240 THE FORMATION 
other, not all invasion produces succession. The number of invaders must be 
large enough, or their 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 unquestionably that these great successions merély 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 disturbances 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 
