STABLE AND MIGRATORY PLANT FORMATIONS 5 
2. Lopographic successions, “ of much greater rapidity, and 
associated with the topographical changes resulting from the 
activities of such agents as running water, wind, ice, gravity, 
and vulcanism, and leading in general to erosion and deposi- 
tion.” In this respect he points out that “the influence of 
erosion is generally destructive to vegetation, or at least 
retrogressive, z.c. tending to cause departure from the meso- 
phytic, while the influence of deposition is constructive or 
progressive, z.c. tending to cause an approach towards the 
mesophytic.” 
3. Biotic successions, where the vegetative changes are 
due to plant and animal agencies. “If in their operation 
regional agencies are matters of zons, and topographical 
agencies matters of centuries, biotic agencies may be expressed 
in terms of decades.” “The influence of biotic agencies is 
not confined to areas that are characterised by a fpre-eroszon 
topography, because the interval between the periods of active 
erosion often is sufficiently long to permit the development 
of an entire biotic cycle.” 
This is a great step forward in plant ecology, since it is the 
first indication of an appreciation of the difference in origin 
between plant communities covering wide areas of the earth’s 
crust, which are, geologically speaking, in a stable condition 
for the time being, and others taking part in the topographic 
successions with which we are now to some extent familiar. 
A point of first importance in defining the habitat of a plant 
association is its present and late condition relative to the 
geological agents of surface change. Is the relation between 
the habitat and the vegetation comparatively stable, and 
inherited from the past as such? or is the vegetation in 
various stages of progressive association owing to constantly 
recurring changes in the geology of the habitat, and can 
stages in the progress of associations be demonstrated ? 
We would therefore distinguish two classes of habitats 
which differ in plant succession and in the limits set to 
stabilisation. We would further distinguish two classes of 
plant formations which differ in their centres of distribution, 
but which tend to overlap and invade one another’s territory 
owing to the migratory nature of the geological agents of 
surface change. The stable formations are those whose plant 
