Possible evolutionary connections between Herbig Ae/Be sources. Three populations of objects are identified: flat continuous disks (group II sources), flat disks with gaps (group II sources), and flaring transition disks (group I sources). Gapped group II sources are most likely descendants of group II sources without gaps. The arrows indicate two possible evolutionary ideas for the ancestors of transitional group I sources: green arrow = influx from a common ancestor; yellow arrow = influx from group II sources with gaps (cf. Fig. 1).