Z THE SCOTTISH BOTANICAL REVIEW 
adjudged the master-factor in plant formation in the “ Types 
of British Vegetation” recently published (3). Many still, 
perhaps advisedly, refer to forest and meadow formations, 
thus falling back on a physiognomic nomenclature in default 
of finding a master-factor other than the sum total of the 
complex relations of these plant associations. 
Starting from another standpoint, and following Warming, 
Cowles and Clements emphasised the succession of plant 
invasions on the appearance of a new habitat. Cowles 
pointed out that successions may be progressive or retro- 
gressive in nature (4). Clements applied the term “forma- 
tion” to certain stages of plant association encountered in the 
successions. He also framed a number of laws of succession, 
of which the following appear to be the most important in 
respect of the present discussion :— 
1. “The initial cause of a succession is the formation or 
appearance of a new habitat, or a striking change in an 
existing one.” It should be remarked here that a change in 
an existing habitat is probably of equal or greater importance 
than the formation of a new one. 
2. “Each stage of a succession reacts upon the habitat in 
such a way as to produce conditions more or less unfavour- 
able to itself, but favourable to the invaders of the next stage.” 
3. “Initial formations are open, ultimate formations are 
closed.” To this might be added, except under semi-desert 
conditions. 
4. “The universal tendency of vegetation is towards 
stabilisation. ” 
5. “The ultimate stage of a succession is determined by 
the dominant vegetation of the region. Lichen formations 
are often final in polar and niveal zones. Grassland is the 
final vegetation for plains and alpine stretches, and for much 
prairie, while forest is the last stage for all mesophytic 
habitats. ” 
6. “The end of a succession is largely brought about by 
the progressive increase and competition, which makes the 
entrance of invaders more and more difficult.” This, of 
course, only embraces closed plant formations (5). 
Moss has recently laid particular stress on the succession 
of associations within the same formation, and defines a 
