Laudator Temporis Acti

Eileen Power (1889-1940), Medieval People, 10th ed. (London: Methuen, 1963; rpt. 1966), p. 15:

But if the gradualness of this process misled the Romans there were other and equally potent reasons for their blindness. Most potent of all was the fact that they mistook entirely the very nature of civilization itself. All of them were making the same mistake. People who thought that Rome could swallow barbarism and absorb it into her life without diluting her own civilization; the people who ran about busily saying that the barbarians were not such bad fellows after all, finding good points in their regime with which to castigate the Romans and crying that except ye become as little barbarians ye shall not attain salvation; the people who did not observe in 476 that one half of the Respublica Romanorum had ceased to exist and nourished themselves on the fiction that the barbarian kings were exercising a power delegated from the Emperor. All these people were deluded by the same error, the belief that Rome (the civilization of their age) was not a mere historical fact with a beginning and an end, but a condition of nature like the air they breathed and the earth they tread. Ave Roma immortalis, most magnificent most disastrous of creeds!

I added a period after "tread."