  We suggest that molecular clouds can be formed on short time scales by
compressions from large scale streams in the interstellar medium (ISM). In
particular, we argue that the Taurus-Auriga complex, with filaments of 10-20 pc
$\times$ 2-5 pc, most have been formed by H I flows in $\lesssim 3$Myr,
explaining the absence of post-T Tauri stars in the region with ages $\gtrsim
3$ Myr. Observations in the 21 cm line of the H I `halos' around the Taurus
molecular gas show many features (broad asymmetric profiles, velocity shifts of
H I relative to $^{12}$CO) predicted by our MHD numerical simulations, in which
large-scale H I streams collide to produce dense filamentary structures. This
rapid evolution is possible because the H I flows producing and disrupting the
cloud have much higher velocities (5-10 kms) than present in the molecular gas
resulting from the colliding flows. The simulations suggest that such flows can
occur from the global ISM turbulence without requiring a single triggering
event such as a SN explosion.
