302 THE STUDY OF PLANT COMMUNITIES ' Chapter XI 



ing early historical time are as nothing compared with those in- 

 volved in determining prehistoric climaxes. 50 Fossils, variously pre- 

 served, are the chief source of our knowledge of ancient floras, 

 many of which have disappeared completely. Considering that 

 different species and even parts of the same plant are unequally 

 preserved, it is surprising that we know as much of these old 



FlG. 160. Interglacial forest relicts on beach below high tide, Glacier Bay, 

 Alaska. These hemlock stumps, probably several thousand years old, repre- 

 sent forest that lived before the last major advance of ice, which buried them 

 under glacial debris (above beach). Tide action has again exposed the stumps. 

 —Photo by W. S. Cooper. 1 



75 



floras as we do. Certainly we know that there have been extreme 

 climatic changes on various parts of the earth and that with them 

 have come modifications in vegetation, which sometimes elimin- 

 ated entire floras. 



More recent vegetational history has been given greater atten- 

 tion because of its direct relationships to our modern flora and, 

 possibly, because it offers greater probability of solution. Post- 

 glacial climate and vegetation have been studied more intensively, 

 therefore, than those of preglacial time. Plant remains, buried and 

 preserved between layers of glacial drift, have yielded much in- 

 formation on the amount of time involved, the climate, and the 



