Mike:
Acton was referring to the urban population. Your graph referred to the founding of new towns, not the urban population. This has some bearing on the question, most likely, but it is not a direct response. Can you make a direct response to his speculation? I think the urban population as a percentage should be the focus, as total population declined temporarily after the Death.
Acton:
I sometimes wonder why it is that, every time I present the idea of technological progress colliding with human conservatism being the cause of the saeculum, it disappears beneath the surface of the pond with nary a response. Or else a response to something peripheral, as last time I mentioned it in conjunction with an idea that the Black Death might have been the first Crisis and started the ball rolling; Mike took issue with that and we got sidetracked. (Whether the Death was the first social moment is irrelevant to what I'm actually getting at. The idea would work as well if the Reformation was the first social moment. I do in fact think the Death was, but that's not the important point for me.)
Is this idea hard to understand? Have I failed to present it adequately?
Mass movements? True, there was slow communication in the Middle Ages, and widespread illiteracy to boot, but it seems to me that the lack of a Medieval saeculum (if there was such a lack) is adequately explained by the glacial pace of technological change, which failed to render values or institutions obsolete.