Symmetry is characteristic of crystals as of bodies of plants and animals, but in the latter the range of forms is much greater. Crystals exhibit figures bounded by plane surfaces; organs and organisms introduce curved surfaces. Symmetry properties are related to material composition. The different types of symmetry can not be understood by isolated consideration of their particular molecular mechanism. Why a particular mechanism should exist in one species and another in a different species, whilst a still other and simpler constitution should be characteristic of non-living matter, can only be explained from a historical or evolutionary point of view, which involves the classification of the kinds of symmetry observed. Organisms do not coincide with geometrical forms; they only approximate thereto. Even the crystal is not mathematically perfect. Various ways of classifying organisms according to their symmetry are discussed, and their application to morphological problems pointed out. Organisms or their parts may be symmetrical in some respects but not in others. The addition of new parts, as in the growth of a plant, necessitates special symmetry considerations, which are dealt with. That the principle of symmetry forms a powerful weapon for analysis of organic form is shown by numerous examples.