Mad Planet Annotated

When the twenty-first century drew to a close the whole human race began to revert to conditions closely approximating savagery. The low-lands were unbearable. Thick jungles of rank growth covered the ground. The air was depressing and enervating. Men could live there, but it was a sickly, fever-ridden existence. The whole population of the earth desired the high lands and as the low country became more unbearable, men forgot their two centuries of peace.