E. V. Wulff — 150 — Historical Plant Geography 



instances of the extinction of species as a result of climatic changes. A 

 good example is that of the finding of fossil remains of subtropical 

 plants in places located beyond the arctic circle and covered with 

 eternal snow. Moreover, even in our present-day flora we may observe 

 such a process of species extinction induced by changes in climatic 

 conditions. 



Braun-Blanquet (1923) has given us a most interesting account of 

 the process of extinction (still in progress) of arctic plants in the moun- 

 tains of southwestern Europe. The occupation of these habitats by 

 arctic species took place during the Ice Age, when a lowering of the 

 temperature in Europe and the advance of the glaciers southward 

 brought about a corresponding movement of the arctic vegetation. 

 The subsequent increase in temperature, as a result of the regres- 

 sion of the glaciers, was accompanied by an analogous movement 

 of the vegetation. But a few of these arctic species remained in 

 the West-European Alps outside their potential areas. Gradually, as 

 the climate became more arid, there occurred an ever greater decrease 

 in the number of stations of these species. Thus, Braun-Blanquet 

 {I.e., p. 164) cites authorities for the fact that Ligularia sibirica had by 

 1878 disappeared from its former habitat in the Massif Central of 

 France and that Saxifraga hirculus, formerly distributed near Nantua, 

 was, in 1897, no longer found there. The species Oxycoccus quadri- 

 petalus, Caltha paluslris, Liparis Loeselii, Rhynchospora fusca, R. alba, 

 and Eriophorum angustifolium became extinct as a result of the drying 

 up of peat swamps near Lake Bientina in Tuscany. Thanks to the 

 disappearance of the first-named species, the genus Oxycoccus altogether 

 disappeared from the flora of central Italy, being preserved only in 

 northern Italy on the southern slopes of the Alps. In a similar way 

 the hmits of distribution of Rhynchospora alba and of the genus Liparis 

 have shifted to the north, for they are no longer found south of the Po. 



Analogous instances of the shifting to the north of the limits of dis- 

 tribution of species that had spread their range to the south during the 

 Ice Age are found in Germany. There are data to show, moreover, 

 that the process of extinction (in southern regions) of these species was 

 not recently initiated but began in prehistoric times. This has been 

 confirmed by the finding of fossil remains of such northern species in 

 central and southern Europe. For example, Andersson reported that 

 in swamps adjoining Lago di Garda there were found a large number of 

 fossilized fruits of Najas flexilis, which no longer grows in Italy. 



In recent years the pollen-statistics method, whereby the pollen con- 

 tained in samples of peat is analyzed to determine the species from 

 which it was derived, has shown that such samples contain pollen of 

 numerous species that no longer grow in the vicinity of the peat 

 swamps investigated, and it has thus provided much valuable testi- 

 mony as to the migration and extinction of species. 



Climatic Changes in the Tertiary Period and the Resultant Shift- 

 ing of Vegetation Zones: — Shifting of the climatic zones of our globe 

 has occurred time and again. A study of the fossil remains of plaiits 

 and their geographical distribution and of the areas of the species 

 making up present-day floras gives ample evidence of this fact. In 



