In Chapter 9, Alec D'Urberville encounters Tess practicing her whistle. Ever predatory in his manner, Alec decides he will teach her to whistle. In doing so, he makes an allusion through his choice of tune:

He suited the action to the word, and whistled a line of "Take, O take those lips away." But the allusion was lost upon Tess.

Alec—and, in turn, Hardy—is referencing a line from a song that appears in Measure by Measure, a play by Shakespeare. The song is as follows: "Take, oh take those lips away, / That so sweetly were foresworn, / And those eyes: the breake of day, / Lights that do mislead the Morn; / But my kisses bring again, bring again, / Seals of love, but sealed in vain, sealed in vain." Despite declaring innocent intentions in his efforts to teach Tess to whistle, the tune Alec chooses betrays his amorous intent, since the song itself is a love song (though it's worth noting that, like most of Shakespeare's work, the song is really about a rather tumultuous, complex kind of love affair). Still, Tess remains unaware of this predatory action for obvious reasons: she lacks worldly experience and a formal education. Alec's unrelenting sexual advances are a form of power he exerts over her, making it very difficult for her to ever free herself from his influence. It's clear, then, that Alec is more interested in this twisted kind of power he holds over Tess than his actual romantic feelings for her—otherwise, he wouldn't purposefully alienate her by making references he knows she won't understand.