Thanks to everyone who made interesting comments on yesterday's post about Dark Ages.

Several people challenged the matching of the economic/population decline to the "fall of Rome". For example, from David Friedman:

On the graph you are citing, 36 million is the population in 200 A.D. The fall of the Western Empire is commonly dated to about 450 A.D. By 400 A.D., on the same graph, population is down to 31 million–say 30 million by 450.