Final answer:
Alison realizes that life is fleeting and short, symbolized by the mirage of an oasis in the desert, which represents life's illusions and the deeper truths that lie beneath the surface.
Step-by-step explanation:
In the context of the story, Alison realizes that life is fleeting and short. This realization often comes as a profound insight to characters in literature as they face situations that challenge their previous understanding of the world. The mirage in the desert, which represents a false vision of an oasis, can be seen as a metaphor for life's illusions. In the given reference, water indeed exists in the valley, but it is hidden beneath the surface, thereby suggesting that reality might be different from initial appearances and that deeper truths often lie below the surface of what we can immediately perceive.