  The linear stability parameter delta is commonly used as a figure of merit
for the nonlinear dynamics of the tearing mode. It is shown, through state of
the art numerical simulations, that factors other than delta can play a very
important role in determining the evolution of nonlinear magnetic islands. In
particular, two different equilibria are analysed and it is shown that, once
perturbed, they have a qualitatively and quantitatively different response
despite the fact that they are characterised by the same delta. The different
behaviour can still be associated with linear properties of the equilibrium. It
is also studied how the nonlinear and saturation phase are affected by an
increasing delta in the two equilibria. As the instability drive is increased,
the systems move from a dynamics characterised by a "universal" generalised
Rutherford equation to a Y-point configuration and then to a plasmoid unstable
Y-point. Finally, for even larger delta the second harmonic overcomes the
fundamental,leading to an interesting double island structure.
