In science, we have to consider two things: power and circumstance. All we know of the egg, from each successive discovery, is, _another vesicle_; and if, after five hundred years, you get a better observer, or a better glass, he finds within the last observed another. In vegetable and animal tissue, it is just alike, and all that the primary power or spasm operates, is, still, vesicles, vesicles. Yes--but the tyrannical Circumstance! A vesicle in new circumstances, a vesicle lodged in darkness, Oken thought, became animal; in light, a plant. Lodged in the parent animal, it suffers changes, which end in unsheathing miraculous capability in the unaltered vesicle, and it unlocks itself to fish, bird, or quadruped, head and foot, eye and claw. The Circumstance is Nature. Nature is, what you may do. There is much you may not. We have two things--the circumstance, and the life. Once we thought, positive power was all. Now we learn, that negative power, or circumstance, is half. Nature is the tyrannous circumstance, the thick skull, the sheathed snake, the ponderous, rock-like jaw; necessitated activity; violent direction; the conditions of a tool, like the locomotive, strong enough on its track, but which can do nothing but mischief off it; or skates, which are wings on the ice, but fetters on the ground. The book of Nature is the book of Fate. She turns the gigantic pages--leaf after leaf--never re-turning one. One leaf she lays down, a floor of granite; then a thousand ages, and a bed of slate; a thousand ages, and a measure of coal; a thousand ages, and a layer of marl and mud; vegetable forms appear; her first misshapen animals, zoöphyte, trilobium, fish, then, saurians--rude forms, in which she has only blocked her future statue, concealing under these unwieldy monsters the fine type of her coming king. The face of the planet cools and dries, the races meliorate, and man is born. But when a race has lived its term, it comes no more again.