The stability conditions in the Landau theory of second order phase transformation are discussed. It is shown that the restrictions imposed by Lifshitz on the change of type of symmetry are not necessary. As a consequence, the second order transformation at the Neél point of metals of the Lanthanum series such as dysprosium, which was thus far regarded as a problem that could not be treated on the basis of the Landau theory, is amenable to theoretical treatment. The linear relationship to temperature of the pitch of the helical spin structure of dysprosium is explained. In the meantime, a new method for the analysis of spin structure is developed employing the theory of space group representation and the minimum conditions of the thermodynamic potential. The results obtained by the application of this method to the magnetic structure of metals of the Lanthanum series are in agreement with the results of neutron diffraction experiments.