We cannot here discuss in detail the question as to why, in the clash between the two, it was civilization which perished and barbarism which prevailed.

But it is important to remember that while the Empire tried to defend its frontiers against the barbarian hosts, it gradually opened them to barbarian settlers.
This peaceful infiltration of barbarians which altered the whole character of the society which it invaded would have been impossible, of course, if that society had not been stricken by disease.

The disease is plain enough to see by the third century.

It shows itself in those internecine civil wars in which civilization rends itself, province against province and army against army.

It shows itself in the great inflationary crisis from about 268 and in the taxation which gradually crushed out the smaller bourgeoisie while the fortunes of the rich escaped its net.