Allegory is a lesson communicated through symbolism, usually involving moral or spiritual ideas. A paradox is a statement that seems to contradict itself but actually contains a truth, or it can be a statement or opinion that is contrary to a belief or idea that is generally accepted by a society.
The two main characters of this story are Mabel and Fergusson. They are from two different classes of society and would therefore be considered incompatible with one another. Society would consider them to be unsuited to each other since he is a doctor and she is a poor horse dealer's daughter. Their love is paradoxical because it goes against the established rules at that time that these two people should not cross the class lines of society.
When Fergusson saves Mabel from drowning in the pond, the author uses metaphor to communicate the passion of the couple as well as to express hope. This is where we see spiritual references, such as Mabel's body had "risen" from the pond and he "rose higher" when he carried her from the pond. The author is referring to Jesus rising from the dead. These spiritual metaphors would be characteristic of allegory.
It is through theme communicated through symbolism that we learn a lesson. Mabel symbolizes instinct while Fergusson represents logic. Mabel feels things deeply and responds to situations instinctively rather than carefully considering what her actions should be before she reacts. Fergusson has always based his decisions on science and logic rather than upon emotions. The personality of these two characters are contradictory in nature, yet their passion is unbridled. Can these two opposing forces coexist? What is the lesson (allegory) we learn? I'll leave that for you to decide.