An interview with Orson Welles taped on 3rd October 1985.
"The classical Western [...] was the dominant form from 1930 to 1955. The classical plot is the one in which a stranger rides into town, resists society (and is resisted by them), but eventually ends up saving the good townsfolk from some villainous threat – and subsequently accepts (and is accepted by) the society he has saved. Wright identifies variations on this theme as well. More significantly, however, he identifies a shift to an appreciably different form of the genre, which was dominant from 1958 to 1970. Wright calls this second form the Professional plot: it is the Western in which a group of heroes – or anti–heroes – (rather than an individual) who fight for money (rather than for justice or some other ideal), exemplified by films such as The Wild Bunch, The Professionals, and Butch Cassidy and The Sundance Kid."