Half the movie conists of this, both the parallels between The Diary Of Adam and Eve and the longing Twain feels for his lost wife, and the ending implication of a budding crush between Tom and Becky.
Doubles as a Tear Jerker, the ending to the Diary Of Adam and Eve. Man has lost Paradise, but is instead given the ability to Love and grow. In their old age, Adam and Eve ponders the journey that has brought them to this point, and considers themselves to have gained far more than they lost.
Eve: The Garden is lost, but I have found him... and am content...
Adam: Wherever she was, there was Eden...
Mark Twain merging with his dark side, and ascends together to whatever awaits him in the hereafter, rejoining his beloved wife.
Twain's final words as he departs, fully whole, with Halley's Comet:
Twain (his face becoming one with the comet's tail) The human race in all its poverty has only one truly effective weapon: Laughter. Against the assault of laughter nothing can stand. (Twain's comet face smiles)
Finally, Tom giving Becky the heartshaped leaf from the Garden Of Eden that Adam preserved for Eve.