Read the excerpt from "Poetry." When they become so derivative as to become unintelligible, the same thing may be said for all of us, that we do not admire what we cannot understand: the bat holding on upside down or in quest of something to eat, elephants pushing, a wild horse taking a roll, a tireless wolf under a tree, the immovable critic twitching his skin like a horse that feels a flea, the base- ball fan, the statistician—What theme does this excerpt express?