316 BOTANICAL GAZETTE [MAY 
give away before the encroaching mesophytic forests. ig. 12 
shows a hemlock-maple forest bordering an arbor vitae swamp. 
In wet springy soils the arbor vitae forest is also found. fig. 73 
shows a young forest in such a situation. 
VI. CLEARING SOCIETIES. 
Thus far what has been called the normal life-history has 
been traced. The influence of human agencies, with few 
exceptions, has been disregarded. By far the greater number 
of areas studied show unmistakable signs of the devastating 
influence of man, though less so on the islands visited than on 
the mainland. These clearing areas cannot be disregarded if a 
complete history of forest development is to be written. At first 
the difficulties of getting any order out of the tangle seemed 
insurmountable, and the notes were usually headed ‘‘clearings, 
conditions artificial.” The literature on the subject of plant 
succession in clearings is confusing. It consists, as a rule, of 
records of isolated observations without any attempt at correla- 
tion, That there is some solution to the problem was the writer's 
firm belief, and after collecting a large number of notes the study 
began to yield results. 
If the factors controlling the normal development as given 
above are correct, they ought to dominate in the clearing socie- 
ties. With two exceptions, the stages observed in the develop- 
ment of these clearing societies are represented in the normal 
history. These exceptions are the “fireweed” and the poplar- 
birch societies. To explain these it was found necessary to 
give prominence to the fact that some plants migrate more easily 
than others. 
If a deciduous forest be destroyed and burned over so that 
the surface débris is partially reduced to ashes, it will usually be 
found that the humus conditions of the soil are then reduced 
toward, but not necessarily to the conditions obtained in the 
heath. The equilibrium that had been established has been dis- 
turbed, and other things being equal all plants will have an equal 
chance provided their seeds are present. Shade plants, however, 
