Nature continues eternally young; the earth adorns itself every spring with leaf and flower, having the same freshness and youthful vigor as when for the first time He "let it bring forth grass, the herb yielding seed, and the fruit-tree yielding fruit after his kind." The grasses and flowers, it is true, which this year are cut or withered, the leaves and blossoms which the wind has to-day blown from the trees, will not assist in forming its robe in the next spring, but Nature draws out new shoots from the old roots, new leaves from the old branches, and thus rejuvenates herself with every new year. And although the human race, although also the other kinds of animals and plants, show as yet no trace of age in spite of the numerous thousands of years in which they have dwelt on the earth, still each individual is perishable, it grows old and dies; but new generations shove themselves uninterruptedly into the gap, so that the whole abide in the freshness of youthful vigor. Rejuvenation so dwells in nature that every individual runs through a limited circle of development, and is finally worn out and cut off, to be replaced by fresh members which pass anew through the accomplished cycle.
If we apply this view which we have gained of the rejuvenation of nature to the consideration of a single being, whether it be man, animal, or plant, we shall perceive that all life rests upon a constant renewal. Life is an uninterrupted contest with death, which attacks it every moment, but is beaten back by rejuvenation. It would be an error to represent a living being as anything constant, its appearance as anything steady; life in truth resembles a water-fall, which only apparently preserves a constant form, while in reality none of the particles of water keep their places, but are continually removed and replaced by new ones. The visible form of stillness is kept up only in perpetual movement. Life resembles a flame, which restlessly consumes itself and can shed an even light only when new particles come up in place of those which have been burned, only to be dissipated in their turn a moment later. So in living bodies the combination and arrangement of the matter on which their outer form and internal disposition depend are at no two instants the same, but an uninterrupted change of matter is taking place. The particles which are together in this moment at one point are in the following moment separated and replaced by others. For only a short time are the atoms of which bodies are built up adapted to the service of life; sooner or later they leave it in order to follow the free play of the forces of attraction which join the elements in the enduring combinations of lifeless nature. Therefore, the living body is obliged constantly to take up from without new elements of nourishment, by means of which it repairs its loss; and these insinuate themselves so closely in the place of the separated ones, that even the eye of the naturalist,