The heading “Souls Belated” already points to the theme of the story. The two main characters, Lydia Tillotson and Ralph Gannett, are deeply in love with each other.
However, Lydia has already been married and at this time society despises divorce and remarriage.
Especially in Lydia´s view, the two could never have a real marriage because only “if [she had] known [him] as a girl- that would have been a real marriage.” (110) Assuming that they finally found their soulmate in each other, one could argue that “Souls Belated” simply means that they met too late in their lives to have a carefree future together.
Resources:
Lewis, R.W.B., ed. The Collected Short Stories of Edith Wharton. New York: Scribner, 1968. Print.104-127.