"We rest; a dream has power to poison sleep. 

We rise; one wandering thought pollutes the day.

We feel, conceive, or reason; laugh or weep,

Embrace fond won, or cast our cares away;

It is the same: for, be it joy or sorrow,

The path of its departure still is free.

Man's yesterday may ne'er be like his morrow;

Nought may endure but mutability (67)"

Mutability is a poem by Shelly's husband, Percy Shelley. The word, mutability means the ability to change, which is most likely why Shelly chose to have this poem in her novel. The novel is about how Frankenstein and his monster change throughout their lives. They each change a significant amount. Victor has a lovely, loving childhood, but ends up alone and depressed. THe monster learns and grows very fast, starting out as an ignorant, kind soul, and ending with an angry, violent view of the world. The two characters constantly change, which is what the poem mutability is about.