92 VEGETATION OF THE PEAK DISTRICT [CH. 
on the formation of moor-pan (Ortstein) in sour soils. 
Graebner pointed out that his explanation does not invalidate 
Krause’s view as a partial explanation. Graebner’s theory is . 
a very reasonable one with regard to woods which occur on com- 
paratively flat plains and plateaux ; but it is scarcely satisfactory 
with regard to forests on many steep hill slopes, for, in such 
places, newer and richer soil from below is often exposed by 
denudation and occasional land slips may bring fresh soils from 
above. As the great majority of the degenerate woods of this 
locality are situated on such steep slopes, some additional 
explanation of forest degeneration must be sought. 
Of course, it is well known that the seedlings of most trees 
fail to develop under dense shade; and, for this reason, some 
forests fail to rejuvenate. For example, in the High Engadine, 
in Switzerland, it has been established by means of long- 
continued observations that the forests of larch (Laria decidua) 
which partly cover the slopes and parts of the valleys of this 
part of the Alps do not everywhere regenerate themselves from 
seed. The seedlings of larch require abundant light; and this 
they do not always find beneath the old forest-growth. But 
the Arolla pine (Pinus Cembra) finds the conditions of light 
more favourable to its development. It sows itself abundantly 
and develops vigorously ; so that under these special and rare 
conditions, the forest of Arolla pine will succeed the forest 
of larch without the intervention of man (cf. Flahault and 
Schroéter, 1910; Rubel, 1911). However, no such explanation 
as this is applicable in the present district. 
A matter which, in my judgment, is not as a rule sufficiently 
emphasized by plant geographers and foresters is that, in a 
closed plant community, seedlings, especially seedlings of plants 
with large seeds such as the oak and beech, are rarely found. 
On the other hand, open and (to a less extent) intermediate 
associations, if the general life-conditions are favourable, permit 
of invasion and rejuvenescence. For example, the elms near 
Cambridge produced an excessive quantity of fertile seeds in 
the summer of 1909. Many of these seeds germinated on more 
or less bare patches of soil, but not on the adjoining closed 
pasture-land. It follows that a wood whose carpet is fully 
occupied by closed ground societies does not tend to rejuvenate 
itself; and, as the more upland ash, oak, and birch woods of 
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