Biological factors were more fundamental. A serious decline of population appears in the West after Hadrian. It has been questioned, but the mass importation of barbarians into the Empire by Aurelius, Valentinian, Aurelian, Probus, and Constantine leaves little room for doubt. Aurelius, to replenish his army, enrolled slaves, gladiators, policemen, criminals; either the crisis was greater, or the free population less, than before; and the slave population had certainly fallen. So many farms had been abandoned, above all in Italy, that Pertinax offered them gratis to anyone who would till them. A law of Septimius Severus speaks of a penuria hominum – a shortage of men. In Greece the depopulation had been going on for centuries. In Alexandria, which had boasted of its numbers, Bishop Dionysius calculated that the population had in his time (250) been halved. He mourned to “see the human race diminishing and constantly wasting away.” Only the barbarians and the Orientals were increasing, outside the Empire and within.
What had caused this fall in population? Above all, family limitation. Practiced first by the educated classes, it had now seeped down to a proletariat named for its fertility; by A.D. 100 it had reached the agricultural classes, as shown by the use of imperial alimenta to encourage rural parentage; by the third century it had overrun the western provinces, and was lowering man power in Gaul. Though branded as a crime, infanticide flourished as poverty grew. Sexual excesses may have reduced human fertility; the avoidance or deferment of marriage had a like effect, and the making of eunuchs increased as Oriental customs flowed into the West. Plantianus, Praetorian Prefect, had one hundred boys emasculated, and then gave them to his daughter as a wedding gift.
Second only to family limitation as a cause of lessened population were the slaughters of pestilence, revolution, and war. Epidemics of major proportions decimated the population under Aurelius, Gallienus, and Constantine. In the plague of 260-65 almost every family in the Empire was attacked; in Rome, we are told, there were 5000 deaths every day for many weeks. The mosquitoes of the Campagna were winning their war against the human invaders of the Pontine marshes, and malaria was sapping the strength of rich and poor in Latium and Tuscany. The holocausts of war and revolution, and perhaps the operation of contraception, abortion, and infanticide, had a dysgenic as well as a numerical effect: the ablest men married latest, bred least, and died soonest. The dole weakened the poor, luxury weakened the rich; and a long peace deprived all classes in the peninsula of the martial qualities and arts. The Germans who were now peopling north Italy and filling the army were physically and morally superior to the surviving native stock; if time had allowed a leisurely assimilation they might have absorbed the classic culture and reinvigorated the Italian blood. But time was not so generous. Moreover, the population of Italy had long since been mingled with Oriental strains physically inferior, though perhaps mentally superior, to the Roman type. The rapidly breeding Germans could not understand the classic culture, did not accept it, did not transmit it; the rapidly breeding Orientals were mostly of a mind to destroy that culture; the Romans, possessing it, sacrificed it to the comforts of sterility. Rome was conquered not by barbarian invasion from without, but by barbarian multiplication within. Moral decay contributed to the dissolution. The virile character that had been formed by arduous simplicities and a supporting faith relaxed in the sunshine of wealth and the freedom of unbelief; men had now, in the middle and upper classes, the means to yield to temptation, and only expediency to restrain them. Urban congestion multiplied contacts and frustrated surveillance, immigration brought together a hundred cultures whose differences rubbed themselves out into indifference. Moral and esthetic standards were lowered by the magnetism of the mass; and sex ran riot in freedom while political liberty decayed.