==Historical Geology==.—­Preparation had been made for this new method of studying life by the formulation of a number of important scientific discoveries.  Prominent among these stood historical geology.  That the earth had left a record of her history in the rocks in language plain enough to be read appears to have been impressed upon scientists in the last of the century.  That the earth has had a history and that man could read it became more and more thoroughly understood as the first decades of this century passed.  The reading of that history proved a somewhat difficult task.  It was written in a strange language, and it required many years to discover the key to the record.  But under the influence of the writings of Lyell, just before the middle of the century, it began to appear that the key to this language is to be found by simply opening the eyes and observing what is going on around us to-day.  A more extraordinary and more important discovery has hardly ever been made, for it contained the foundation of nearly all scientific discoveries which have been made since.  This discovery proclaimed that an application of the forces still at work to-day 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.