  Among the tracers of the earliest phases in the massive star formation
process, methanol masers have gained increasing importance. The
phenomenological distinction between Class I and II methanol masers is based on
their spatial association with objects such as jets, cores, and ultracompact
HII regions, but is also believed to correspond to different pumping
mechanisms: radiation for Class II masers, collisions for Class I masers. In
this work, we have surveyed a large sample of massive star forming regions -
296 objects divided into two groups named 'High' and 'Low' according to their
[25-12] and [60-12] IRAS colours - in Class I and II methanol masers. Previous
studies indicate that the High sources are likely more evolved. Therefore, the
sample can be used to assess the existence of a sequence for the occurrence of
Class I and II methanol masers during the evolution of a massive star forming
region. We observed the 6 GHz (Class II) CH3OH maser with the Effelsberg 100-m
telescope, and the 44 GHz and 95 GHz (Class I) CH3OH masers with the Nobeyama
45-m telescope. We have detected: 55 sources in the Class II line (12 new
detections); 27 sources in the 44 GHz Class I line (17 new detections); 11
sources in the 95 GHz Class I line (all except one are new detections). Our
statistical analysis shows that the ratio between the detection rates of Class
II and Class I methanol masers is basically the same in High and Low sources.
Therefore, both masers are equally associated with each evolutionary phase. In
contrast, all maser species have about 3 times higher detection rates in High
than in Low sources. This might indicate that the phenomena that originate all
masers become progressively more active with time, during the earliest
evolutionary phases of a high-mass star forming region.
