(In). One by one. The American Indians, when they go on an expedition, march one by one. The one behind carefully steps in the footprints of the one before, and the last man of the file obliterates the footprints. Thus, neither the track nor the number of invaders can be traced.
“Each man followed his leader in Indian file.” —Captain Burnaby: On Horseback through Asia Minor.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894