  Linear analysis of the formation of protostellar cores in planar magnetic
interstellar clouds shows that molecular clouds exhibit a preferred length
scale for collapse that depends on the mass-to-flux ratio and neutral-ion
collision time within the cloud. We extend this linear analysis to the context
of clustered star formation. By combining the results of the linear analysis
with a realistic ionization profile for the cloud, we find that a molecular
cloud may evolve through two fragmentation events in the evolution toward the
formation of stars. Our model suggests that the initial fragmentation into
clumps occurs for a transcritical cloud on parsec scales while the second
fragmentation can occur for transcritical and supercritical cores on subparsec
scales. Comparison of our results with several star forming regions (Perseus,
Taurus, Pipe Nebula) shows support for a two-stage fragmentation model.
