The human spirit, the brute spirit, the vegetable spirit—­all are mere names to fill a void.  The spirit of the oak, the beech, the pine, the palm—­how different! how different the plan or idea or interior economies of each, though the chemical and mechanical processes are the same, the same mineral and gaseous elements build them up, the same sun is their architect!  But what physical principle can account for the difference between a pine and an oak, or, for that matter, between a man and his dog, or a bird and a fish, or a crow and a lark?  What play and action or interaction and reaction of purely chemical and mechanical forces can throw any light on the course evolution has taken in the animal life of the globe—­why the camel is the camel, and the horse the horse? or in the development of the nervous system, or the circulatory system, or the digestive system, or of the eye, or of the ear?

A living body is never in a state of chemical repose, but inorganic bodies usually are.  Take away the organism and the environment remains essentially the same; take away the environment and the organism changes rapidly and perishes—­it goes back to the inorganic.  Now, what keeps up the constant interchange—­this seesaw?  The environment is permanent; the organism is transient.  The spray of the falls is permanent; the bow comes and goes.  Life struggles to appropriate the environment; a rock, for example, does not, in the same sense, struggle with its surroundings, it weathers passively, but a tree struggles with the winds, and to appropriate minerals and water from the soil, and the leaves struggle to store up the sun’s energy.  The body struggles to eliminate poisons or to neutralize them; it becomes immune to certain diseases, learns to resist them; the thing is alive.  Organisms struggle with one another; inert bodies clash and pulverize one another, but do not devour one another.

Life is a struggle between two forces, a force within and a force without, but the force within does all the struggling.  The air does not struggle to get into the lungs, nor the lime and iron to get into our blood.  The body struggles to digest and assimilate the food; the chlorophyll in the leaf struggles to store up the solar energy.  The environment is unaware of the organism; the light is indifferent to the sensitized plate of the photographer.  Something in the seed we plant avails itself of the heat and the moisture.  The relation is not that of a thermometer or hygrometer to the warmth and moisture of the air; it is a vital relation.

Life may be called an aquatic phenomenon, because there can be no life without water.  It may be called a thermal phenomenon, because there can be no life below or above a certain degree of temperature.  It may be called a chemical phenomenon, because there can be no life without chemical reactions.  Yet none of these things define life.  We may discuss biological facts in terms of chemistry without throwing any light on the nature of life itself.  If we say the particular essence of life is chemical, do we mean any more than that life is inseparable from chemical reactions?