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.
Charles Yang, The Infinite Gift: How Children Learn and Unlearn the Languages of the World
language, language-acquisition