GEOLOGICAL EXTERMINATIONS. 185 



the excessively limited range of the various species of Ammonites 

 throughout the Jurassic and Liassic periods, so that their life's 

 history seems limited to the time necessary for the deposition of a 

 few inches or feet of strata. The succession of the Ammonite forms 

 without any apparent change in the environment, as far as it is 

 possible to carry observation, is one of the most curious problems in 

 the life history of oceanic forms. It is otherwise with land forms 

 and those which inhabited estuaries and shallow waters, there, 

 slight physical changes may easily have brought about the destruc- 

 tion of whole races. 



P.S. — On reading Dr. Woodward's important remarks, it seems 

 to me that he has rather mistaken the views of the author of this 

 paper. It does not seem to me that Dr. Warring wished to be 

 understood as holding that all life was at any time exterminated 

 over the globe after its original appearance, and was subsequently 

 reintroduced, but that from time to time, certain genera and species 

 were exterminated, or failed to leave descendants. 



n 2 



