desertsThe term is a generic locution for an area uninhabited by humanity (see II:2:13 and note). Thus, the Creature's plan to seek "the vast wilds of South America" is compatible with this meaning.
Compare the OED:
1. An uninhabited and uncultivated tract of country; a wilderness: . . . b. formerly applied more widely to any wild, uninhabited region, including forest-land.
See also Johnson's definition from the Dictionary of 1755:
DESERT. n.s. [desertum, Latin.] A wilderness; solitude; waste country; uninhabited place.
Be alive again,
And dare me to the desert with thy sword.
Of trembling I inhibit; then protest me
The baby of a girl. Shakespeare's Macbeth.
He, looking round on every side, beheld
A pathless desert, dusk with horrid shades. Paradise Reg.