NATURAL HISTORY NINETEENTH CENTURY. 647 



FIG. 321. PASTEUR. 



descriptive of the present actual state of things on the surface of our 

 planet the strata, their nature, organic remains, order of arrangement, 

 the changes visibly going on by the action of waters, winds, volcanoes, 

 etc. Second, systematic inquiry into the laws and principles of the 

 actual arrangements and changes which can be arrived at by induction 

 from observation and experiment. And third, theoretical, that most 

 difficult but most fascinating department of the science, which under- 

 takes to trace the past history of our globe and of the life upon it. 

 This last problem is the subject of theoretical or speculative geology, 

 and it is not surprising that the general principles on which its solution 

 has been attempted have been very diverse. During the first part of 

 the present century two antagonistic doctrines were contending for 

 supremacy. One held that the changes which have obviously taken 

 place in the earth's crust have been occasioned by great and sudden 

 convulsions, such as upheavals or depressions of land, earthquakes, 

 huge floods, etc. Cuvier was the most distinguished supporter of 



