LATER GEOLOGICAL HISTORY OF NORTPI AMERICA 27 



It seems probable that the great majority of our arborescent flora 

 or forest trees were already in existence before the close of the 

 Pleistocene, if not in the preceding Pliocene, such changes as 

 have occurred since that time relating almost entirely to their 

 distribution. 



The making available for occupation of large areas covered by 

 the ice sheet or submerged along the continental border and the 

 resulting complexity of glacial soils was doubtless a great stimulus 

 to evolutionary activity, which was, however, confined almost 

 entirely to the herbaceous vegetation. 



In the preceding very brief sketch the aim has been to convey 

 in a concrete manner the cardinal fact, often overlooked, that our 

 present forest flora is but one of many that have flourished in this 

 country and that its beginnings may be traced back several millions 

 of years, rather than to present a detailed picture of the floral 

 history of North America since the Cretaceous. No attempt has 

 been made to make the following sketches exhaustive even for the 

 forms that are discussed, and while the separate discussion of the 

 different forest trees necessitates a certain amount of repetition 

 tliis will serve to amplify and fix the facts of the foregoing brief 

 outhne of the later history of our continent and its relations to 

 the other continents. 



