240 THE FORMATION 



Other, not all invasion produces succession. The number of invaders must be 

 large enough, or tiieir effect must be sufficiently modifying or controlling to 

 bring about the gradual decrease or disappearance of the original occupants, 

 or a succession will not be established. Partial or temporary invasion can 

 never initiate a succession unless the reaction of the invaders upon the habitat 

 is very great. Complete and permanent invasion, on the other hand, regularly 

 produces successions, except in the rare cases where a stable formation en- 

 tirely replaces a less stable one without the intervention of other stages. 

 Succession depends in the first degree upon invasion in such quantity and of 

 such character that the reaction of the invaders upon the habitat will prepare 

 the way for further invasion. The characteristic presence of stages in a 

 succession, which normally correspond to formations, is due to the peculiar 

 operation of invasion with reaction. In the case of a denuded habitat, for 

 example, migration from adjacent formations is constantly taking place, but 

 only a small number of migrants, especially adapted to somewhat extreme 

 conditions, are able to become established in it. These reach a maximum 

 development in size or number, and in so doing react upon the habitat in 

 such a way that more and more of the dormant disseminules present, as well 

 as those constantly coming into it, find the conditions favorable for germina- 

 tion and growth. The latter, as they in turn attain their maximum, cause 

 the gradual disappearance of the species of the first stage, and at the same 

 time prepare the way for the individuals of the succeeding formation. It is 

 at present impossible to determine to what degree this substitution is due to 

 the struggle for existence between the individuals of each species and be- 

 tween the somewhat similar species of each stage, and to what degree it 

 arises out of the physical reaction. 



It is evident that geological succession is but a larger expression of the 

 same phenomenon, dealing with infinitely greater periods of time, and pro- 

 duced by physical changes of such intensity as to give each geological stage 

 its peculiar stamp. If, however, the geological record were sufficiently com- 

 plete, we should find imquestionably that these great successions merely rep- 

 resent the stable termini of many series of smaller changes, such as are 

 found everywhere in recent or existing vegetation. 



289. Kinds of succession. The fundamental causes of succession are in- 

 vasion and reaction, but the initial causes of a particular succession are to 

 be sought in the physical or biological disturliances of a habitat or forma- 

 tion. With reference to the initial cause, we may distinguish normal succes- 

 sion, 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 



