And thus the three great nature-philosophers of the old world, Thales, Anaximenes, and Heracleitus, have been our guides, so to speak, in surveying the most striking phenomena of water, air, and fire. The fourth member of the ancient group of "elements" has received but incidental treatment. Obviously it could hardly be otherwise, especially within the limits which such a study as this imposes. The varied and wondrous forms of vegetable and animal life have likewise made but brief and transient appearances; but this omission has been due to a definite intention expressed at the outset. It may nevertheless be well, before concluding, to cast a glance over the rich provinces which still lie open to the nature-mystic for further discovery and research. The more striking features of the landscape have always arrested attention and stimulated the mystic sense. The peculiar influence of heights has been noted at an earlier stage, though but cursorily. Much might be said of the enormous effect of mountain scenery. The most direct form of nature-feeling finds expression in Scott and Byron; and the description of crags, ravines, peaks and gorges, bulks largely in their writings. Typical are these lines from "Manfred": "Ye crags upon whose extreme edge I stand, and on the torrent's brink beneath Behold the tall pines dwindled as to shrubs In dizziness of distance." or Shelley with his "Eagle-baffling mountain Black, wintry, dead, unmeasured, without herb, Insect, or beast, or shape, or sound of life." Indeed there are few poets, even those who are chiefly concerned with man and his doings, who do not often turn to mountain scenery at least for similes. And it could not be otherwise; for the immanent ideas here manifested are self-assertive in character and specially rich in number and variety. As it has been well expressed, nature's pulse here seems to beat more quickly. In olden days the high places of the earth associated themselves with myths of gods and Titans. Fully representative of the world of to-day, Tennyson asks: "Hast thou no voice, O Peak, That standest high above all?" And his answer turns on the mystic bonds that bind the deep and the height into a cycle of interdependent activities. "The deep has power on the height, And the height has power on the deep. A deep below the deep And a height beyond the height! Our hearing is not hearing, And our seeing is not sight." Or Morris gives the mysticism a more personal turn: "Oh, snows so pure! oh, peaks so high! I lift to you a hopeless eye, I see your icy ramparts drawn Between the sleepers and the dawn; I see you when the sun has set Flush with the dying daylight yet. . . . Oh, snows so pure! oh, peaks so high! I shall not reach you till I die."