  To understand the formation of stellar groups, one must first document
carefully the birth pattern within real clusters and associations. In this
study of Taurus-Auriga, we combine pre-main-sequence ages from our own
evolutionary tracks with stellar positions from observational surveys. Aided by
the extensive, millimeter data on the molecular clouds, we develop a picture of
the region's history. Star formation began, at a relatively low level and in a
spatially diffuse manner, at least 10 Myr in the past. Within the last few
million years, new stars have been produced at an accelerating rate, almost
exclusively within a confined group of striated cloud filaments. The gas both
inside and around the filaments appears to be in force balance. Thus, the
appearance of the filaments is due to global, quasi-static contraction of the
parent cloud material. Gravity drives this contraction and shock dissipation
mediates it, but the internal motion of the gas does not appear to be
turbulent. The accelerating nature of recent star formation means that the
condensation of cloud cores is a threshold phenomenon, requiring a minimum
background density. Other, nearby cloud regions, including Lupus and
Chamaeleon, contain some locales that have attained this density, and others
that have not. In the latter, we find extensive and sometimes massive molecular
gas that is still devoid of young stars.
