We explain the results recently obtained for the damage-spreading behaviour
in the Bak-Sneppen (BS) model (Tamarit er al 1998 Eur. Phys. J. B 1 545).
We do this by relating the BS model to a much simpler one, which includes m
any features of the BS model and provides a clear explanation for the occur
rence of power-law growth of the distance.