406 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



THE PLEISTOCENE FLORAS. 



The Pleistocene, because of the widespread glaeiation which gives 

 it a distinctive place in geological chronology, is, for humanity, the 

 Ice age or Glacial period, although a similar period of climatic rigor 

 has already been described in connection with the Permian Glos- 

 sopteris flora, and evidence of similar glaciations in the early 

 Paleozoic and pre-Paleozoic times has been discovered in recent 

 years. That the Pleistocene glaeiation was contemporaneous with 

 the evolution of the human stock and exercised a profoundly modify- 

 ing influence on the noble races of mammals and forest trees of 

 the Northern Hemisphere enhances its interest, as does the obvious- 

 ness of its modification of the topography, resulting in numerous 

 lakes, ponds, and bogs. The freshness of its moraines, bowlder till, 

 and sand plains — all scarcely modified in the few thousands of 

 years that have elapsed since the last ice sheets disappeared — also 

 emphasized its nearness to human history. 



The inauguration of glacial conditions found an essentially similar 

 flora in all three of the continents of the Northern Hemisphere. The 

 retreat of the last ice sheet left an impoverished flora in Europe and 

 two great asylums of survivors in eastern North America and east- 

 ern Asia. The explanation, broadly speaking, is most simple. 

 Neither America nor Asia with their extensive coastal plains and 

 north and south mountain chains offered insuperable barriers to 

 migration southward and back, while in Europe the mountain 

 ranges (Pyrenees, Alps, Carpathians, Balkans, Caucasus) all trend 

 east and west, many were lofty enough to be local centers of glaeia- 

 tion, while the sea effectually stopped the gaps between the various 

 mountain systems. Hence many of the plants of the Pliocene forests 

 of Europe were unable to escape extinction and so perished. 



There were at least four separate times when ice sheets accu- 

 mulated over the land. Each of these lasted for from 10,000 to 

 20,000 years, and they were separated by long intervals of genial 

 climate known as Interglacial periods, of thousands of years' dura- 

 tion, during which the floras spread northward to even beyond 

 their present range. Many such Interglacial floras have been de- 

 scribed from Europe, where the subject has been diligently investi- 

 gated in connection with the economic study of peat bogs. The 

 best known Interglacial flora of North America, where the extensive 

 peat resources have been largely neglected, is that found in the 

 Don Valley near Toronto. Here are found the plane tree, maple, 

 osage orange, and other types that do not quite reach that latitude 

 at present. Other traces of the Pleistocene floras are found in cave 

 deposits associated with a partially extinct fauna, and buried swamp 

 deposits overwhelmed by a mantle of sand during changes along the 



