Accordingly, let the temperature be different, and the species of plant will be different. Suppose conditions entirely opposite to those just mentioned; take the summit of a mountain swept by violent winds, with a thin scanty soil, a cold climate, a short summer, and snow during the winter; not only will the orange not thrive here, but the greater part of other trees will perish. Of all the seeds scattered haphazard by the wind only one will survive, and you will see but one species to endure and be propagated, the only one adapted to these severe conditions; the fir and the pine will cover the lonely crags, the abrupt precipices, and long, rocky ridges, with their stiff colonnades of tall trunks and vast mantles of sombre green, and there, as in the Vosges, in Scotland and in Norway, you may travel league after league, under silent arches, on a carpet of crisp leaves, among gnarled roots obstinately clinging to the rocks, the domain of the patient energetic plant which alone subsists under the incessant attacks of gales, and the hoar-frosts of long winters. We may accordingly regard temperature and physical circumstances as _making a choice_ amongst various species of trees, all owing a certain species to subsist and propagate, to the exclusion, more or less complete, of all others. Physical temperature acts by elimination and suppression, in other words, by _natural selection._ Such is the great law by which we now explain the origin and structure of diverse existing organisms--a law as applicable to moral as to physical conditions, to history as well as to botany and zoology, to genius and to character, as well as to plant and to animal.