bout all its adaptations, which
adjusts it to its environment, which sees to its repairs, heals its
wounds, meets its demands, provides more force when more is needed,
which makes one organ help do the work of another, which wages war on
disease germs by specific ferments, which renders us immune to this or
that disease; in fact, which carries on all the processes of our
physical life without asking leave or seeking counsel of us,--all this
is on another plane from the mechanical or chemical--super-mechanical.

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