In Seeley's striking phrase: 'The human harvest was bad,' It was bad in all classes, but the decline was most marked in the upper ranks, the most educated, the most civilized, the potential leaders of the race.

In the terrible words of Swift, facing his own madness, the Roman Empire might have cried: 'I shall die like a tree--from the top downwards.' Why (the insistent question forces itself) did this civilization lose the power to reproduce itself?
Was it, as Polybius said, because people preferred amusements to children or wished to bring their children up in comfort?
Hardly, for it is more marked among the rich than the poor and the rich can have the best of both worlds.

Was it because people had grown discouraged and disheartened, no longer believing in their own civilization and loath to bring children into the darkness and disaster of their war-shattered world?
We do not know.

But we can see the connection of the falling population with the other evils of the empire--the heavy cost of administration relatively heavier when the density of the population is low; the empty fields, the dwindling legions which did not suffice to guard the frontier.
To cure this sickness of population the Roman rulers knew no other way than to dose it with barbarian vigour.

Just a small injection to begin with and then more and more till in the end the blood that flowed in its veins was not Roman but barbarian.