In the following a brief summary is given of some of the essentials of the theory of the secondary structure. This structure is very sensitive to external disturbing effects which frequently cause the transition of crystals from thermally stable into thermally pseudostable configurations for which the term mosaic structure is proposed. A discussion of the relation of the mosaic structure to the secondary structure is given. The essentially new points of this paper are brought forth in §5, where it is shown that several very fundamental causes are operative during the growth of crystals which endanger the formation of thermally stable configurations. The most important of these causes is due to the fact that on solidification the heat of fusion must be carried away from the growing surface. The resulting temperature gradient very often is great enough to produce plastic deformations in the crystal in status nascendi. Finally a short review is given which intends to show that the adoption of the conception of a secondary structure and the closely related mosaic structure provides an adequate basis for the understanding of the totality of phenomena which is characteristic for the solid state of matter.