  Motivated by recent proposals (Bialynicki-Birula, Mycielski; Haag, Bannier;
Weinberg; Doebner, Goldin) for nonlinear quantum mechanical evolution equations
for pure states some principal difficulties in the framework of usual quantum
theory, which is based on its inherent linear structure, are discussed. A
generic construction of nonlinear evolution equations through nonlinear gauge
transformations is indicated.
