And nature, moreover, does this on a scale so stupendous as to render all human contrivances for this purpose utterly insignificant in comparison.

The great agent which nature employs in this work is vegetation.

Indeed, it may truly be said that the great function of vegetable life, in all the infinitude of forms and characters which it assumes, is to _receive and store up force_ derived from the emanations of the sun.
Animal life, on the other hand, exists and fulfills its functions by the _expenditure_ of this force.

Animals receive vegetable productions containing these reserves of force into their systems, which systems contain arrangements for liberating the force, and employing it for the purposes it is intended to subserve in the animal economy.
The manner in which these processes are performed is in general terms as follows: The vegetable absorbs from the earth and from the air substances existing in their natural condition--that is, united according to their strongest affinities.

These substances are chiefly water, containing various mineral salts in solution, from the ground, and carbonic acid from the air.