128 THE SCOTTISH BOTANICAL REVIEW 



coastal and travelling dunes. Mud-flats have always 

 appeared in the protected estuaries of the larger rivers, or 

 behind sand-bars in deltas, or shingle-spits traversing 

 shallow bays where small streams find exit seaward. 



These various coastal formations have repeatedly evolved 

 under certain recurring conditions of physiography, the 

 results of the action and reaction of the static and dynamic 

 factors of geology, and, apart from the effects of differences 

 of climate and geography on plant growth and distribution, 

 have, further, led to a similar range of habitats for the 

 evolution of plant adaptation and association. 



With the same reservations the other agents of erosion 

 and deposition have as constantly originated defined series 

 of habitats for plant evolution the world over and throughout 

 geological histor\'. 



The streams have ceaselessly cut gorges and waterfalls, 

 and laid down alluvial deposits of gravels, sands, and silty 

 loams. So long as mountains have arisen, and passed away 

 under the influence of erosion, so long have frost, insolation, 

 and gravity conspired to produce alpine crags, screes, and 

 landslips. 



But the sphere of these operations has undergone endless 

 change. The geological agents desert their work barel)^ 

 begun, or but half accomplished, and start afresh elsewhere, 

 only to return once again, after intervals measured by days, 

 years, or centuries. 



From the very nature of their operations, these agents 

 must have caused frequent disturbances in the more stable 

 types of vegetation, whose character of stability, indeed, 

 depends upon their absence. But other types, adapted to 

 profit b)' less stable conditions of existence, or quicker to 

 migrate and take possession of newly formed surfaces, have 

 been ever ready to escape the biological or physical struggle 

 for existence intensified in the areas of least disturbance, 

 and thus have arisen plants and plant communities especially 

 accommodated to the comparatively rapid changes of environ- 

 ment attached to such conditions. 



With a climate favourable for vigorous vegetation, the 

 plants that follow the track of the migratory agents include 

 many that are forced to avoid the intense biological com- 



