Ecology. 29 
peat mosses during such phases, but it has also restricted the range and 
curtailed the number of species of the alpines incapable of competing with 
acid humus-dwelling plants, and those requiring snow cover or protection 
from growth stimulus in winter-time, and greater warmth and light in the 
summer season. In other words, the wet moorland phases favoured the 
humus-dwelling alpines and arctic types, while the drier continental phases 
would favour the more delicate alpines that shun competition with humus 
plants and are chiefly chomophytes, or mesophytes, so far as their soil 
requirements and underground organs are concerned. 
As a further result of these regional changes of plant associations, there 
were doubtless changes in the constitution and distribution of our country’s 
fauna, a waning or expansion in the distribution of the smaller species 
depending on forest and alpine, or moorland and arctic-tundra, conditions, 
as the case might be, and various migrations amongst the larger forms. 
One at least of the latter, the Irish Elk, became extinct before historic times. 
Before the advent of civilisation the country was, therefore, chiefly moor- 
land in the north and west, with certain alpine formations in the higher 
parts of the mountains. Birch woods, pine woods, and various types of 
northern and western heaths flanked and dissected the areas where the 
formation of peat was prevalent. The oak forest, which occupied nearly 
the whole of the lower ground to the south and east, gave place to woodlands 
with a dwarf type of “sessile oak” and birch in steep subalpine districts of 
the moorland exposed to the Atlantic type of climate. On the other hand, 
in the drier eastern lowland areas, the oak forest gave place to what has 
been termed “dry oak wood,” also with Q. sessiliflora, but with a different 
ground flora, wherever the porous condition of the soil or rapid drainage 
resulting from the nature of the physiography had led to leaching of the 
surface; and this in turn to various forms of heath where, from the 
dominating sterility of the ground, the oak was unable to obtain a footing. 
Ash woods were dominant on subalpine valley slopes where limestone was 
the rock basis. The chalk downs were occupied by chalk grassland. Beech 
woods, though now so widely distributed on well-drained, firm surfaces of 
rock, stiff boulder clay or other consolidated deposits, were originally almost 
restricted to certain slopes of chalk and other limestones in the south of 
England (2). 
But besides these stable or regional types of plant association so 
dependent on geographical and climatic conditions, we had others more 
dependent on locally recurring specific conditions in their habitat which, 
I have suggested, should be grouped together under the name Migratory 
Formations, These also are usually segregated by geographical boundaries, 
