TJie Method 13 



constant changes are made from one form to 

 others. 



9. A law having parallel expressions in two 

 fields has greater validity than a complete in- 

 duction in one field. 



10. If the causes of present changes are 

 sufficient to account for past changes, they 

 should also be accepted for the latter. 



There is in this last rule an importance 

 not at first apparent. The principle of paral- 

 lelism is well illustrated by the relation of 

 geography to geology. Geography is a study 

 of the earth as it is, including all its present 

 changes. Geology is a study of it as it has 

 developed through successive stages. Geology 

 is thus a long series of past geographies. 

 There were no forces at work in any of 

 them that are not now active. Forces may, 

 it is true, be obscure and subordinate at the 

 present time, but they are here, and it is 

 through their study that the great changes 

 of the past are made manifest. We have 

 few glaciers, but it is through them that the 

 puzzles of the glacial epoch were solved. 

 The denuding forces of air and water seem 



