  We study the behavior of the extended states of a two-dimensional electron
system in silicon in a magnetic field, B. Our results show that the extended
states, corresponding to the centers of different Landau levels, merge with the
lowest extended state as B --> 0. Using our data, we construct an
experimental-based ``disorder vs filling factor'' phase diagram for the integer
quantum Hall effect (QHE). Generalizing this diagram to the case of the
fractional QHE, we show that it is consistent with the recently observed direct
transitions between insulator and FQHE at 2/5, 2/7, and 2/9.
