130 THE SCOTTISH BOTANICAL REVIEW 



found refuge in stable areas, subject to climatic extremes, as 

 drought or great humidity accompanied b\' low temperatures, 

 but with amelioration of climate were liable to extinction by 

 competition, unless they underwent changes rendering them 

 more capable of rapid migration. 



Migratory formations do not, however, always show a 

 complete change of species from the neighbouring stable 

 formations, but may be partly stamped by a change in the 

 type of growth, i.e. the habit of certain species, or perhaps by 

 forms or variations, such as have not yet been sufficiently 

 explained as fixed and inherited, as purely physiological and 

 anatomical adaptations, or selected mutations. It seems, in 

 fact, to be the rule rather than the exception that a species 

 common in a stable formation, on appearing as a dominant 

 in a neighbouring migratory formation, is under a different 

 form, which is retained everywhere within the limits of the 

 migratory formation. This, at least, appears to hold with 

 Calluna vulgaris, species of Sphagnum, grasses, and some 

 other plants. 



The migratory formations are naturally distributed in the 

 field into : — 



1. Those of the coastal belt. 



2. Those of the stream belt. 



3. Those of the foci where the physiography curtails 

 vegetation and prevents stabilisation, owing to its influence 

 on drought, wind exposure, low temperatures, or the 

 instability of surface incurred through gravitation. 



In the coastal belt we find various formations depending 

 chiefly upon reactions between the following geological 

 factors : — 



1. The nature of the rocks, and the initial topography of 

 the surface entering the belt of erosion. The resulting 

 configuration of the coast-line and coastal profile. Their 

 comparative permanence or tendency to rapid alteration. 



2. Recent positive or negative displacements of the coast- 

 line and their influence on plant succession. 



3. The submarine profile as influencing the fetch of the 

 waves, and thereby, in particular, the range and character of 

 the cliff vegetation. 



4. The nature of the coastal currents in their influence on 



