-This is an exert from a poem by Samuel Taylor Coleridge written in 1797. The poem is about a mariner coming back from sea with his crew and all of the adventures that he went through. The one exert that Shelley chose to use matches exactly with what is happening in in Frankenstein at the moment. Victor has just created the monster and is frequently checking behind his back to see if the monster is close behind him. But he knows he is, even when he doesn't look, because the monster will always be there from then on, no matter how physically far away is actually is from Victor. 

http://www.ram.org/contrib/rime_ancient_mariner.html

http://en.wikipedia.org/wiki/The_Rime_of_the_Ancient_Mariner

http://en.wikipedia.org/wiki/Samuel_Taylor_Coleridge