The Earth as Modified by Human Action eBook

In such half-civilized regions, too, windfalls are more frequent than in those where the forest is unbroken, because, when openings have been made in it for agricultural or other purposes, the entrance thus afforded to the wind occasions the sudden overthrow of hundreds of trees which might otherwise have stood for generations and have fallen to the ground, only one by one, as natural decay brought them down.

[Footnote:  Careful examination of the peat mosses in North Sjaelland—­which are so abundant in fossil wood that, within thirty years, they have yielded above a million of trees—­shows that the trees have generally fallen from age and not from wind.  They are found in depressions on the declivities of which they grew, and they lie with the top lowest, always falling towards the bottom of the valley.—­Vaupell, Bogens Indvandring i de 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 vegetable covering, and the natural face of the earth may be considered as virtually permanent.  These conditions are well exemplified in Ireland, in a great part of England, in extensive districts in Germany and France, and, fortunately, in an immense proportion of the valley of the Mississippi and the