GEOLOGY 143 



certain group of remains in certain rocks and an entirely new 

 assemblage of different forms in the immediately overlying 

 rocks. This is no longer regarded as an evidence of the anni- 

 hilation of the older fauna and the sudden recreation of the 

 new, but to the migration from the region of the one and the 

 immigration into it of the other, due to a change in the en- 

 vironment which had become unfavorable to the older and 

 favorable to the newer fauna. 



On the other hand, gradual changes in the character of a 

 fauna in ascending through a series of conformably overlying 

 beds is a common occurrence. A specially significant fact is 

 that such conditions are found in localities which show evi- 

 dence of gradual change of conditions, making organic change 

 necessary in order to bring about adaptations to the changing 

 environment. 



I have stated the fundamental concepts with which geol- 

 ogy is concerned and some great principles of geological sci- 

 ence which have been constructed from the facts that have 

 been discovered in the study of the earth's crust. Not all of 

 the problems, however, have been solved. Our roadway has 

 been traced for only a short distance. The darkness always 

 exists at either one end, or both. In fact, the roadway itself 

 is traceable with more or less difficulty. Its general trend 

 only is known. It is merely blazed out, or at most roughly 

 chopped out, and is still narrow. Before the wheels of geo- 

 logical progress can travel over it without hindrance, innu- 

 merable obstructions must be removed and the grades 

 changed. In other words, a large number of doubtful ques- 

 tions of detail concerning the great principles stated in this 

 paper are still unsolved. The work of removing these diffi- 

 culties will require years and possibly ages to accomplish. 



In addition to problems of detail concerning principles 

 whose general trend is known, there are other great problems 

 on which we have scarcely begun the accumulation of facts, 

 or at least have not yet accumulated enough to build even the 



