The productive capacity of a country is, however, in one respect very different from those great physical features--such as temperature, rainfall, coast configuration, surface character, geological structure, and river system--which have been previously noted. Those features are permanent qualities which man can affect only to a limited extent, as when he reduces the rainfall a little by cutting down forests, or increases it by planting them, or as when he unites an isle, like that of Cadiz, to the mainland, cuts through an isthmus, like that of Corinth, or clears away the bar at a river mouth, as that of the Mississippi has been cleared. [Sidenote: Exhausting the Mineral Wealth] But the natural products of a country may be exhausted and even the productive capacity of its soil diminished. Constant tillage, especially if the same crop be raised and no manure added, will wear out the richest soils. This has already happened in parts of Western America. Still the earth is there; and with rest and artificial help it will recover its strength. But timber destroyed cannot always be induced to grow again, or at least not so as to equal the vigour of primeval forests. Wild animals, once extirpated, are gone for ever. The buffalo and beaver of North America, the beautiful lynxes of South Africa and some of its large ruminants, are irrecoverably lost for the purposes of human use, just as much as the dinornis, though a few individuals may be kept alive as specimens. So, too, the mineral resources of a country are not only consumable, but obviously irreplaceable. Already some of the smaller coalfields of Europe have been worked out, while in others it has become necessary to sink much deeper shafts, at an increasing cost. There is not much tin left in Cornwall, not much gold in the gravel deposits of Northern California. The richest known goldfield of the world, that of the Transvaal Witwatersrand, can hardly last more than thirty or forty years. Thus in a few centuries the productive capacity of many regions may have become quite different from what it is now, with grave consequences to their inhabitants.