HISTORY OF THE WILLOWS AND POPLARS 21 



tains. 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 advance 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 con- 

 ditions 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 

 tcdaj^ 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, associ- 

 ated with other plants of the far north such as Dryas and animals 

 like the Arctic fox and lemming. 



The accompanying 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 

 localities throughout central and northern Europe, where the 

 deposits of this age have been so intensively studied. The 

 conditions were duplicated 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. 



Over twenty kinds of willows have been discovered in the 

 Pleistocene deposits and only two or three of these are extinct 

 species. The details of their present range and Pleistocene 

 occurrences are too extensive for the present sketch so that only 

 a few will be mentioned. One or the other of the four herbace- 

 ous small leafed arctic and alpine species — Salix herbacea, S. 

 polaris, S. retusa and S. reticulata are found in Pleistocene 



