INTRODUCTION. 7 



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 in- 

 evitable that problems of life should come to the 

 front, and that the study of life from the dynam- 

 ical standpoint, rather than a statical, should en- 

 sue. 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 recog- 

 nize 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, 



2 



