32 Proceedings of the Royal Physical Society. 
therefore have formed the ultimate phase of plant succession in this country 
wherever the migratory factors or the physiography aided its stabilisation 
and hindered that of other types of vegetation. 
Steep, rocky, and uneven stony surfaces have tended rather towards some 
degree of stability in dwarf shrub or thorn bush, since the nature of the 
surface, with its poverty in pasture grasses, leads to neglect by grazing 
animals, but forms a cover for the smaller mammals and especially birds 
on migration. These often bring with them the fruits which this type of 
vegetation (eg. whitethorn, rose, sloe, rowan, elder, bramble, ete.) especially 
provide for their consumption, and the fruits thus sown develop into a 
thicket which is unattractive to the grazing fraternity, and may even form 
a barrier to its ingress. 
When the primitive forests disappeared before the axe, many of the 
plants of the migratory formations, and especially those entering into the 
composition of grassland and bushland, acquired greater freedom. Coastal 
species, those of the river-belt and others, particularly the “camp-followers ” 
of prehistoric man and the larger animals, spread widely across the face of 
the country to occupy the ground as patches of annual or herbaceous weeds, 
or to form secondary associations of grassland or bushland ; and with them, 
in many cases, went the insects dependent on them for food, while these 
in turn were followed as prey by other insects, reptiles, birds and mammals. 
Those species that were completely dependent on the forest for their 
habitat gradually became more and more restricted, and now are only to be 
found where remnants of their primitive forests persist. Many plants, and 
especially animals, that were common in the virgin forests thus became 
extinct, or are now looked upon as the greatest treasures of the collector, 
only to be obtained in certain circumscribed districts. 
Others, however, appear to have been more plastic under the changed 
conditions owing to an innate ability to adopt a wider range of conditions of 
habitat and competition. Certain plants and insects were originally confined 
to the patches of grassland, scrub, marsh or swamp occupying the more 
open parts of the forests where migratory factors checked full stabilisation, 
and these found greater freedom for migration, and became even more widely 
spread on the artificially generated open formations. Many insects that 
are polyphagous on various bushes and low-growing plants, according to the 
season, were perhaps original to these habitats. Others, perhaps, evolved such 
habits under the changed conditions. Similar changes, too, may have induced 
certain insects to leave their original food plants for garden or field produce. 
The surviving plants and animals mingled with other vagrants migrating 
from the coast-line and river-belt, and with the planted or self-sown trees 
