6 THE SCOTTISH BOTANICAL REVIEW 
associations have their centres of distribution on ground 
which has been for a long period comparatively stable from 
the geological standpoint, and under climatic conditions 
favourable to the type of vegetation. The mzgratory forma- 
tions are those whose plant associations have their centres of 
distribution in areas within the sphere of influence of the 
geological agents of surface change, and only naturally initi- 
ate and progress on habitats directly generated and periodically 
or continuously influenced by such agents. 
The factors defining these two classes of habitats are as 
follows :— 
In the stable formations they are: 1, the climate, as due 
to geography, the amount and seasonable combinations 
of light, heat, wind, atmospheric moisture, and rain ; 2, the 
nature of the soil, resulting from the geological distribution 
of the rocks in their present and past relations to climate, 
vegetation, and geographical change. 
In the mzgratory formations: 1, the relation of habitat to 
climate is chiefly topographical; and 2, the soil varies in 
each case with the nature of the geological agent of surface 
change, its topographical relations to erosion or deposition, 
its constant, periodical, or occasional character, and the stage 
and kind of succession of plant association growing upon it. 
The plants of the dominant stable formations are con- 
stantly attempting to colonise the habitats of the adjacent 
migratory formations, but only succeed in so doing after the 
geological agents have ceased for some time to exert their 
influence. On a rock cliff, for instance, species which are 
normal to the habitat, as lithophytes, or as chomophytes (either 
endemic species or those incapable of withstanding competi- 
tion), are found side by side with invading species from the 
neighbouring stable formations, but the latter can never 
succeed in displacing the former species until the habitat is 
permanently altered in its relations to the geological agents 
of surface change. In deserted quarries, at a distance from 
naturally formed rock formations, invasion is rapid, and the 
species other than lithophytes may entirely consist of such 
invaders. Closed migratory formations, as the reed-belt, 
marsh, fen, and alluvial meadow, are generally free from 
invaders until degeneration sets in. 
