  We simulate the dynamics of fractal star clusters, in order to investigate
the evolution of substructure in recently formed clusters. The velocity
dispersion is found to be the key parameter determining the survival of
substructure. In clusters with a low initial velocity dispersion, the ensuing
collapse of the cluster tends to erase substructure, although some substructure
may persist beyond the collapse phase. In clusters with virial ratios of 0.5 or
higher, initial density substructure survives for several crossing times, in
virtually all cases. Even an initially homogeneous cluster can develop
substructure, if it is born with coherent velocity dispersion. These results
suggest that the simple initial conditions used for many sophisticated $N$-body
simulations could be missing a very important and dramatic phase of star
cluster evolution.
