STABLE AND MIGRATORY PLANT FORMATIONS 7 
On the other hand, interference by man or beast with the 
habitat of a stable formation often permits of a temporary 
invasion by plants which are normally excluded from it, and 
which have their natural station on migratory habitats. 
Stable formations that are undergoing retrogression through 
climatic change may also show partial invasion by plants 
from the neighbouring migratory formations. The present 
invasion of the moorland by extensive colonies of Wardus 
stricta is probably to be looked upon as of this nature. 
Nardus is normally confined in moorland districts to the 
flood-rims of burns and “flushes,” where the drainage comes 
from acid peat or peaty hollows subject to snow-lie. 
From this tendency on the part of the plants of the stable 
formations to eventually obtain possession of areas formerly 
occupied by migratory formations on the one hand, and of 
certain plants of the migratory formations to invade disturbed 
areas of the stable formations on the other, a feeling appears 
to have arisen that the succession of plant associations is 
always from those characteristic of the neighbouring migratory 
formations towards the full stabilisation met with in the 
stable formations. But most of the plant associations of the 
migratory formations, such as those of rock, marsh, aquatic, 
and saline habitats, are bound down to sharply defined 
physical environments limited to the immediate neighbour- 
hood of the migratory agents of change ; and, with regard to 
many of the plants of the migratory formations, it is doubtful 
if at the initiation of the existing stable formations they had 
yet migrated to their present limits of distribution. It is, 
moreover, fairly certain that with change of climate stable 
types of vegetation may gradually replace other stable types 
without complete initiations of succession such as are met 
with in the migratory habitats. Where large regions devoid 
of vegetation arise rapidly de novo, as in volcanic areas, the 
whole surface consists of bare rock, and the succession of 
plant associations must initiate as lithophytes. These cases 
are, however, exceptional, since large regions of the globe 
are known to have existed chiefly under a deep load of soils 
and subsoils for such long periods that they must have 
undergone many vicissitudes of climate and changes of plant 
formations. 
