166 DIRECTION OF DEVELOPMENT. 



Cooper (1912 : 198) has used the term "subsuccession" for the seres which 

 begin on rock-surfaces, in crevices and in rock-pools, and terminate in the 

 formation of a heath-mat. Thus, he distinguishes a rock-surface subsuc- 

 cession, a crevice subsuccession, and a rock-pool subsuccession of the rock- 

 shore succession. However, he does not apply the term to seres which con- 

 verge later in the development. The phenomenon is the same whether it 

 appears early or late in succession. It is here proposed to apply the term 

 ddsere {ad-, to, implying convergence) to that portion of a sere which precedes 

 its convergence into another at any time before the climax stage. While it 

 is possible to distinguish adseres with respect to convergence in the initial, 

 medial, or subclimax stages, at present it does not seem wise to do so. Like- 

 wise, a developmental line formed by the convergence of two or more adseres 

 may itself converge and become an adsere (fig. 5). The use of subsiiccession 

 in this connection seems undesirable because of the fundamental distinction 

 already drawn between succession and sere. 



Normal movement. — It is probable that the large majority of all the seres of 

 a climax association pass through their development in the normal manner. 

 All the stages are represented; they follow each other in the usual sequence 

 and progress at about the same rate. But the normal course of development 

 may be disturbed or changed in various ways. Frequently the modification 

 is merely one of rate, and succession takes place in the usual way, but at a 

 faiW or slower pace. Distinctions upon the rate of movement can hardly be 

 made at present, as our exact knowledge of succession is still small. There 

 are many seres, however, in which it has been shown that artificial or topo- 

 graphic changes have hastened or retarded the normal course. This disturb- 

 ance may be so great that the sere is held for a long time in some associes, 

 which in consequence takes on the appearance of a climax. Or, as a result 

 of the absence of the usual cUmax species, the subfinal stage may become the 

 actual climax. 



Apart from such modifications as these in which the sequence is not affected, 

 there are those in which stages are dropped out or interpolated, or in which 

 there is a deflection of the course of movement. The failure of a particular 

 stage to develop is a frequent occurrence in seres with many stages, particu- 

 larly when the reaction of each is not especially marked. In such cases, the 

 sequence is determined largely by migration, and the relative abundance and 

 nearness of the dominants of two or three associes is decisive. On the other 

 hand, the interpolation of an unrelated stage occurs but rarely, since it can 

 take place only when a new dominant enters the region, as in the case of 

 weeds. A complete change in the course of development apparently can 

 result only from a change of climate. Such changes necessarily affect the 

 climax vegetation, and hence are considered in later chapters. 



These various modifications have previously been recognized and distin- 

 guished by terms (Clements, 1904 : 107, 122; 1905 : 240, 254). Normal suc- 

 cession begins with nudation, and passes through the regular sequence to the 

 climax association. Anomalous succession occurs when the sequence is 

 destroyed by addition or subtraction, or when the succession is deflected. 

 Imperfect succession results when one or more of the ordinary stages is omitted 

 anywhere in the course and a later stage appears before its turn. It will 

 occur at any time when a serai area is so surrounded by dense vegetation that 



