Page:Popular Science Monthly Volume 3.djvu/88

78 their structure just alluded to, and which enables them to bear their part bravely in the conflict.

It is easy to understand how an alteration of the conditions under which plants grow influences very materially the struggle we have been alluding to. A very slight change in climatal conditions—produced, for instance, by the growth of sheltering trees, or by the drainage of the soil—may be followed by the growth of quite a different set of plants from those that occupied the ground previously. The altered conditions have been advantageous to the one and disadvantageous to the other set of plants.

As an illustration of the complexity of the checks and relations between organic beings struggling together, Darwin mentions the case of a barren heath which fell under his observation, part of which was left intact, while another portion had been enclosed and planted with Scotch fir. The change in the native vegetation of the planted part of the heath was most remarkable. "Not only the proportional numbers of the heath-plants were wholly changed, but twelve species of plants, not counting grasses and carices, flourished in the plantations, which could not be found on the heath."

This sort of change was pointedly referred to by Dureau de la Malle, who relates how, after the felling of the timber in forests of a particular district of France, broom, foxglove, heaths, birch-trees, and aspens sprang up, replacing the oaks, the beech, and the ash, felled by the woodman. After thirty years, the birch and poplars were felled in their turn. Still very few of the original possessors of the soil, the oaks, etc., made their appearance: the ground was still occupied with young birch and poplar. It is not till after the third repetition of the coppicing—after an interval of ninety years—that the oaks and beech reconquer their original position. They retain it for a time, and then the struggle begins again.

Antiquarian researches also have proved that, in the natural state of things, without any violent change in external conditions, the nature of forests becomes altered. The Hercynian forests, of which Cæsar speaks, and which then consisted of deciduous-leaved trees, are now made up principally of conifers. A forest which, in the middle ages, was of beech, is now stocked with oak, and vice versa. Again, we have the evidence afforded by submerged forests and peat-bogs, according to which certain plants, now extinct in particular localities, once flourished there. We are not alluding to plants that may have required a different climate from what they now experience, but to such cases as the silver fir, the Scotch fir, Pinus Mughus, etc., which are found in this partially-fossilized condition in spots where there is apparently nothing to prevent them from growing now, where, in fact, they do grow well when planted.

Foresters in all countries are perfectly well aware of these facts, and botanists watch with interest the appearance of a different