(7) At present, successions are generally mesotropic, grassland and forest being the ultimate stages, though many are xerostatic or hydrostatic. If erosion continue until the sea level is reached, the ultimate vegetation of the globe will be hydrophytic. Should the heat of the sun decrease greatly before this time, the last vegetation will be xerophytic, i. e., crymophytic. (8) The operation of succession was essentially the same during the geological past as it is to-day. From the nature of their vegetation forms, the record deals largely with the ultimate stages of such successions. _CLASSIFICATION AND NOMENCLATURE_ =325. Basis.= New or denuded habitats arise the world over by the operation of the same or similar causes, and they are revegetated in consequence of the same reactions. Similar habitats produce similar successions. The vegetation forms and their sequence are usually identical, and the genera are frequently the same, or corresponding in regions not entirely unrelated. The species are derived from the adjacent vegetation, and, except in alpine and coast regions, are normally different. The primary groups of successions are determined by essential identity of habitat or cause, e. g., aeolian successions, erosion successions, burn successions, etc. When they have been more generally investigated, it will be possible to distinguish subordinate groups of successions, in which the degree of relationship is indicated by the similarity of vegetation forms, the number of common genera, etc. For example, burn successions in the Ural and in the Rocky mountains show almost complete similarity in the matter of vegetation forms and their sequence, and have the majority of their genera in common. A natural classification of successions will divide them first of all into normal and anomalous. The former fall into two classes, primary and secondary, and these are subdivided into a number of groups, based upon the cause which initiates the succession. [Illustration: Fig. 69. Aspen forest formation (_Populus-hylium_), the typical stage of burn successions in the Rocky mountains; it is sometimes an anomalous stage in primary successions, interpolated in place of the thicket formation.]