28 Proceedings of the Royal Physical Society. 
cold steppe climate may have invaded this country in late glacial times, but 
since then the climate appears rather to have shown alternating phases that 
were (1) of a more insular moorland type, cold and humid with rather equable 
summer and winter temperatures and atmospheric precipitations, or (2) of 
a more continental forest type with heavy seasonal precipitations, warmer, drier 
summers and frosty winters with longer snowlie. The former would favour 
the moorland type of vegetation and accumulation of humus, but with lower 
temperatures during such a phase an approach to tundra conditions would 
have occurred. 
The continental phases, on the other hand, favoured a forest vegetation in 
the lower-lying and sheltered districts, and an alpine vegetation on the higher 
mountain slopes and summits. 
Repeated insular or moorland phases have resulted in a gradually increas- 
ing accumulation of humus to form the humus barrier aforesaid. The 
continental phases, on the other hand, have tended to destroy the humus 
barrier. 
On the whole, the moorland phases have prevailed in the north and west, 
since the humus barrier has become widely established there, even at low 
levels, but steep sheltered slopes have provided a retreat for a subalpine 
forest vegetation, while a precipitous topography at high elevations has 
afforded a sanctuary for a truly alpine vegetation. 
In the lowlands of the south and east the continental phases have 
prevailed, since the humus accumulation is but local, and woodlands have 
been enabled to retain wide possession, but certain areas of porous soils and 
isolated eminences have suffered such prolonged leaching that heath has 
locally become the dominant vegetation on a sparse, raw humus soil (6). 
The moorland phases resulted in a widening of the humus barrier, a 
general retreat of the forests and an advance of the moorland, a partial 
substitution of lowland forest by subalpine forest, and a retreat of the true 
alpine flora to the steepest topography where humus accumulation was 
delayed. The continental phases, on the other hand, caused a removal of 
peat from steep surfaces and exposed tops at high altitudes, a retrogression of 
the plateau peat mosses into bog and their invasion by pine forest and Calluna 
moor, an expansion of the lowland forest and a limitation of the subalpine 
forests to the mountain slopes and western coastal regions subject to the 
moistest climate. 
Each recurring moorland phase has doubtless favoured alpine colonisation 
by the cold-tundra and acid humus-dwelling species like Salix herbacea, 
Loiseleuria, Arctostaphylos alpina, Empetrum, Vaccinium Vitis-idea, and 
Letula nana, since some of these species became widespread on the plateau 
