  We study the effect of crystal symmetry and step-edge diffusion on the
surface current governing the evolution of a growing crystal surface. We find
there are two possible contributions to anisotropic currents, which both lead
to the destabilization of the flat surface: terrace current (j_t), which is
parallel to the surface slope, and step current (j_s), which has components
parallel (j_pa) and perpendicular (j_pe) to the slope. On a high-symmetry
surface, terrace and step currents are generically singular at zero slope, and
this does not allow to perform the standard linear stability analysis. As far
as a one-dimensional profile is considered, (j_pe) is irrelevant and (j_pa)
suggests that mound sides align along [110] and [1-10] axes. On a vicinal
surface, (j_s) destabilizes against step bunching; its effect against step
meandering depends on the step orientation, in agreement with the recent
findings by O.Pierre-Louis et al. [Phys. Rev. Lett. 82, 3661 (1999)].
