96 VEGETATION OF THE PEAK DISTRICT [CH. 
ScRUB IN OTHER DISTRICTS 
Clements (1905: 287) has maintained that “in forests, 
while many vegetation forms can still enter, none of these 
produces a reaction sufficient to place the trees at a disadvantage ; 
and the ultimate forest stage, though it may change in com- 
position, cannot be displaced by another.” If my contention 
in the previous section of this chapter be sound, it follows that 
this generalization of Clements is not of universal application. 
In this district, and indeed in very many other districts, 1t would 
appear to be indubitable that woodland is frequently displaced 
by associations of scrub, grassland, heath, and moor. In all parts 
of the British Islands, there has, within the historical period, 
been a pronounced diminution of the forest area, a diminution 
which, in my judgment, is in addition to and apart from any 
artificial disforestation or any change of climate. The decay 
of forests in central Europe and the conversion of many of 
them into grassland and heaths is admitted by most phyto- 
geographers; and there are not wanting authorities who have 
gone so far as to assert that prairies and even steppes have 
been derived from pre-existing forest (cf. Warming, 1909: 282), 
though it is difficult to accept this view, especially with regard 
to the origin of climatic steppes. In practically all cases of the 
ascertained conversion of forest into grassland, it would seem 
certain that an intermediate stage of open scrub occurred. It 
has also been urged by some plant geographers that some 
tropical forests have degenerated into savana-forest and scrub ; 
and whilst this degeneration must obviously be accelerated by 
a diminishing rainfall, it is by no means improbable that the 
retrogressive succession may also take place in districts where 
such a decrease is imperceptible. In Great Britain, the con- 
version of woodland into scrub, and of scrub into grassland, 
heath, or moor is seen not only on the Pennines, but in Wales, 
in the Lake District, and in Scotland; and some of these 
districts have a mean annual rainfall of 80 inches (203 cm.) 
and occasionally more. Such successions are not exceptional 
in this country, but widespread and general; and whilst they 
are without doubt often due, in part, to artificial causes, it 1s at 
least conceivable that this is not always and wholly the case. 
