Besides making soil themselves, the lichens help accumulate soil by holding grains of rock broken up by their fibres and loosened by the action of the heat and cold of day and night and change of season. These little grains become entangled in the larger lichens and are kept, many of them, from being washed away by the heavy rains. So held, they are in time crumbled into soil by the action of the acids and by mixture with the products of plant decay. To this day, go where you will, over the whole face of the earth, and you'll find the lichens there ahead of you, dressed in their sober suits, some gray as ashes, others brown, but some are as yellow as gold; for even these old people like a little color once in a while. As travellers they beat all. "Their geographical range is more extended than that of any other class of plants." That's how the learned lichenologists put it. For these lichens, these humble little brothers of our dust, that many of us never looked at twice on the stones of the field, or the gray stumps and dead limbs in the wood, are so interesting when you've really met them--been properly introduced--that a whole science has grown up around them called "lichenology." And exciting! You ought to hear the hot discussions that lichenologists get into. You read, for instance, that such and such a theory "was received with a storm of opposition" (as most new theories are, by the way, particularly if they are sound). But the tumults and the strifes of science, of politics, or of wars don't disturb little old Mr. Lichen himself. There on his rock he'll sit, overlooking the scenery and watching life and the seasons come and go for 100, 200, 500 years, and more. For while they grow so slowly the lichens make up for it by living to an extreme age. THE LICHENS AND THE ROMAN EMPIRE Why, do you know that during the lifetime of certain lichens that are still hale and hearty, not only a long line of Cæsars might rise, flourish, die, and, with their clay, stop holes to keep the wind away, as Mr. Shakespere put it, but the vast Roman Empire could and did come into being, move across the stage with its banners and trumpets and glittering pomp and go back to the dust again. Some lichens, growing on the highest mountain ranges of the world, are known to be more than 2,000 years old! [Illustration: THE SEQUOIAS; THE SUNLIGHT AND THE SHADE Wonderful sunlight effect, isn't it? We are here in Sequoia National Park and those big trees are sequoias, members of the pine-tree family.] II. THE MARCH OF THE TREES