  We investigate the dynamics of spin glasses from the `rheological' point of
view, in which aging is suppressed by the action of small, non-conservative
forces. The different features can be expressed in terms of the scaling of
relaxation times with the magnitude of the driving force, which plays the role
of the critical parameter. Stated in these terms, ultrametricity loses much of
its mystery and can be checked rather easily. This approach also seems a
natural starting point to investigate what would be the real-space structures
underlying the hierarchy of time scales. We study in detail the appearance of
this many-scale behavior in a mean-field model, in which dynamic ultrametricity
is clearly present. A similar analysis is performed on numerical results
obtained for a 3D spin glass: In that case, our results are compatible with
either that dynamic ultrametricity is absent or that it develops so slowly that
even in experimental time-windows it is still hardly observable.
