Ii] SCRUB ASSOCIATIONS 97 
In districts where the rainfall is low, as on the borders 
of steppes and at very high altitudes, where the amount of 
precipitation is insufficient to permit of the growth of large 
trees, there can be no doubt that static, climatic scrub occurs; 
and, on certain very dry soils in moderately rainy localities, 
it is also certain that static, edaphic scrub occurs. 
The relations of the most important types of scrub are 
shown in the following scheme :— 
Scrub 
E 
Kinetic Static 
| 
Laermer mL SRE 
(1) Retrogressive (2) Progressive (3) Edaphic (4) Climatic 
Of these, the examples of scrub met with in the Peak 
District belong mostly to the first type; and no doubt the 
great majority of the British examples of scrub should be 
placed in the same class. 
RELATION OF THE GROUND VEGETATION OF WOODLAND 
TO RETROGRESSIVE SCRUB 
Although nearly all the examples of scrub on the Pennines 
belong to the retrogressive type, they are important, and no 
account of the vegetation of a district which failed to account 
for them, could be regarded as complete. At the same time, it 
does not appear to be reasonable to regard subordinate asso- 
ciations such as retrogressive or progressive scrub as of the 
same ecological rank as chief associations like flourishing 
tracts of woodland. 
From the standpoint of succession, the study of the ground 
vegetation of woodland is a matter of prime importance both 
to the phytogeographer and to the forester. By such a study, 
an inkling may be gained of the possible fate of particular 
tracts of degenerating forest. For example, where the ground 
vegetation consists of heathy undershrubs, such as bilberry 
(Vaccinium Myrtillus) and heather (Calluna vulgaris), and of 
humus-loving grasses, such as purple moor-grass (Molinia 
caerulea) and silver hair-grass (Deschampsia flexuosa), it may 
M. 7 
