INVASION AND SUCCESSION 275 



species, or one in which the development departs radically from 

 the normal course. Normal successions occur everywhere, anoma- 

 lous ones are infrequent. 



Normal successions may arise upon soils or habitats newly 

 formed, or upon those laid bare by the destruction of the plants 

 upon them. New soils are more or less unfavorable to ecesis. 

 They possess few or no seeds, and many changes are necessary 

 before a stable formation can take possession. Consequently a 

 succession in a new soil takes place slowly and exhibits many 

 stages. It is termed a 'primary succession. Denuded soils as a 

 rule offer much better conditions for ecesis owing to the action 

 of the previous vegetation. Dormant seeds are more or less 

 abundant. Revegetation takes place rapidly and shows few stages. 

 Such a succession is termed secondary. 



A normal succession is usually perfect, i.e., its stages occur 

 in the usual sequence from initial to final formations without an 

 omission. Imperfect succession results when one or more of the 

 stages is omitted anywhere in the course, and a later stage appears 

 before its turn. It may develop at any time when a new or de- 

 nuded habitat is so surrounded by other vegetation that the usual 

 invaders are unable to enter, or when the abundance, nearness, or 

 mobility of certain species enables them to take possession before 

 their turn. 



294. Primary successions. These arise on newly formed soils 

 or upon those exposed for the first time. In general they are 

 characteristic of mountain regions, where weathering is the rule, 

 and of lowlands and shores where sedimentation is constantly 

 occurring. The physical processes that bring about the for- 

 mation of new soils are: (1) elevation, (2) volcanic action, 

 (3) weathering. The first two arc infrequent and often local, 

 and hence need not be considered here.^ Weathering is prac- 

 tically universal, and in connection with the disposition of the 

 weathered material furnishes the basis for distinguishing new 

 soils. In accordance, the latter are divided into the following 

 groups: . (1) residuary soils, which are formed in the positif)n 

 occupied by the original rock, (2) colluvial soils, duo to the action 

 of gravity upon the weathered material, (3) alluvial soils, those 

 arising by deposit in water, (4) a^olian soils, wliicli are formed 

 or deposited by winds, (5) glacial soils, due to the action of gla- 



' Research Methods, 241. 



