There will come soft rains and the smell of the ground, And swallows circling with their shimmering sound; And frogs in the pools singing at night, And wild-plum trees in tremulous white; Robins will wear their feathery fire(5) Whistling their whims on a low fence-wire; And not one will know of the war, not one Will care at last when it is done. Not one would mind, neither bird nor tree If mankind perished utterly;(10) And Spring herself, when she woke at dawn, Would scarcely know that we were gone. How does the poet develop the theme of this poem?