Why plagiarism isn't "intertextuality".

Suppose I set a novel in Wyoming, and have my main characters be Frodo and his uncle Bilbo. That would set up intertextuality with Lord of The Rings. Suppose Bilbo left a valuable object to his nephew in his will...   Or I wrote a micro fiction in which a cockroach turns into a human being. Bolaño wrote a sequel to Kafka's story of Josephine the mouse singer. Graham Greene has a novel based obviously on the Quijote. Ashbery has a cento in which each line is a line from a famous poem. Koch parodies "This is Just to Say" in a poem called "Variations on a Theme by William Carlos Williams."  A play called "Lorca in a Green Dress" uses material from Lorca's plays and poems.

Intertexuality depends on the reader recognizing the source. It so happens I don't know the origins of all the lines in Ashbery's poem, but I can tell very early on that it's a cento (and even labelled as such!).  A concealed "intertextuality" with an unknown text is plagiarism. Concealing the origins, and using a "intertext" that most people would not recognize, means that there is no longer any intertextuality at work, because you don't have two things playing off each other.

Most intertextuality is with the canon. I'd say more: by using a text as intertext, if it is not already canonical, one is canonizing it, or treating it as a work that ought to be recognized by the reader.

As the cat climbed over the top of the jamcloset

Silent, upon a peak in Darien

To soothe a time-worn man

Silent, upon a peak in Darien

Caminante, son tus huellas el camino, y nada más. Caminante, no hay camino

Silent, upon a peak in Darien

As I sd to my friend, because I am always talking, John, I sd, which was not his name

Silent, upon a peak in Darien

Stout Cortez!

Silent, upon a peak in Darien 

First the right forefoot carefully

Silent, upon a peak in Darien 

Yo quiero ser llorando el hortelando de la tierra que ocupas y estercolas 

Silent upon a peak in Darien

Bright star, would I were steadfast as thou art, not in mute splendor

Silent, upon a peak in Darien...