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 was even greater, was the doctrine of evolution. It is true
that the doctrine of evolution was no new doctrine with the middle of
this century, for it had been conceived somewhat vaguely before. But
until historical geology had been formulated, and until the idea of the
unity of nature had dawned upon the minds of scientists, the doctrine of
evolution had little significance. It made little difference in our
philosophy whether the living organisms were regarded as independent
creations or as descended from each other, so long as they were looked
upon as a distinct realm of nature without connection with the rest of
nature's activity. If they are distinct from the rest of nature, and