30 Proceedings of the Royal Physical Society. 
but are further limited by the specific conditions mentioned below. They 
include various formations limited to the stream and coastal belts, or to the 
foci where the physiography curtails vegetation and prevents full stabilisation 
owing to its influence on drought, wind exposure, low temperatures, or the 
instability of surface incurred through gravitation. Thus we have various 
alluvial grasslands, marshes, fens, reedswamp and aquatic formations with 
those of shingles, sand dunes, mudflats, crags, screes, etc. 
The truly alpine formations are, therefore, for the most part migratory, 
at least in this country, while the sub-arctic peat mosses and the plant 
associations of the semi-desert alpine plateaux are more of the nature of 
outliers of the circumpolar tundra. But the latter, in this country, also 
chiefly owe their persistence to purely local migratory factors. 
To these migratory formations, as limited above, must be added others 
where distinctly local changes occur in the vegetation as a result of repeated 
interference by animals. We may thus define the migratory formations 
as resulting from markedly local disturbances in the more stable types of 
vegetation leading to displacement of their associations by others which 
followed in the track of the geological agents of surface change, or on that 
of the more social animals, including primitive man, who, for long ages, 
had formed, by their pathways and places of congregation, the specialised 
haunts of various parasites, weeds and camp-followers. 
The migratory formations included plant associations with differences 
in the history of their segregation. Thus they provided sanctuaries for 
plants ousted by other plant associations advancing with a change of climate. 
These formed relict associations from past conditions, when they had a wider 
range of distribution. Others formed outposts for pioneer plants attempting 
to invade the country from the coast-line, or by lines of migration along the 
river-belt. A third, and more inclusive group, was formed of plants peculiar 
to the various classes of migratory habitat. 
There can be little doubt that the associated insects and other invertebrate 
creatures also formed defined groups of species which might, similarly, be 
divided into (1) alpine and recessive, (2) coastal species and late immigrants, 
and (3) species peculiar to migratory habitats on account of the nature of 
their food, the aquatic or other conditions of their larval stages, or from 
other causes. 
The vertebrates too, though so often powerful migrants, were doubtless 
then, as now, chiefly confined to definite habitats by their food, enemies, 
breeding or other habits. 
Many species have ceased to inhabit our islands, and the range of the 
survivors is, to a considerable extent, governed by artificially imposed 
