What specific words and phrases does she use to link the conclusion to her introduction? How do these words and phrases support her argument?

Quindlen links the conclusion to the introduction of her essay with the words "like many improbable ideas, when it works, it's a wonder", which is very close to her claim in the overall text, "America is an Improbable idea", it isn't supposed to work but it does despite all of the differences, specifically racial or ethnic, within it.