That life does not supply energy, that is, is not an independent source of energy, seems to me obvious enough, but that it does not manifest energy, use energy, or "exert force," is far from obvious. If a growing plant or tree does not exert force by reason of its growing, or by virtue of a specific kind of activity among its particles, which we name life, and which does not take place in a stone or in a bar of iron or in dead timber, then how can we say that any mechanical device or explosive compound exerts force? The steam-engine does not create force, neither does the exploding dynamite, but these things exert force. We have to think of the sum total of the force of the universe, as of matter itself, as a constant factor, that can neither be increased nor diminished. All activity, organic and inorganic, draws upon this force: the plant and tree, as well as the engine and the explosive--the winds, the tides, the animal, the vegetable alike. I can think of but one force, but of any number of manifestations of force, and of two distinct kinds of manifestations, the organic and the inorganic, or the vital and the physical,--the latter divisible into the chemical and the mechanical, the former made up of these two working in infinite complexity because drawn into new relations, and lifted to higher ends by this something we call life. We think of something in the organic that lifts and moves and redistributes dead matter, and builds it up into the ten thousand new forms which it would never assume without this something; it lifts lime and iron and silica and potash and carbon, against gravity, up into trees and animal forms, not by a new force, but by an old force in the hands of a new agent. The cattle move about the field, the drift boulders slowly creep down the slopes; there is no doubt that the final source of the force is in both cases the same; what we call gravity, a name for a mystery, is the form it takes in the case of the rocks, and what we call vitality, another name for a mystery, is the form it takes in the case of the cattle; without the solar and stellar energy, could there be any motion of either rock or beast?