This is the sentence in which this phrase appears:
The great Northern cycle of which he is the returning unit has almost completed its round, accomplished itself.
("Herman Melville's Typee and Omoo", Ch. 10 of Studies in Classic American Literature, by D.H. Lawrence)
Lawrence wrote in the period after the First World War, where hundreds of thousands of men would die in a single battle (e.g.Somme, 1915).
The term returning unit would be used to denote a single unit from a larger formation that survived and returned. A platoon or squad from a regiment maybe, or a battalion from an entire division.
For the readers of that period, returning unit would have meant something immediate and personal.
