Das mine!' protested Ava, Bennie's daughter, affirming Alex's recent theory that language acquisition involved a phase of speaking German. She snatched a plastic skillet away from his own daughter, Cara-Ann, who lurched after it, roaring, 'Mine pot! Mine pot!
Universal grammar is about what language is: it is to be distinguished from prescriptive grammars, often distilled in newspaper columns, which tell us what language should be. We are all entitled to our own opinions of what is appropriate, be it in the arrangement of words or flowers - as long as we keep in mind that these are just opinions. The properties of universal grammar linguists have unearthed, however, are a useful defense when language "authorities" try to rationalize their pontifications: none of the don'ts they advertise can be found in the book of universal grammar.
Now we have come full circle to the subtitle of this book: children learn by unlearning other languages. Viewed in the Darwinian light, all humanly possible grammars compete to match the language spoken in the child's environment. And fitness, because we have competition, can be measured by the compatibility of a grammar with what a child hears in a particular linguistic environment. This theory of language takes both nature and nurture into account: nature proposes, and nurture disposes.