The Law of Civilization and Decay: An Essay on History by Brooks Adams

The evidence, however, seems to point to the conclusion that, when a highly centralized society disintegrates, under the pressure of economic competition, it is because the energy of the race has been exhausted. Consequently, the survivors of such a community lack the power necessary for renewed concentration, and must probably remain inert until supplied with fresh energetic material by the infusion of barbarian blood.