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4l
just as in the domain of physics from the same reasons ihe com-
mon gas laws are not valid for very small quantities of a gas.
Some years ago, there was a pronounced maximum in the num-
ber of voles on Svartlóga. In the spring of 1915, almost all
fruit-trees on the island stood loose in the ground and later died
because their roots had been eaten through by the voles during
the winter. In the same summer, the observation was made that
Sphagna had come into the meadows and meadow-woods, occur-
ring in a very peculiar manner, viz. forming an irregular mosaic
and stripes in the shape of a system of veins on certain spots of
the grass lawns. A nearer invesligation showed that on the spots
mentioned the ground was almost completely undermined by the
voles, and the system of Sphagnum veins corresponded to a system
of real veins in the ground, mined by the voles.
As to the causal relation between the two effects, it seems unli-
kely to suppose that the mining of the voles should locally cause
an increased humidity in the ground. The more probable con-
jecture is, I believe, that the peat-moss could come in simply
because the grass-roots wore eaten up and the grasses killed by
the voles. It may be supposed that on the moist meadows of
Svarllóga, in a humid sea-climate, the advantage in the competi-
tion between the grass vegetation and the peat-moss is always but
slight in favor of the former, so that a slight injury befalling the
successful competitor may give the other one the advantage.
The phenomena observed, however, were but temporary. In
the following years, the peat-mosses have again disappeared and
so have the voles almost completely. The latter fact is probably
caused partially by the weather in the spring of 1916, which was
not suitable for their young, on the other hand by the bringing in
of hedgehogs (Erinaceus europaeus).
The fact that the Sphagna disappeared as quickly again as they
did, would perhaps permit a conclusion that goes against the con-
cepts of PALMGREN on competition in the islands of Aland only a
few miles distant from Svartlöga. This author concludes from
certain facts (the chief of which I have critizised elsewhere from
another point of wiew; ROoMELL 1920) that the ecological factors
have little or no importance for the distribution of plants in the
meadow-woods of Aland, so that competition there would be merely
a contest of being the first on a free place. The rapid appearence
and disappearence of the Sphagna under the conditions described
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