PRIMARY AND SECONDARY SUCCESSION. 169 



soils, except in cases of excessive erosion. Denuded soils as a rule offer 

 optimum conditions for ecesis, as a result of the action of the previous suc- 

 cession; dormant seeds and propagules are abundant, and the revegetation of 

 such habitats takes place rapidly and shows relatively few stages. The great 

 majority of secondary sucessions owe their origin to fire, floods, animals, or 

 the activities of man. They agree in occurring upon soils of relatively medium 

 water-content, which contain considerable organic matter and a large number 

 of dormant migrants. 



Successions were also classified as imperfect, continuous, intermittent, abrupt, 

 and interpolated upon the basis of the nature of development. Initial causes 

 were classified as (1) weathering, (2) erosion, (3) elevation, (^ subsidence, 

 (5) climatic changes, (6) artificial changes. The reactions of succession were 

 summarized as (1) by preventing weathering; (2) by binding aeolian soils; 

 (3) by reducing run-off and preventing erosion; (4) by filling with silt or plant 

 remains; (5) by enriching the soil; (6) by exhausting the soil; (7) by accumu- 

 lating humus; (8) by modifying atmospheric factors. It was further stated 

 that a natural classification of successions wiU divide them first of all into normal 

 and anomalous. The former fall into two classes, primary and secondary, 

 and these are subdivided into a number of groups, based upon the cause which 

 initiates the succession. 



Normal and anomalous succession. — ^The persistent study of successional 

 development for the decade since the preceding views were enunciated seems 

 to have confirmed and emphasized the distinction between normal and anoma- 

 lous succession. Normal succession is unit succession, that is, the development 

 from an initial bare area to a climax. It is represented by the sere, with its 

 distinctions of prisere and subsere. Anomalous succession may be termed 

 compound succession, i. e., that in which similar or related seres are combined 

 into a cosere as a consequence of climatic action. It is represented by the 

 cosere and clisere, and in its major expression by the great successions of 

 geological eras, the eoseres. Since climate rarely if ever produces a denuded 

 area of any extent, the earlier distinction of normal and anomalous successions 

 conforms closely to the present division into seres and cliseres. The former 

 are essentially topographic or biotic as to cause, the latter are fundamentally 

 climatic. Cowles (1911 : 170) has also recognized the validity of this distinc- 

 tion in contrasting climatic or regional successions with topographic and 

 biotic ones. 



Primary and secondary succession. — ^Further investigation appears to show 

 conclusively that the distinction between primary and secondary seres is the 

 outstanding fact of the development of existing formations. It is inherent in 

 the organic nature of the formation (Chapter I), and is no more subjective 

 than the reproduction by seed and propagation by offshoots in the case of an 

 individual plant. The original distinction was somewhat confusing, as it 

 placed too much weight upon the initiative process. In the case of erosion it 

 was particularly difficult to determine offhand whether the new area was 

 primary or secondary. The concept has now been definitized by basing it 

 wholly upon development, though this basis necessarily includes reaction and 

 the general influence of the denuding agent. From the developmental view- 

 point, primary and secondary seres are wholly distinct. There is little or no 

 possibility of confusing one with the other. At the same time it must be recog- 



