PRIMARY SUCCESSIONS 107 



successions merely represent the stable termini of many 

 series of smaller changes, such as are found everywhere in 

 recent or existing vegetation. 



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 {successio nw- 

 malis), which begins with nudation, and ends in stabilis- 

 ation, and anomalous succession {successio anomala), 

 in which the facies of an ultimate stage of a normal suc- 

 cession are replaced by other species, or in which the direc- 

 tion of movement's radically changed. The former is of 

 universal occurrence and recurrence ; the latter operates up- 

 on 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 wav 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 import- 

 ant 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 pre- 

 sent 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 pro- 

 pagules are abundant, and the revegetation of such habitats 

 takes place rapidly and shows few stages. The former may 

 be termed primary succession {successio primitiva), 

 the latter, secondary succession {successio secuta). 

 PRIMARY SUCCESSIONS 



These arise on newly formed soils, or upon surfaces ex- 

 posed for the first time, which have in consequence never 

 borne vegetation before. In general they are character- 

 istic of mountain regions, where weathering is the rule. 



