It has been known that there are two schemes in the framework of three
flavor neutrinos to accommodate the global features of the hot dark m
atter neutrinos, the solar neutrino deficit and the atmospheric neutri
no anomaly in a manner consistent with terrestrial neutrino experiment
s, i.e,, hierarchical mass neutrinos and almost degenerate neutrinos.
We demonstrate that the recent result by the CHOOZ experiment excludes
the scheme of hierarchical neutrinos. We also point out in the scheme
of almost degenerate neutrinos that if neutrinos are Majorana particl
es then the double beta decay experiments must see positive signals on
their way to reach a limit more stringent than the present one by a f
actor of 5. (C) 1998 Elsevier Science B,V.