[Illustration: FIG. 134.--Sheep Grazing on Holy Cross National Forest, Colorado. The drove consists of 1600 sheep, of which only part are shown in the photograph.] The grazing of sheep, goats and cattle in the forest is another important source of injury to which foresters must give attention. In the West this is quite a problem, for, when many thousands of these animals pass through a forest (Fig. 134), there is often very little young growth left and the future reproduction of the forest is severely retarded. Grazing on our National Forests is regulated by the Government. As a means of protection against insects and fungi, all trees infested are removed as soon as observed and in advance of all others, whenever a lumbering operation is undertaken. [Illustration: FIG. 135.--A Typical Montana Sawmill.] How forests are harvested: Forestry and forest preservation require that a forest should be cut and not merely held untouched. But it also demands that the cutting shall be done on scientific principles, and that only as much timber shall be removed in a given time as the forest can produce in a corresponding period. After the cutting, the forest must be left in a condition to produce another crop of timber within a reasonable time: see Fig. 122. These fundamental requirements represent the difference between conservative lumbering and ordinary lumbering. Besides insuring a future supply of timber, conservative lumbering, or lumbering on forestry principles, also tends to preserve the forest floor and the young trees growing on it, and to prevent injury to the remaining trees through fire, insects and disease. It provides for a working plan by which the kind, number and location of the trees to be cut are specified, the height of the stumps is stipulated and the utilization of the wood and by-products is regulated. Conservative lumbering provides that the trees shall be cut as near to the ground as possible and that they shall be felled with the least damage to the young trees growing near by. The branches of the trees, after they have been felled, must be cut and piled in heaps, as shown in Fig. 122, to prevent fire. When the trunks, sawed into logs, are dragged through the woods, care is taken not to break down the young trees or to injure the bark of standing trees. Waste in the process of manufacture is provided against, uses are found for the material ordinarily rejected, and the best methods of handling and drying lumber are employed. Fig. 135 shows a typical sawmill capable of providing lumber in large quantities.