Brewer's: Hark Back
(To). To return to the subject. “Revenons à nos moutons ” (q.v.). A call to the dogs in fox-hunting, when they have overrun the scent, “Hark [dogs] come back”; so “Hark forards!” “Hark away!” etc.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894