=289. Kinds of succession.= The fundamental causes of succession are invasion and reaction, but the initial causes of a particular succession are to be sought in the physical or biological disturbances of a habitat or formation. With reference to the initial cause, we may distinguish _normal succession_, which begins with nudation, and ends in stabilization, and _anomalous succession_, in which the facies of an ultimate stage of a normal succession are replaced by other species, or in which the direction of movement is radically changed. The former is of universal occurrence and recurrence; the latter operates upon relatively few ultimate formations. In the origin of normal successions, nudation may be brought about by the production of new soils or habitats, or by the destruction of the formation which already occupies a habitat. In a few cases, the way in which the habitat arises or becomes denuded is not decisive as to the vegetation that is developed upon it, but as a rule the cause of nudation plays as important a part in the development of a succession as does the reaction exerted by the invaders. The importance of this fact has been insisted upon under invasion. New soils present extreme conditions for ecesis, possess few or no dormant disseminules, and in consequence their successions take place slowly and exhibit many stages. Denuded soils as a rule offer optimum conditions for ecesis as a result of the action of the previous succession, dormant seeds and propagules are abundant, and the revegetation of such habitats takes place rapidly and shows few stages. The former may be termed _primary succession_, the latter _secondary succession_. _PRIMARY SUCCESSIONS_ =290.= These arise on newly formed soils, or upon surfaces exposed for the first time, which have in consequence never borne vegetation before. In general they are characteristic of mountain regions, where weathering is the rule, and of lowlands and shores, where sedimentation or elevation constantly occur. The principal physical phenomena which bring about the formation of new soils are: (1) elevation, (2) volcanic action, (3) weathering, with or without transport.