decomposition of the leaves of the birch the battle begins. The birch
still flourishes on the borders of lakes and other marshy places, where
its enemy cannot exist. In the same way, in the forests of Zeeland, the
fir forests are disappearing before the beech. Left to themselves, the
firs are soon displaced by the beech. The struggle between the latter
and the oak is longer and more stubborn, for the branches and foliage of
the oak are thicker, and offer much resistance to the passage of light.
The oak, also, has greater longevity; but, sooner or later, it too
succumbs, because it cannot develop in the shadow of the beech. The
earliest forests of Denmark were mainly composed of aspens, with which
the birch was apparently associated; gradually the soil was raised, and
the climate grew milder; then the fir came and formed large forests.
This tree ruled for centuries, and then ceded the first place to the
holm-oak, which is now giving way to the beech. Aspen, birch, fir, oak,
and beech appear to be the steps in the struggle for the survival of the
fittest among the forest-trees of Denmark.

It may be added that in the time of the Romans the beech was the
principal forest-tree of Denmark as it is now, while in the much earlier
bronze age, represented by the later remains found in the peat bogs,
there were no beech-trees, or very few, the oak being the prevailing
tree, while in the still earlier stone period the fir was the most
abundant. The beech is a tree essentially of the temperate zone, having
its northern limit considerably southward of the oak, fir, birch, or
aspen, and its entrance into Denmark was no doubt due to the
amelioration of the climate after the glacial epoch had entirely passed
away. We thus see how changes of climate, which are continually
occurring owing either to cosmical or geographical causes, may initiate
a struggle among plants which may continue for thousands of years, and
which must profoundly modify the relations of the animal world, since