When reeds are dead and a straw to thatch the marshes, And feathered pampas-grass rides into the wind Like aged warriors westward, tragic, thinned Of half their tribe, and over the flattened rushes, Stripped of its secret, open, stark and bleak, Blackens afar the half-forgotten creek,— Then leans on me the weight of the year, and crushes My heart. I know that Beauty must ail and die, And will be born again,—but ah, to see Beauty stiffened, staring up at the sky! Oh, Autumn! Autumn!—What is the Spring to me? What is the central idea of this poem?