106 SUCCESSION 



be large enough, or their effect must be sufficiently modify- 

 ing 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 in- 

 vaders upon the habitat is very great. Complete and per- 

 manent invasion, on the other hand, regularly produces suc- 

 cessions, except in the rare cases where a stable formation 

 entirely replaces a less stable one without the intervention 

 of other stages Succession depends in the first degree up- 

 on 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 form- 

 ations, is due to the peculiar operation of invasion with re- 

 action. In the case of a denuded habitat, for example, mi- 

 gration 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 pre- 

 sent, as well as those constantly coming into it, find the con- 

 ditions favorable for germination and growth. The latter, 

 as they in turn attain their maximum, cause the gradual dis- 

 appearance 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 between 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 ex- 

 pression of the same phenomenon, dealing with infinitely 

 greater periods of time, and produced by physical changes 

 of such intensity as to give each geological stage its peculiar 

 stamp. If, however, the geological record were sufiiciently 

 complete, we should find unquestionably that these great 



