SUCCESSION 253 
“is cut over for certain species ‘alone, the undisturbed trees soon take full pos- 
session, though the causes effective in the beginning will ultimately restore 
the original facies in many instances. Such successions are anomalous, ‘and 
-will be treated under that head. 
309. Succession by cultivation. The clearing of forests and the “break- 
ing” of grassland for cultivation destroy the original vegetation; the tempo- 
rary or permanent abandonment of cultivated fields then permits the entrance 
‘of ruderal species, which are the pioneers of new successions. This phe- 
nomenon takes place annually in fields after harvest, resulting in the second- 
ary formations of Warming, in which practically the same species reappear 
year after year. In fields that lie fallow for several years, or are perma- 
nently abandoned, the first ruderal plants are displaced by newcomers, or 
certain of them become dominant at the expense of others. In a few years, 
these are crowded out by invaders from the adjacent formations, and the 
field is ultimately reclaimed by the original vegetation, unless this. has en- 
tirely disappeared from the region. The number of stages depends chiefly 
upon whether the final formation is to be grassland or woodland. Other 
activities of man, such as the construction of buildings, roads, railways, 
canals, etc., remove the native vegetation, and make room for the rapid de- 
velopment of ruderal formations. In and about cities, where the original 
formations have entirely disappeared, the chance for succession is remote, 
and the initial ruderal stages become more or less stabilized. Elsewhere 
the usual successions are established, and the ruderal formation finally gives 
way to the dominant type. In mountain and desert. regions, where ruderal 
plants are rare.or lacking, their place is taken by subruderal forms, species 
of the native vegetation capable of rapid movement in them. These, like 
ruderal plants, are gradually replaced by other native species of less mobil- 
ity, but of greater persistence, resulting in a short succession operating often 
within a single formation. From the nature of cultivated plants, succession 
after cultivation generally operates within the mesophytic series. 
310. Succession by drainage. Successions of this kind show. much the 
same stages as are found in those due to flooding. They proceed from 
aquatic or swamp formations to mesophytic termini, either grassland or 
woodland. When drainage takes place rapidly and completely, the pioneer 
stages are usually xerophytic; cases of this sort, however, are infrequent. 
311. Succession by irrigation. Irrigation produces short successions of 
peculiar stamp along the courses of irrigating canals and ditches, and in the 
vicinity of reservoirs, These are recent, as a rule, and are usually found in 
