uva uvam vivendo varia fit

most agree that the phrase generally means something along the following lines: A grape changes color (i.e.,ripens) when it sees another grape.

From there, any number of interpretations have arisen to explain why McMurtry chose to communicate that particular idea. Probably the soundest theory is that the phrase serves as a metaphor for the group’s journey, as many of the story’s characters go through a process of personal maturation and development. Much like grapes ripen in the presence of others.