Every five years in baseball, there is a new generation of ballplayers that becomes the staple of the major leagues. It could be because of a shift in emphasis. Maybe pitchers had a new pitch of choice (the cutter, the split). Maybe the stadiums changed (AstroTurf came on the scene, then it quietly went away). Maybe the game opened up to new markets (enter Japan, the advent of Latin American academies, the ebb and flow of African-American participation).