4 THE SCOTTISH BOTANICAL REVIEW 
tions of the lower ground. Where such soils form the lower 
ground in Britain, they have all the advantages of shelter 
from the wind and of summer warmth, and they also lie 
in the direct route of migration from the Continent, factors 
favouring the presence of the later post-glacial immigrants 
and a forest vegetation. Clays and sands in the northern 
and western parts of our islands, and those at high levels, 
have suffered encroachment by moorland almost as much as 
any other type of soil ; but sands and clays everywhere differ 
in the former being more subject to leaching and consequent 
encroachment by heath, while the clays tend rather to 
stagnation of soil waters in low-lying positions. 
On the other hand, the so-called siliceous soils derived from 
the harder sandstones, grits, and metamorphic rocks usually 
form sloping, hilly ground, and in this country are therefore 
more subject to atmospheric humidity, cool summer tempera- 
tures, and leaching. This is due to the past relations of the 
rocks to denudation, z.e. to physiography. 
The relations of the calcareous soils to physiography and 
vegetation are very complex, and will be referred to later. 
In the “Types of British Vegetation” the retrogressive 
changes due to human and other interference in the wood- 
lands are discussed in detail, but we find no account of pro- 
gressive association in this connection, and have to turn to 
other formations for their demonstration. Analysis of any 
published list of plant formations shows, indeed, that the 
progressive associations commonly met with are those 
belonging to the types of successions to which Cowles has 
given the names “topographic ” and “ biotic,” while the retro- 
gressive associations are chiefly distinctive of successions 
which have at least partly completed stabilisation, and whose 
initial stages are in most cases unknown. 
Cowles has recently defined three cycles of vegetative 
succession :— 
1. Regional successions, “due to secular change, and in 
rate of development bearing some comparison with the suc- 
cession of geological periods.’’ Of these the most important 
to the plant ecologist in this country is “the post-glacial 
invasion of southern forms into boreal regions, accompanying 
and following the retreat of the ice” (7). 
