APPARITION OF SPECIES 113 



The reptiles of the Mesozoic utterly fail to give us 

 the necessary links. If they were changing into any 

 thing, it was into birds, not into mammals. 



Again, the time in which the horse and its sup 

 posed progenitors have lived is one of continuous, 

 unbroken succession of species. More especially in 

 the later Tertiary there seems the best evidence of 

 gradual extinction and introduction of species, with 

 out any very widespread and wholesale destruction, 

 and this notwithstanding the intervention of that 

 period of cold and of submergence of land in the 

 northern hemisphere which has given rise to all the 

 much-agitated glacial theories of our time. Can we 

 affirm that such piecemeal work has continued 

 throughout geological time? At this point opens 

 the battle between the catastrophists and uniformi- 

 tarians in geology, a battle which I am not about to 

 fight over again here. I have elsewhere stated reasons 

 for the belief that neither view can be maintained 

 without the other, and that geological time has con 

 sisted of alternations of long periods of physical re- &quot; 

 pose and slow subsidence, in which our more important 

 fossiliferous formations have been deposited, with 

 others of physical disturbance and elevation, with 

 extinction of species. Dana has well shown how 

 completely this view is established by the series of 

 geological formations as seen on the broad area of 

 the American continent. Now the question arises, 

 How would the law of derivation operate in these two 



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