How did we get from barbarians to Gregorian chants?

From The Seven Story Mountain by Thomas Merton

How did it ever happen that, when the dregs of the world had collected in western Europe, when Goth and Frank and Norman and Lombard had mingled with the rot of old Rome to form a patchwork of hybrid races, all of them notable for ferocity, hatred, stupidity, craftiness, lust, and brutality—how did it happen that, from all of this, there should come Gregorian chant, monasteries and cathedrals, the poems of Prudentius, the commentaries and histories of Bede, the Moralia of Gregory the Great, St. Augustine’s City of God, and his Trinity, the writings of Anselm, St. Bernard’s sermons on the Canticles, the poetry of Caedmon and Cynewulf and Langland and Dante, St. Thomas’ Summa, and the Oxoniense of Duns Scotus? How does it happen that even today a couple of ordinary French stonemasons, or a carpenter and his apprentice, can put up a dovecote or a barn that has more architectural perfection than the piles of eclectic stupidity that grow up at the cost of millions of dollars on the campuses of American universities?