70 THE BRONTOSAUR AND ITS COUSINS 



There is a corresponding change in the earth's 

 vegetation. Up to this point it has been an evergreen 

 earth. There was no winter, no need to shed the 

 leaves once a year. Now, in the Chalk Period, we 

 find in America the traces of trees which shed their 

 leaves periodically. Winter has become an institution 

 of the northern hemisphere. Before the end of the 

 period we find the older types of trees giving way to 

 the willow and birch, the oak and mulberry, the 

 laurel and myrtle, the maple and elm, the walnut, 

 and dozens of other familiar trees. New flowers in 

 great variety appear. We will consider this opening 

 of our modem world in the next few chapters. 



