the bulldog”perseverance” and “indomitable spirit” remained within the artificially selected greyhounds.  I did not know this until I read the paragraph beginning with “As changes in domestic breeds,” in in Volume I of his “Variations book”, 1897.