The theme of separation plays a large role in the story.  Ratan's entire life in the story is bookended by separation.  On one hand, she is an orphan.  The opening of the story represents how Ratan has been separated from social community or context and a personal one in her orphaned state.  In this, there is separation evident as part of her own being in the world.  Additionally, I think that the theme is relevant as the postmaster, himself, has been separated from his own world and social context.  It is this separated sense of consciousness that brings both Ratan and the postmaster together in the first place.  Separation has allowed bonds to be forged.  This is ruptured in the ending, where Ratan again feels the pain of separation when the postmaster laughs in taking her with him.  Both of them going separate ways is something in the ending that enables Tagore to discuss the philosophical implications of how human beings break bonds in order to be separate from one another.  Ratan's plight at the end is one where she has almost come to peace with this condition of separation, wandering about wondering for her "dada- babu."  For his part, the postmaster recognizes the intense pain of separation, but he rationalizes it away with his feeling that this is the way of the world.  In the end, Tagore presents the theme of separation as something not only for both the characters, but an element of real life and a part of being in the modern setting.