254 THE FORMATION 



the midst of cultivated lands, so that their complete history is still a matter 

 of conjecture. The original xerophytes are forced out not only by the dis- 

 turbance of the soil, but also by its increased water-content. A few of them 

 often thrive under the new conditions, and, together with the usual ruderal 

 plants and a large number of lowland mesophytes and amphibious forms 

 derived from the banks of the parent stream, constitute a heterogeneous as- 

 sociation. This is doubtless to be regarded as an initial stage of a succes- 

 sion, but it is an open question whether the succession will early be stabilized 

 as a new formation, or whether the original vegetation will sooner or later 

 be reestablished under somewhat mesophytic conditions. From the number 

 of mesophytes and from the behavior of valleys, it seems certain that the 

 banks of such canals will ultimately be occupied by a formation more meso- 

 phytic than hydrophytic, into which some of the surrounding xerophytes of 

 plastic nature have been adopted. 



312. Anomalous successions are those in which the physical change in the 

 habitat is relatively slight, resulting in a displacement of the ultimate stage, 

 or the disturbance of the usual sequence, merely, instead of the destruction 

 and reconstruction of a formation, or the gradual development of a new 

 series of stages on new soil. In nature, the ultimate grass or forest stage 

 of a normal succession is often replaced by a similar formation, especially 

 if the facies be few or single. It is evident that certain trees naturally re- 

 place others in the last stages of a forest succession, without making the lat- 

 ter anomalous. The last occurs only when a normal stage is replaced by 

 one belonging properly to an entirely different succession, as when a conif- 

 erous forest replaces a deciduous one in a hardwood region. The presence 

 and development of such successions can be determined only after the nor- 

 mal types are known. The interpolation of a foreign stage in a natural suc- 

 cession, or a change of direction, by' which a succession that is mesotropic 

 again becomes hydrophytic, is easily explained when it is the result of arti- 

 ficial agents, as is often the case. In nature, anomalous successions are 

 commonly the result of a slow backward and forward swing of climatic 

 conditions. 



313. Perfect and imperfect successions. A normal succession will regu- 

 larly be perfect ; it passes in the usual sequence from initial to ultimate con- 

 ditions without interruption or omission. 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 new or 

 denuded habitat becomes so surrounded by other vegetation that the forma- 

 tions which usually furnish the next invaders arc unable to do so, or when 



