86 TREE ANCESTORS 



marine marls so that the American Pliocene plant record is almost 

 a blank, although we know that the familiar types must have been 

 present since willows are abundant in the next or Pleistocene period. 



The Pleistocene, or period of continental glaciers, was an epic 

 time for all plants and animals, for it was a time during which ice 

 sheets many feet in thickness gradually accumulated in northern 

 America and Europe and in the more elevated mountains. After 

 fluctuating near a maximum for some thousands of years these 

 ice sheets gradually disappeared and were followed by a long genial 

 interglacial stage. This great accumulation and southward ad- 

 vance of the ice was repeated four times and the last ice sheet has 

 been gone only a few thousand years. During these changing 

 times all life forms were subjected to new competitions and great 

 stresses, hence many forms succumbed. Others shifted back and 

 forth with the shifting climatic conditions and still survive. A 

 great many still existing species of willows, as well as other trees, 

 make their appearance in the Pleistocene bogs, lake beds and river 

 terrace deposits and thus serve to record the gamut of changing 

 environments. We find for example the tiny Arctic willows like 

 Salix polaris, which today occurs in the Scandinavian mountains 

 and reaches its southern limits in the tundras along the Arctic 

 coast of Russia, present in the Transylvanian and Swiss Alps, 

 throughout Britain, southern Sweden, Denmark and the north 

 German plain, associated with other plants of the far north such 

 as Dryas and animals like the Arctic fox and lemming. 



The accompanjdng sketch map of Europe shows the unusual 

 climatic conditions which enabled this far northern form, now 

 confined to the lined area on the map, to extend southward almost 

 to the Mediterranean. This and other herbaceous or shrubby 

 species of Arctic willows are found at innumerable locaHties through- 

 out central and northern Europe, where the deposits of this age 

 have been so intensively studied. The conditions were dupHcated 

 in North America but as we have devoted so little study to the 

 life of our Pleistocene deposits it is not possible to obtain adequate 

 records of the distribution of our Pleistocene plants. 



