A quick word about apostrophes

Apostrophes, as you know, indicate possessives rather than plurals. Hence "the cat's" means "belonging to the cat" rather than "three or more cats." For some reason, this distinction is one of the first to fly out the window when we write papers: I have seen A papers with misused apostrophes.

When you are creating the possessive of a proper noun -- for instance, an author's name -- you always add 's, even if the name ends in an s already. Hence:

Yeats = The last name of William Butler Yeats
Yeats's = Belonging to Yeats
Yeats' = Utterly, utterly meaningless

Seriously, reading sentences like Yeats poem responds to the crisis of intimacy, etc., is a little... distressing.

(It's possible you were taught that names of Biblical figures can be given a possessive without adding an s after the apostrophe: Jesus' cross, Moses' tablets, etc. This is technically true, although this option has fallen into disuse. In any case, Yeats is not a Biblical figure, however much he might have seen himself as one.)

For further information on this topic, consult Bob the Angry Flower's Quick Guide to the Apostrophe.