The most marked result of cold and snow is the extreme shortness of the growing-season which they allow the trees. Many inclined trees are broken off by snow, while others are prostrated. Though the trees are flattened upon the earth with a heavy load for months, the snow cover affords the trees much protection, from both the wracking violence of the winds and their devitalizing dryness. I know of a few instances of the winter snows piling so deeply that the covered trees were not uncovered by the warmth of the following summer. The trees suspended in this enforced hibernating sleep lost a summer's fun and failed to envelop themselves in the telltale ring of annual growth. Snow and wind combined produce acres of closely matted growth that nowhere rises more than three feet above the earth. This growth is kept well groomed by the gale-flung sand, which clips persistent twigs and keeps it closely trimmed into an enormous bristle brush. In places the surface of this will support a pedestrian, but commonly it is too weak for this; and, as John Muir says, in getting through, over, under, or across growths of this kind, one loses all of his temper and most of his clothing! Timber-line is largely determined by climatic limitations, by temperature and moisture. In the Rocky Mountains the dry winds are more deadly, and therefore more determining, than the high winds. During droughty winters these dry winds absorb the vital juices of hundreds of timber-line trees, whose withered standing skeletons frequently testify to the widespread depredations of this dry blight. A permanent advance, too, is made from time to time. Here and there is a grove, a permanent settlement ahead of and above the main ranks. In advance of these are a few lone trees, heroes scouting in the lead. In moist, sheltered places are seedlings and promising young trees growing up in front of the battle-scarred old guard. Advances on dry, wind-swept ridges are more difficult and much less frequent; on a few dry ridges these trees have met with a repulse and in some places have lost a little territory, but along most of its front the timber-line is slowly advancing into the heights. With this environment it would be natural for these trees to evolve more hardiness than the present trees have. This would mean trees better fitted to contend with, and more likely to triumph over, the harsh conditions. Evolutionary development is the triumphing factor at the timber-line.