  We discuss the lifetimes and evolution of clumps and cores formed as
turbulent density fluctuations in nearly isothermal molecular clouds. In the
non-magnetic case, clumps are unlikely to reach a hydrostatic state, and
instead are expected to either proceed directly to collapse, or else
``rebound'' towards the mean pressure and density of the parent cloud.
Rebounding clumps are delayed in their re-expansion by their self-gravity. From
a simple virial calculation, we find re-expansion times of a few free-fall
times. In the magnetic case, we present a series of driven-turbulence,
ideal-MHD isothermal numerical simulations in which we follow the evolution of
clumps and cores in relation to the magnetic criticality of their ``parent
clouds'' (the numerical boxes). In subcritical boxes, magnetostatic clumps do
not form. A few moderately-gravitationally bound clumps form which however are
dispersed by the turbulence in < 1.3 Myr. An estimate of the ambipolar
diffusion (AD) time scale t_AD in these cores gives t_AD > 1.3 Myr, only
slightly longer than the dynamical times. In supercritical boxes, some cores
become locally supercritical and collapse in typical times ~ 1 Myr. We also
observe longer-lived supercritical cores that however do not collapse because
they are smaller than the local Jeans length. Fewer clumps and cores form in
these simulations than in their non-magnetic counterpart. Our results suggest
that a) A fraction of the cores may not form stars, and may correspond to some
of the observed starless cores. b) Cores may be out-of-equilibrium structures,
rather than quasi-magnetostatic ones. c) The magnetic field may help reduce the
star formation efficiency by reducing the probability of core formation, rather
than by significantly delaying the collapse of individual cores.
