Danske Skove, pp. 10,14.] Besides this, the flocks bred by man in the
pastoral state keep down the incipient growth of trees on the half-dried
bogs, and prevent them from recovering their primitive condition. Young
trees in the native forest are sometimes girdled and killed by the
smaller rodent quadrupeds, and their growth is checked by birds which
feed on the terminal bud; but these animals, as we shall see, are
generally found on the skirts of the wood only, not in its deeper
recesses, and hence the mischief they do is not extensive.

In fine, in countries untrodden by man, the proportions and relative
positions of land and water, the atmospheric precipitation and
evaporation, the thermometric mean, and the distribution of vegetable
and animal life, are maintained by natural compensations, in a state of
approximate equilibrium, and are subject to appreciable change only from
geological influences so slow in their operation that the geographical
conditions may be regarded as substantially constant and immutable.


NATURAL CONDITIONS FAVORABLE TO GEOGRAPHICAL CHANGE.

There are, nevertheless, certain climatic conditions and certain forms
and formations of terrestrial surface, which tend respectively to impede
and to facilitate the physical degradation both of new countries and of
old. If the precipitation, whether great or small in amount, be equally
distributed through the seasons, so that there are neither torrential
rains nor parching droughts, and if, further, the general inclination of
ground be moderate, so that the superficial waters are carried off
without destructive rapidity of flow, and without sudden accumulation in
the channels of natural drainage, there is little danger of the
degradation of the soil in consequence of the removal of forest or other