E, V. Wulff —20— Historical Plant Geography 



ing the succeeding years this geographical principle was lent serious 

 support in a number of excellent monographs (on Acaniholimon, 1872; 

 Labiatae persicae, 1873; Chenopodiaceae, 1880), published by Bunge 

 in the transactions (Memoires) of the Academy of Sciences of St. 

 Petersburg. In 1872 there also appeared Engler's monograph on the 

 genus Saxifraga, in the title of which it is noted that special attention is 

 given to the geographical phase. A number of very important propo- 

 sitions regarding the geographical distribution of plants were advanced 

 by Kerner in many of his later works. 



This new trend in plant taxonomy, which during the ensuing years 

 was extensively developed, was of very great service to historical plant 

 geography, since, on the basis of such thoroughly investigated material, 

 work in the field of the study of the interrelations and development of 

 floras acquired a firm foundation. 



Around Engler there developed a whole school of botanical geog- 

 raphers, who directed their work on historical plant geography along 

 the Hnes indicated by him. A great memorial to this school is the 

 collection of botanico-geographical monographs edited by Engler and 

 Drxide under the general title "Vegetation der Erde". 



These investigations were provided with an even firmer foundation 

 and developed more rapidly after the pubHcation, in 1898, of the 

 work we have already mentioned by Wettstein on the botanico-geo- 

 graphical method in plant taxonomy and its further elaboration by 

 V. L. KoMAROV in the introduction to this "Flora of Manchuria" 

 (1901). 



The number of works in our field of science from this time on in- 

 creases so rapidly that it is no longer possible to review them as fully 

 as we have up to this point, and we shall now limit ourselves to an 

 enumeration of the chief problems and investigations that have played 

 an important role in the development of historical plant geography. 



Among such investigations are those of Nathorst and Andersson 

 on fossil plants of the Ice Age. The first irrefutable proof of the 

 former extension of arctic flora considerably further south than its 

 present southern Hmits was established in 1842 by Steenstrup, who 

 found in Danish peat bogs a number of fossil remains of plants giving 

 undoubted indications as to the past history of the vegetation of Den- 

 mark. However, the fact of the finding of fossil flora of the Ice Age 

 received full recognition only later, when in 1870 Nathorst found 

 representatives of this flora in eight different localities in southern 

 Sweden. By his detailed studies and those of other investigators the 

 presence of glacial flora was established in very many localities not only 

 in Sweden but also in Norway, Denmark, the Baltic Region, Germany, 

 England, Scotland, Switzerland, Hungary, France, etc. These findings 

 gave entirely unexpected indications as to the migrations of species 

 during and immediately following the Ice Age. Moreover, these in- 

 vestigations showed that the fossil remnants of Ice Age flora found 

 in a single locality were not identical but showed variations depending 

 on the age of the deposits in which they were embedded; the more 

 recent a deposit the more closely did the species composition of its 

 fossil flora resemble that of the flora of the locahty. The numerous 

 investigations along this line which followed directly after those of 



