The White Whale swam before him as the monomaniac incarnation of all those malicious agencies which some deep men feel eating in them, till they are left living on with half a heart and half a lung. What theme of the novel does this excerpt best address?

- The Self-destructive nature of obsession.

Explanation:

Theme is demonstrated as the key idea or message that an author intends to convey or reflect through a work. Such ideas function to unify/combine all the elements of the plot together and are often universal in nature so that the readers could associate themselves to it.

In the given excerpt, the theme that is being described here would be 'the self destructive nature of obsession' through which the author connects to the audience by suggesting that the excessive passion or obsession with anything leads to invite disastrous consequences as reflected by the phrase 'they are left with half a heart and half a lung'.