Chapter II —21— History 



Nathorst enabled Andersson in 1897 to outline the history of 

 the vegetation of Sweden and of northern Europe in general. 



Closely linked with the problem of the Arctic flora is that of the 

 origin and history of development of the Alpine flora, of the finding in 

 the Alps of species of Arctic flora and the finding of Alpine plants far 

 to the north. On these problems many papers have appeared by vari- 

 ous investigators. First among these papers was that by Christ 

 (1867). 



Another important question of historical plant geography, the inter- 

 relations between the floras of North America and eastern Asia, was 

 raised as early as 1846 by Asa Gray and discussed by him in greater 

 detail in a number of subsequent papers. The accumulation of 

 paleobotanical data pointing to the fact that a number of genera, such 

 as Liquidamhar, Sassafras, Aralia, Magnolia, Liriodendron, Taxodium, 

 Sequoia, etc., represented in our day in North America by various 

 species but absent in Europe, had in the Tertiary Period an extensive 

 distribution, being found as far north as Greenland, gave grounds for 

 Gray to conclude that the flora of North America in the Tertiary 

 Period was closely allied and in many respects identical to the flora 

 then found in Greenland, Spitsbergen, and northern Europe. 



A second very important service of Gray was that he pointed out 

 the existence of an undoubted connection between the flora of the 

 Atlantic states of North America and that of northeastern Asia. This 

 remarkable fact, confirmed by an extensive list of species, Gray ex- 

 plains as follows: The flora, of which these species constitute remnants, 

 was distributed in the Miocene in what is now the arctic and subarctic 

 zone. As the cUmate became colder and the glaciers advanced, this 

 flora retreated toward the south and survived until the present time in 

 those regions that preserved climatic conditions approximating those 

 of former times, as occurred in eastern Asia and Japan, on the one 

 hand, and on the western and eastern shores of North America, on the 

 other. These data indicate that during the Tertiary Period there was a 

 direct connection and interchange of forms between Asia and America, 

 a land-bridge presumably existing at that time where the Bering Strait 

 now lies. In 1859 Maximovicz presented data showing that the flora 

 of eastern Asia had preserved much of its Tertiary character. 



Among other investigations of importance for historical plant 

 geography we may mention those concerned with the problem of peat 

 bog vegetation, which have made it possible not only to establish the 

 succession of floras in the post-glacial period but also to determine the 

 past areas of existing species of woody plants. These data, gleaned 

 from a study of microscopic plant remains in peat-deposit profiles, 

 acquired a firmer foundation and developed more rapidly after Weber 

 (1896) and Lagerheim (1905) had proposed a special microscopic 

 method of investigating the plant remains in peat bogs. This method 

 is based on a study of pollen found by taking a number of successive 

 samples at different depths in peat deposits. Thanks to the taxonomic 

 specificity of the structure of pollen, it is relatively easy to determine 

 to which genus and sometimes to which species it belongs. On the 

 basis of such investigations it is possible to judge as to the species 

 composition of a past forest, as to the quantitative interrelations of the 



