Le Conte.] 



Critical Periods. 



zoic reptiles on the appearance of the Tertiary mammals, and of the 

 great Palaeozoic ganoids on the appearance of the Mesozoic rep- 

 tiles. But in the case of the appearance of man, this effect was far 

 greater than in any other. The changes of life-forms produced 

 during the glacial period in the manner already explained have 

 been continued and increased by man. The whole fauna and flora 

 of the earth are now being changed by his agency and readjusted 

 to his wants, and the change will be completed only when the whole 

 earth is occupied by civilized man. Evidently, then, there is now 

 going on under our eyes and by human agency, a change in organic 

 forms, more complete and more rapid than has ever before taken 

 place in the whole history of the earth. Shall we ignore it because 

 we are in the midst of it, or because we are ourselves the main 

 agent in bringing it about? Must not man be accounted among 

 the agencies of nature? 



4. Among the characteristics of critical periods we mentioned 

 also the birth of great mountain ranges. Have we an)- such born 

 at this time? — I think we have. 



On account, I suppose, of the increasing rigidity of the earth's 

 crust, the great movements of this time were mainly epeirogenic not 

 erogenic. Mountains of the usual type, i. c, by strata-crushing, 

 were not formed unless we make an exception in case of plications 

 of Pliocene strata in the Coast Ranges of California, mentioned by 

 Lawson.* But on the western margin of the American continent, 

 mountains of the monoclinal type, i. c, by block-tilting, were formed 

 on a grand scale. Examples of these are found in the Basin-ranges, 

 in the Sierra Nevada and Wahsatch (which border the basin on 

 either side), and in Alaska, as shown by Russell, in Mt. St. Elias 

 Range. All these mountains had been already formed at a previous 

 time as mountains of the usual type, but at the end of the Tertiary 

 or beginning of the Quaternary, they were rejuvenated and their 

 present forms and heights given as mountains of the monoclinal 

 type. 



It is almost impossible to overstate the greatness of the changes 

 which took' place during this wonderful period. Besides the eleva- 



*This Br [.let r n, Veil. 1, No*. 4 rind S. 



