The concentration of wealth is natural and recurrent, driven by differences in human ability, and it builds until the disparity reaches a breaking point. At that point society either redistributes wealth peacefully through legislation and taxation or violently through revolution, and this oscillation repeats across history.
Moral codes are not fixed absolutes but adaptations of behavior to the economic conditions of an era: hunting societies, agricultural societies, and industrial societies each produce different and appropriate moralities. What looks like moral decay in a transition is often the lag between old codes and changed material conditions.
Freedom and equality are sworn enemies: when one prevails the other dies, because leaving people free to act allows the naturally unequal in ability to produce unequal results. Only the average person favors equality, while the able favor liberty, so a society cannot maximize both at once.
The Durants point to Rome's decline as caused less by barbarian invasion than by internal factors: falling birth rates among the capable classes, moral disintegration, economic exhaustion, and dependence on imported populations and mercenaries. The barbarians did not conquer a healthy civilization; they entered one already hollowed from within.
View human history as a branch of biology, subject to the same laws governing all life: individuals and groups compete for resources, nature selects the fit according to shifting circumstances, and populations rise and fall by differential reproduction. Through this lens war, inequality, and group rivalry appear not as aberrations but as expressions of biological competition.