What is the importance of the title "By the Waters of Babylon" and what is the problem with the Forest People?
The title "By the Waters of Babylon" is a clear allusion to Psalm 137 of the Bible, which begins "By the Waters of Babylon I sat down and wept." This Psalm is a lament of the Israelites for their lost "promised land" of Israel from which they have been exiled. Their homeland was destroyed and its people scattered. There is a clear parallel therefore between the lament of the Psalmist for his home and the realisation of John of what has happened and his sadness at what has been lost. Using this allusion therefore reinforces the message of the short story: the eventual threat of self-destruction if we are unable to curb our thirst for knowledge - and not "eat it too fast."
The problem with the Forest people demonstrates the kind of primal, savage society that John and his tribe live in. Despite the incredible advances that man made, the resulting cataclysm reduced mankind to a neanderthal-like state, robbing him of ideals such as the capacity for mutual understanding and the ability to co-exist with different people and replacing them with a Darwinian survival of the fittest code of law. This likewise reinforces the dangers of knowledge by showing us that any future man could have after such a cataclysm can only be bleak, barbaric and bellicose.
The allusion in the title is to the 137th Psalm. This psalm describes the despair of the exiled Israelites after Nebuchadnezzar II deported them from their lands in Babylon, after which he had the temple in Jerusalem destroyed. This allusion is symbolic because the despair of the Israelites is now being felt by the survivors of the Great Burning, who also live in exile. Like the Jews' continued desire to return to Jerusalem, John in the novel states a desire to return to, and rebuild, their ruined cities.
The Forest People are something of a cautionary tale. They are ignorant, and they fight against the more civilised people who, like John, regret the destruction of their great society and seek a return to that greatness. They indicate that human civilisation may advance to a very high level, only to be reduced to barbarism by the vicissitudes of such events as the Burnings.