64 VEGETATION OF THE PEAK DISTRICT [cH. 
not continuous under the peat of the southern Pennines, at all 
events: the birch remains do not exist, for example, on the ex- 
posed ridges, and they are absent from certain exposed hill sides: 
they frequently follow the hollows worn out by the streams; 
and, at their highest limit, they are practically limited to the 
stream banks. It seems to me unreasonable to elevate these 
discontinuous birch remains almost to the rank of a geological 
formation, as is done by some writers on peat; and it seems best 
to speak of them simply as the remains of a former birch forest, 
since their occurrence is exactly what one would expect them to 
be assuming they are the remains of a thin and open forest 
which once occurred at the upper local limit of woodland. 
This ancient birch forest is wholly a post-glacial affair; and 
the reduction in altitude of the forest limit illustrates what 
is perhaps a general law that in any district where a forest 
exists at its extreme limits, climatic or otherwise, the forest 
will as time goes on exhibit retrogressive tendencies. The 
latter are usually intensified by human interferences, such as 
by felling and by the grazing of domestic animals, and, on the 
other hand, they may be retarded by human interference, as 
by the careful replanting of the indigenous trees; but, left to 
itself, any forest which exists at its climatic or edaphic limits 
will, in all probability, become degenerate in time. The causes 
of this degeneration are discussed rather more fully in the next 
chapter (see page 91). 
It seems to be the case that, in this primitive Pennine 
birch forest, the Scots pine (Pinus sylvestris) occurred. How- 
ever, as pine timber is only rarely met with under the peat 
of the Pennine moors, and as birch timber is abundant, it is 
impossible to postulate a general zone of pine forest at a 
different altitude from the birch forest. Probably the pine 
occurred much more rarely than the birch, either as an 
occasional associate in the birch association, or 1t formed smaller 
associations or societies here and there. On these assumptions, 
the ancient forest on the upper slopes of the Pennines would 
be regarded as part of the forest region of north-western 
Europe, but not, as is the case of the woodlands with birch 
and pine in southern England, as part of the forest region 
including the north German plain. The pine probably became 
extinct here at an early date; and the existing trees have, in 
