Sometimes a great theory and its believers are like the parties to a marriage that fails after just a year or two. Maybe the two of them always looked great together, they seemed to be destined for a life together. But when they break up, the have to pick up the pieces and face their respective post-destiny lives. So it has been with one of my favorite theories of medieval history (yes, I have a top ten list in my mind of favorite theories of medieval history -- doesn't everybody? -- this one is my fourth). Associated with the Stanford University historian Lynn White, the theory posits that the invention of the stirrup, a neat way of maintaining the balance of the rider of a horse, was critical to the development of the feudal system. It was an adventurous speculation. The stirrup made possible shock combat (riders aiming lances at one another); the new form of combat required an infrastructure, armorers, pages, and so forth. The good lance bearers became knights, the low b...