214 TRANSACTIONS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
when a clearing is made. It then rapidly passes the pine, and for a 
time holds it in check. 
We can observe a struggle going on between Calluna and its 
associate the blae-berry {Vaccinium Myrtillus, L,.). Where the two 
grow together Calluna has almost always the upper hand. The 
distribution of Vaccinium^ however, is much wider than Calluna. It 
reaches a much higher altitude on our mountains, and it can grow in 
shadier woods. In both these stations it forms associations un¬ 
molested by Calluna. On the Sidlaws, where trees sheltering 
Vacciniw 7 i have been felled, Calluna can be observed gradually 
encroaching upon the area of Vaccinium and confining it to the 
remaining wood. The great range of altitude possessed by Vaccinium 
Myrtillus is shared by Vacciniuni uliginosufn, L., both surpassing 
Calluna^ Erica.^ and other Vaccinium species. This is probably 
connected with the fact that these two are the only species with 
deciduous leaves. This enables them in winter to better withstand 
the cold. (We can compare this with the fact, already mentioned, 
that the thin-leaved deciduous birch reaches a higher altitude than 
the evergreen Coniferse.) The deciduous leaves of the Vacciniu 7 n 
Myrtillus and Vacciniu 77 i uliginosu 7 n are thinner and more delicate 
than the evergreen leaves of the other species mentioned. This in 
part accounts for their dominance over the other species in shady 
woods. {Vacciniu 77 i uliginosu 77 i is not one of our wood plants, 
but in Norway I have found it, along with Vaccinhwi Myrtillus^ to 
be one of the most characteristic plants of the pine woods. We 
have already referred to a possible explanation of its rarity in our 
country.) 
Other interesting observations can be made on the changes 
brought about by Sphagmwi —in the filling up of lochs or in 
the burying of old forests. Over the felled trees in wet places 
Sphagnu 7 n gradually creeps, and soon completely covers them. 
With the return of drier conditions, Calluna may spring up on 
the surface of the moss, and finally the old forest may be reinstated 
(Geikie, etc.). 
One principle stands out in the consideration of these successions 
of vegetation. In the changes of the Spliagnu 7 n bog, the growth of 
the seedlings on the moor, the succession of plants on a forest 
clearing, in the peopling of the sand dunes—in all alike, we see a 
natural tendency towards the regeneration of the forest, which must in 
our region be looked upon as the primitive state of stable equilibrium. 
These changes afford us the biological evidence for the belief founded 
on history, tradition, and geology, that formerly a much larger area 
was covered by our forests; and they also show that all natural 
tendency is in favour of the reforestation of our mountains. 
