You could think of "The Circus Animals' Desertion" as Yeats cleaning out his closet, sifting through old socks and last year's coats. In this case, though, it's last year's characters (or, er, last decade's characters). Chances are that a journey through the past will yield things that don't seem like the brilliant ideas they appeared to be when you first came up with them. As this poem proves, thoughts (like coats) have a shelf life. What seemed useful and honest in 1907 might just seem contrived and complicated in 1939. At least, that's the conclusion Yeats reaches.
In this poem, Yeats proves that exploring old topics with fresh eyes can be just as valuable as charting new territory.
Yeats proves that exploring the human heart is more important than exploring myth when it comes to writing poetry.