140 UNIVERSITY OF MISSOURI BULLETIN 



decline of vitality, but interposes special opposing or extin- 

 guishing forces when the vital forces have dropped below the 

 point of efficiency. The same is possible in land forms, but 

 they do not necessarily operate. 



The study has included more than the mere form of the 

 land. It has been extended to the operation of the forces 

 that produce the succession of land forms — the forces of 

 erosion, particularly the action of rivers. It has been found 

 that these have a characteristic development in time both as 

 to the particular manner of their action as well as to the rela- 

 tion of the stream itself to location. Rivers have, in other 

 words, the ability to adjust themselves to a changing environ- 

 ment. They may change their location in order to avoid 

 unforeseen difficulties and to improve their right of way, so 

 to speak. They may adjust themselves to the line of least 

 resistance, even though that may make important changes of 

 course necessary. 



The past history of a land area may now be interpreted 

 through the form it has assumed under the action of erosion. 

 Its stage of topographic development, whether youth, infancy, 

 maturity, or old age, can be determined and its forms traced 

 back to the initial stage. The length of time it has existed as 

 a land surface may be determined through the correlation of 

 its forms with some period of deposition of known age. 



The working out of these principles has lighted up, at 

 least partially, the dark side of geological history. We are 

 now able to read the order of events that have taken place 

 on the land as recorded in the land forms, while before this 

 advancement we were enabled to interpret only that part of 

 the record that was written in terms of deposition. 



The first one of the great subsidiary principles of geology 

 to be worked out and the one that has the most profound 

 influence in the subsequent development of the science was 

 what I shall call the Doctrine of the Parallelism of Organic 

 Evolution and Earth Development. Without the discovery 



