The cumulative distribution of stellar ages showed a distinct difference
among star forming regions in NGC 2264. A Kolmogorov-Smirnov test gave a very low probability of them
being from the same population. The results indicate that star formation in NGC 2264
started at the surface region (Halo and Field regions) about 6 -- 7 Myr ago,
propagated into the molecular cloud and finally triggered the recent star
formation in the Spokes cluster. The kind of sequential star formation that
started in the low-density surface region (Halo and Field regions) implies
that star formation in NGC 2264 was triggered by an external source.