rikes have been heated, and thus the motion of
the ball has simply been transformed into a different form of motion,
which we call heat. Or, again, the heat set free under the locomotive
boiler is converted by machinery into the motion of the locomotive. By
still different mechanism it may be converted into electric force. All
forms of motion are readily convertible into each other, and each form
in which energy appears is only a phase of the total energy of nature.

A second condition of energy is energy at rest, or potential energy. A
stone on the roof of a house is at rest, but by virtue of its position
it has a certain amount of potential energy, since, if dislodged, it
will fall to the ground, and thus develop energy of motion. Moreover, it
required to raise the stone to the roof the expenditure of an amount of
energy exactly equal to that which will reappear if the stone is allowed
to fall to the ground. So in a chemical molecule, like fat, there is a
store of potential energy which may be made active by simply breaking
the molecule to pieces and setting it free. This occurs when the fat
burns and the energy is liberated as heat. But it required at some time
the expenditure of an equal amount of energy to make the molecule. When
the molecule of fat was built in the plant which produced it, there was
used in its construction an amount of solar energy exactly equivalent to
the energy which may be liberated by breaking the molecule to pieces.
The total sum of the active and potential energy in the universe is thus
at all times the same.

This magnificent conception has become the cornerstone of modern
science. As soon as conceived it brought at once within its grasp all
forms of energy in nature. It is primarily a physical doctrine, and has
been developed chiefly in connection with the physical sciences. But it
shows at once a possible connection between living and non-living
nature. The living organism also exhibits motion and heat, and, if the
doctrine of the conservation of energy be true, this energy must be
correlated with other forms of energy. Here is a suggestion that the
same laws control the living and the non-living world; and a suspicion
that if we can find a natural explanation of the burning of a piece of
coal and the motion of a locomotive, so, too, we may find a natural
explanation of the motion of a living machine.

==Evolution==--A second conception, whose influence upon-the development
of biology