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 
cften 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 are unable to do so, or when 
