Some of Nature's strange ways are exhibited in the interrelation of insects and fires in tree-killing. It is common for the attack of one of these tree-enemies to open the way for the depredations of the other. The trees that insects kill quickly become dry and inflammable and ready kindling for the forest fire. On the other hand, the injuries that green trees often receive from forest fires render them most susceptible to the attacks of insects. This interrelation--almost coöperation--between these arch-enemies of the forest was impressed upon me during my early tree studies. One day I enjoyed a splendid forest sea from the summit of a granite crag that pierced this purple expanse. Near the crag a few clumps of trees stood out conspicuous in robes of sear yellow brown. Unable to account for this coloring of their needles, I went down and looked them over. The trees had recently been killed by insects. They were Western yellow pine, and their needles, changed to greenish yellow, still clung to them. In each clump of these pines there were several stunted or deformed trees, or trees that showed a recent injury. The stunted and injured trees in these clumps were attacked and killed by beetles the summer before my visit. In these injured trees the beetles had multiplied, and they emerged the following summer and made a deadly attack upon the surrounding vigorous trees. Although this latter attack was made only a month or two before my arrival, the trees were already dead and their needles had changed to a sickly greenish yellow. Amid one of these clumps was a veteran yellow pine that lightning had injured a few years before. Beetles attacked and killed this old pine about a year before I appeared upon the scene. It was the only tree in this now dead clump that was attacked on that first occasion; but some weeks before my visit the beetles in multiplied numbers swarmed forth from it and speedily killed the sound neighboring trees. These conclusions were gathered from the condition of the trees themselves together with a knowledge of beetle habits. Not a beetle could be found in the lightning-injured pine, and its needles were dry and yellow. The near-by dead pines were full of beetles and their eggs; the needles, of a greenish yellow, were slightly tough and still contained a little sap.