The events of The Mayor of Casterbridge span more than twenty years – most of Henchard's adult life. In a sense, you can read Henchard's downfall as a commentary on the way the past always comes back to haunt you. There's no way to outrun your past actions. Lucetta's death is likewise caused by past events coming to light. In the world of the novel, history always repeats itself, and your past actions will always come back to bite you.
In The Mayor of Casterbridge, the past continually comes back to haunt the present.
The Casterbridge Ring represents the tendency for history – even ancient history – to repeat itself.