  We show that the emergence of criticality in the locally-defined Bak-Sneppen
model corresponds to separation over a hierarchy of timescales. Near to the
critical point the model obeys scaling relations, with exponents which we
derive numerically for a one-dimensional system. We further describe how the
model can be related to the glass model of Bouchaud [{\em J. Phys. I France
{\bf 2}, 1705 (1992)}], and we use this insight to comment on the usual
assumption of stationarity in the Bak-Sneppen model. Finally, we propose a
general definition of self-organised criticality which is in partial agreement
with other recent definitions.
