ay on the earth's surface,
but continued throughout long ages, will furnish the interpretation of
the history written in the rocks, and thus an explanation of the history
of the earth itself. The slow elevation of the earth's crust, such as is
still going on to-day, would, if continued, produce mountains; and the
washing away of the land by rains and floods, such as we see all around
us, would, if continued through the long centuries, produce the valleys
and gorges which so astound us. The explanation of the past is to be
found in the present. But this geological history told of a history of
life as well as a history of rocks. The history of the rocks has indeed
been bound up in the history of life, and no sooner did it appear that
the earth's crust has had a readable history than it appeared that
living nature had a parallel history. If the present is a key to the
past in interpreting geological history, should not the same be true of
this history of life? It was inevitable that problems of life should
come to the front, and that the study of life from the dynamical
standpoint, rather than a statical, should ensue. Modern biology was the
child of historical geology.

But historical geology alone could never have led to the dynamical phase
of modern biology. Three other conceptions have contributed in an even
greater degree to the development of this science.

==Conservation of Energy==.--The first of these was the doctrine of
conservation of energy and the correlation of forces. This doctrine is
really quite simple, and may be outlined as follows: In the universe, as
we know it, there exists a certain amount of energy or power of doing
work. This amount of energy can neither be increased nor decreased;
energy can no more be created or destroyed than matter. It exists,
however, in a variety of forms, which may be either active or passive.
In the active state it takes some form of motion. The various forces
which we recognize in nature--heat, light, electricity, chemism,
etc.--are simply forms of motion, and thus forms of this energy. These
various types of energy, being only expressions of the universal energy,
are convertible into each other in such a way that when one disappears
another appears. A cannon ball flying through the air exhibits energy of
motion; but it strikes an obstacle and stops. The motion has apparently
stopped, but an examination shows that this is not the case. The cannon
ball and the object it st