1 60 EVOL UTION AND NA TUEAL THEOL G Y. 



subordinate rank in comparison, and must 

 therefore give way whenever his direct or 

 indirect influence interferes to a sufficient extent 

 with the conditions of their life. Thus, an 

 animal may be extirpated either because it is 

 dangerous, or because it is good for food ; and 

 if a forest is felled, or a marsh drained, all 

 the plants and animals which require such con- 

 ditions for their existence must necessarily 

 disappear. 



Sometimes species die out slowly and gradu- 

 ally ; but when rapid progress is needed, forces 

 appear to be put in action, which work with 

 much greater intensity. At a former period of 

 the history of geology, it was supposed that the 

 earth was periodically devastated by cataclysms ; 

 and although this theory has long been aban- 

 doned in its entirety, yet there is good reason 

 to believe that volcanic disturbances, floods, 

 changes of climate, etc. were far more frequent 

 and violent at an earlier period than at present ; 

 for the further we go back in time, the thinner 

 and more liable to disturbance would be the 

 crust of the earth. 



For a long period the climate appears to have 

 been nearly uniform over the whole earth, and 



