LIFE-HISTORY STAGES. 103 



govern ecesis and hence the degree of occupation. The habitat determines 

 the character of the initial stage by its selective action in the ecesis of the 

 migrules. In all secondary areas, however, it must be recognized that the 

 conditions of the habitat are largely due to the reactions of the original vege- 

 tation. After the initial stage the development of succeeding ones is pre- 

 dominantly, if not wholly, a matter of reaction, more or less affected by compe- 

 tition. In addition, some stages owe their presence to the fact that certain 

 species develop more rapidly and become characteristic or dominant, while 

 others which entered at the same time are growing slowly. This is a frequent 

 explanation of stages of annuals, as also of stages of perennials preceding scrub 

 or forest in secondary succession (plate 27 a). 



Reasons why plants appear before their proper time. — The appearance of a 

 species before its usual place in the sequence is generally due to migration 

 in such amount that the handicap of more or less unfavorable conditions is 

 overcome. It is most frequent in secondary seres, where the factors are less 

 extreme, and the majority of the species can become dominant as soon as a 

 sufficient number of migrules appear. In primary succession, especially, 

 species can become characteristic only after the reactions have reached a cer- 

 tain point. In the great majority of cases where a species appears out of 

 order, it is due to local variations in the area. The premature development 

 of an entire stage is caused by agencies which suddenly or rapidly change the 

 habitat in the direction of the reaction. This is particularly true of areas which 

 are affected in this way by animals or man. The nimiber of stages omitted 

 will depend upon the rate and degree of change. It is not unusual for this 

 telescoping effect to eliminate two or more stages. The agencies which accel- 

 erate reaction may also retard it, so that stages may be delayed by the imdue 

 persistence of an earlier one. In all secondary successions the time of appear- 

 ance of shrub and tree stages depends in the first degree upon the action of 

 the denuding agent. When this destroys all seeds and propagules the sequence 

 of stages wiU be determined as tisual by the mobiUty of migrules and by the 

 habitat. When seeds or Uving parts of dominants escape destruction the 

 species concerned will take possession at once, or as soon as their development 

 permits. Thus when an aspen forest is burned the root sprouts often make 

 the aspen again dominant the following season, and succession is foimd only 

 in the renewal of the undergrowth. As noted in other connections, the seeds of 

 lodgepole pine and similar pines are available in large numbers after fire, with 

 the result that lodgepole pine reappears the first season, though its slow growth 

 to dominance permits the rapid development of several stages. A similar 

 effect has been noted by Hofmaim in the forests of the Pacific slope when 

 burned. The seeds of several species he dormant for several years at such 

 a depth in the forest duff or soil that they escape the fire and are ready for 

 germination the year following (plate 27 b). 



Initial stages. — No sharp line exists between initial and medial stages. The 

 distinction, though convenient, can be only relative. Seres vary greatly in 

 the number of stages and especially in the number and character of initial 

 stages. The number of stages may range from one to twenty or more, and a 

 large number of secondary seres consist of not more than three or four. 

 Furthermore, since secondary succession always begins after the pioneer stage 

 of a primary sere, and usually at a medial or climax stage, the initial stages of 



