This fact has its effect upon the next generation, because it is these dominant surviving trees which bear seed most abundantly. After the tree has finished growing in height and diameter most vigorously--the pole stage--and proved to be fitted for the place, its energy is largely spent in raising seed. As this process goes on generation after generation, only the best coming to maturity in each, the poorer sorts are sifted out, and each region and continent has those species best fitted to meet the conditions of life there. This is the reason why exotics are very likely to be sensitive and perhaps succumb to influences to which native trees are immune. Standards and veterans are the survivors of all the lower stages, each of which has had its especial dangers. If left alone, the tree gradually dies and at last falls and decays, adding somewhat to the fertility of the forest soil. From the point of view of human use, it would far better have been cut when ripe and turned into lumber. It is a mistake to suppose that the natural virgin forest is the best possible forest, and that it should therefore be left alone. In the National Forests the ripe lumber is sold and a considerable revenue is thus available. But nature's way with the dead tree is to use it to produce more life. How she does so will be explained in the next chapter, on the enemies of the forest. [Footnote 1: Gifford Pinchot, _Primer of Forestry_, p. 44.] THE FOREST ORGANISM. REFERENCES:[A] Pinchot, _For. Bull._ No. 24, I, pp. 25-66. Bruncken, pp. 13-31. _For. Circ._ No. 36, p. 8. Fernow, _Economics_, pp. 140-164. [Footnote A: For general bibliography, see p. 4] CHAPTER VI. NATURAL ENEMIES OF THE FOREST. The natural enemies of the forest--as distinct from its human enemies--fall into three groups: (1) Meteorological, (2) Vegetable, (3) Animal. METEOROLOGICAL FORCES. [Illustration: Fig. 74. Effect of Wind, July, 1902, Cass County, Minnesota. _U. S. Forest Service._] _Wind._ "Windfalls" are not an uncommon sight in any forest. Frequently only small areas are blown down, one large tree upsetting a few others, or again a vast region is destroyed by great storms, Fig. 74. An area of many square miles in Florida covered with long-leaf pine was thus destroyed several years ago. The "slash" thus formed, when well dried, is particularly liable to catch fire and burn furiously. Windfalls are especially common among shallow-rooted trees, as hemlock, basswood and spruce, on sandy soil and on shallow soil underlaid with solid stone, especially where open spaces give the wind free sweep. It follows that an unbroken forest is a great protection to itself. The only precautions against wind therefore, that can be taken by the forester, are to keep the forest unbroken by selecting only the larger trees for felling or to cut down a given tract by beginning at the side opposite the direction of prevailing storms and working toward them.