In conventional drift wave theory, the density gradient kappa n=d ln n/dx determines the linear phase velocity, and the (electron) temperature gradient kappa T=d ln T/dx gives rise to a nonlinear term which leads to the existence of soliton-type solutions and solitary waves, Lakhin et al. (1987, Phys. Lett. A119, 348; 1988, Plasma Phys. Contr. Fusion 30, 457) claimed that it is not kappa T but essentially the derivative of the density gradient, d kappa n/dx, that is relevant. This claim is refuted by means of an expansion scheme in epsilon =e Phi /T<<1, where Phi is the drift wave potential.