Final answer:
Fitzgerald shifts the mood in the paragraph by using descriptive language.
Step-by-step explanation:
In the paragraph that begins "Already it was deep summer", Fitzgerald shifts the mood by using descriptive language. This is evident in the intensely detailed descriptions of various human figures and the movement of butterflies. The use of terms like "curiously irregular" and "zig-zag" creates a sense of vagueness and randomness, shifting the mood from the previous one. Additionally, the change of focus to the positioning and attitudes of a man and a woman alerts the reader to anticipate that they will be the subject of what follows, further shifting the mood of the paragraph.