"The neighbours began to form mutual alliances(1), wishing neither to do nor suffer violence among themselves. They appealed on behalf of their children and womenfolk, pointing out with gestures and inarticulate cries that it is right for everyone to pity the weak. It was not possible to achieve perfect unity of purpose. Yet a substantial majority kept faith honestly. Otherwise the entire human race would have been wiped out there and then instead of being propagated, generation after generation, down to the present day."

Lucretius
from "On the Nature of the Universe"
trans. R. E. Latham, 1951, revised by John Godwin, 1994.

1. "mutual alliances" apparently refers to the social contract outlined by Plato in "Republic" and is the opposite of Cicero's idea of inborn justice from "On Duties".

This is not Lucretius at his poetic best, but Plato would have preferred it that way.