THE DEVELOPMENT OF COMMUNITIES 325 



Progressive and Retrogressive, Primary and Secondary Successions, — 

 All seres which lead toward the climax are progressive. Retrogressive 

 successions, which recede from the cHmax, are often caused by man or 

 animals (Fig. 145). The spread of moors following a rise of the water 

 table is also a retrogressive succession {cf. p. 327). 



The Anglo-American school distinguishes between primary suc- 

 cessions or sequences of communities which originate independently 

 of man and secondary successions (Clements, 1905, 1916) caused by 

 human intervention. The most important causes of secondary suc- 



FiG. 162. — High dune advancing over swamp and forest. The willows surmount' 

 the advancing sand but many of the trees are killed. Dune Park, Indiana. {Photo 

 by Meyers.) 



cession are the complete or partial disturbance of original vegetation by 

 fire, clearing, pasturing, or cultivation and consequent human utiliza- 

 tion. Freed from the hand of man the vegetation again moves toward 

 the climax (Fig. 147). Secondary successions are abundant in all 

 civilized countries on abandoned arable land. In the "deflected 

 succession" (Godwin and Tansley, 1929) normal development is 

 prevented by anthropogenic influences but is not completely checked. 

 For example, by grazing, the normal succession toward forest may be 

 directed toward a grassland community. 



Climax Fluctuations. — The mature climax of vegetation may suffer 

 injuries through human or natural agencies, such as wind, snow, 

 flooding or the advance of sand dunes (Fig. 162), which may 



