for those of us who are linear minded, the fault lies in Anu's definition, which is a bit misdirecting. actually, the phrase hearken back takes the second meaning, as is perhaps better explained here:
http://www.bartleby.com/68/26/2926.html.

the "usually" as given in sense two probably just shortcuts an explanation such as: sometimes people leave off the back but the context tells you that they refer to something said or done earlier. well, it is mostly a poetic (non-linear?) usage these days.

p.s. - in looking for recent citations, I found one with what I thought to be an odd phrase:

Nowadays... musical comedies all seem to hearken back to Old Vienna or Gay Paree, and they don't exactly contribute to the gaiety of nations. -Guardian, Dec. 15, 1967

the gaiety of nations, the cheerfulness or pleasure of a large number of people; general gaiety or amusement (freq. used hyperbolically or ironically).