8 THE SCOTTISH BOTANICAL REVIEW 
The plant associations of stable formations are stabilised 
to the extent allowed by the prevailing climate and edaphic 
conditions, and are of long persistence on the same habitat, 
but their boundaries are subject to frequent retraction and 
expansion owing to the migratory nature of the habitats of 
the migratory formations. Moreover, under a continuance 
of the same climatic and geographic conditions, their closed 
plant associations appear capable of resisting invasion. In 
open stable formations the physical conditions limit invasion. 
Change in stable formations is therefore in all probability to 
be chiefly accounted for by change of climate or geography, 
inducing accommodative changes in the plant associations or 
dislocation in the balance of relations between the associa- 
tions and their habitats and those of neighbouring plant 
formations. 
Migratory formations are of comparatively short persist- 
ence on the same habitat, which sooner or later undergoes 
change or destruction, with renewal elsewhere. On retreat 
of the geological agent, their associations tend to rapid 
degeneration from plant invasion. All stages of progressive 
successions of associations are encountered, owing to the 
frequent formation of new habitats and the zonal nature of 
the influence of the geological agents. 
PART II. THE REGIONAL SUCCESSION OF STABLE 
FORMATIONS. 
The replacement of one type of vegetation by another over 
wide areas of the earth’s crust has doubtless always been 
partly due to the secular migration and competition of plants 
and animals, but probably chiefly originates in extensive 
geological transformations or secular changes in climate and 
geography. Such changes may occur :— 
1. From tangential movements along lines of weakness in 
the earth’s crust, leading to mountain-building, and thus 
erecting barriers to moisture-laden winds, and new belts or 
foci involving climatic zonation due to altitude. The more 
deeply seated rocks implicated in these tangential movements 
usually undergo more or less metamorphism and injection by 
igneous magma, and cores of ancient crystalline rocks are 
