INTRODUCTION. 15 



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 dis- 

 covery has hardly ever been made, for it con- 

 tained 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 geo- 

 logical history, should not the same be true of 

 this history of life 1 It was inevitable that pro- 

 blems of life should come to the front, and that 

 the study of life from the dynamical standpoint, 



