"And all times are one time, and all those dead in the past never lived before our definition gives them life, and out of the shadow their eyes implore us. That is what all of us historical researchers believe. And we love truth." [p. 342]
Jack's development is the most important them in the novel. He begins his journey as a man who looks at both history and life as a roller coaster of uncontrollable events and ends with a firm belief in historical connectedness. His initial belief allowed him freedom from personal responsibility, but we eventually see his acceptance of that same responsibility. Jack's passion becomes finding history's truths, and the way each of those truths builds a bridge to the one that follows.
The "great twitch" is a crutch he uses to get over a love affair that ended badly. He was hurt............ the concept of the "great twitch" allowed him time to ruminate because it supported his opinion (very real to him) that life has no meaning, and that no one is responsible for their choices or their destiny; everything is preordained and thus every day we're alive is simply fate.