Seeds are the chief means by which the forest is extended or produced. They are sown by wind and gravity, by water, by birds and beasts. I have dwelt at length upon the romance of seed-scattering in "The Spell of the Rockies," in the chapter concerning "The Fate of a Tree Seed." Each species of tree has its own way of scattering its seeds. Once upon the earth, they and the seedlings that may spring from them have peculiar limitations and special advantages. In some cases--as, for instance, with most willows and poplars--these seeds must in an extremely short time find a place and germinate or they perish; the seeds of few trees will stand exposure for two years and still be fertile. It is only a question of a few years until seeds are carried to every treeless locality. They may journey down-stream or across lakes on a log, fly with birds across mountain-ranges, ride by easy stages clinging to the fur of animals, or be blown in storms across deserts; but these adventurous seeds may find grass in possession of the locality and so thickly sodded that for a century or longer they may try in vain to establish a forest. Commonly wind-blown seeds are first upon the ground and the most numerous. Though it is of advantage to be the first upon the ground, it is of immense importance that the seed which falls in an opening produce a seedling which thrives in the sun-glare,--which grows without shade. The seedlings of our great oaks and most strong and long-lived trees cannot thrive unless shielded from the sun, sheltered from the wind, and protected from the sudden temperature-changes which so often afflict openings. While these maintain the forest areas, they extend it but little. Only a small number of trees have the peculiar frontier characteristics. Young trees which cannot live in the sun are called tolerant,--they tolerate shade and need it. Species which conquer sunny territory are called intolerant,--they cannot stand shade and need sunlight. It will thus be seen that the acquirement of treeless territory by any species of tree demands not only that the tree get its seeds upon the earth in that territory, but also that the seeds, once there, have the ability to survive in the sunlight and endure the sudden changes of the shelterless opening. Most species of oaks, elms, firs, and spruces require shade during their first few years, and though they steadfastly defend possessions, they can do but little toward winning new territory. On the other hand, aspens, willows, gray birch, cottonwood, old-field pine, and lodge-pole pine produce seedlings that glory in the sunlight and seek to gain more territory,--to push forward the forest-frontier.