how is buck and spitz first confrontation interrupted?

Spitz is driven off by Francois who administers a severe lashing.

Buck staggered over against the sled, exhausted, sobbing for breath, helpless. This was Spitz’s opportunity. He sprang upon Buck, and twice his teeth sank into his unresisting foe and ripped and tore the flesh to the bone. Then Francois’s lash descended, and Buck had the satisfaction of watching Spitz receive the worst whipping as yet administered to any of the teams.